Skin

This article primarily describes the skin of mammals.

Skin (from Old Norse skinn "animal hide, fur,") (= cutaneous membrance) is a thin layer of tissue that forms the natural outer covering of vertebrates. (Other animal coverings such as the arthropod exoskeleton have different developmental origin, structure and chemical composition.) It sits above and protects the flesh, which attaches the skin to underlying bone, muscle, blood vessels, and nerves.

The skin is the largest organ in the human body by surface area and weight. In adults, the skin covers an area of about 2 square meters and weights 4.4-5 kg (10-11 lb). [7] In humans, it ranges in thickness from 0.5mm on the eyelids to 4.5mm on the heels.

Contents

1   Functions

Skin has several functions:

  1. Protection against oxidative stressors, such as `ultraviolet radiation`_, ozone_, and chemicals
  2. Retention of body water
  3. Helps regulate body temperature
  4. Provide sensory information about the surrounding environment
  5. Excrete small amounts of salt and other organic compounds
  6. Production of `vitamin D`_

2   Properties

2.1   Color

The color of skin is determined by melanin. People who lived in parts of the world that had less ultraviolet radiation (due to higher latitude, ozone concentration, atmospheric turbudity, and the perihelion effect), evolved lighter skin in order to better absorb `vitamin D`_.

Changes in skin color may indicate imbalances in homeostatis_. For example, a bluish color indicates oxygen deficiency.

2.1.2   Birthmarks

There are two types of birthmarks.

  1. Pigmented birthmarks result from when there is an excess of skin pigment cells, the cells that give your skin color, in one place. Examples of these would include moles, café au lait spots, and Mongolian spots.
  2. Vascular birthmarks are from an increased amount of blood vessels in a given location. These are red, such as salmon patches and port-wine stains.

It is still unknown why birthmarks form.

2.2   Hydration

Skin dries out by a natural process called trans-epidermal water loss ("tewl"). Blood vessels only supply moisture to the dermis. From there, water migrates to the epidermis before it evaporates.

3   Substance

3.1   Glands

3.1.1   Sebaceous gland

A sebaceous gland is a microscopic exocrine_ gland_ in the skin that secrete sebum. Sebum (from Latin grease) is an oil made of triglycerides_, `wax esters`_, squalene_, and free fatty acids.

In humans, sebaceous glands occur in the greatest number on the face and scalp, but are on all parts of the skin except the palms and soles.

Sebum sits between the dead cells of the straum corneum and living ones (which?). It has two functions: lock in moisture and prevent allergens from entering lower layers of the skin.

There are two types of sebaceous glands, those connected to hair follicles, and those that exist independently.

Androgens_ such as testosterone have been shown to stimulate secretion, and estrogen has been show to inhibit secretion.

The skin covering the lips lacks sebaceous glands, which is why people require chapstick.

4   Layers

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The skin is full of pore holes which penetrate into the skin, called follicles. In pigskin, they penetrate deep into the lower flesh area, and as a result when you look at the reverse side of a pig skin, you can see the pore holes. [1] (Think of a football.) Hair grows in the skin through the follicles.

Mammalian skin consists of two primary layers: the epidermis and the dermis.

4.1   Epidermis

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Layers of the epidermis.

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The epidermis (from Greek epi "on" + derma "skin") is the most superficial layer of the skin. The epidermis keeps water in the body and prevents pathogens_ from entering.


[50]

When you look at someone's skin, you see the top layer, made up of cells that reflect light. When that top layer is smooth, it reflects light evenly so that the skin looks more uniform and radiant than it does when the surface is rougher.

As the lowest portion of the epidermis are "mother cells", called basal cells, which produce all the other skin cells. They divide into "daughter cells", which rise up to the higher levels of the epidermis. As they travel, they age and evenutally die off in a process called the "cell cycle", which can take anywhere from twenty-six to for forty-two days. Between age 30 and 80, the cell slows from 30 to 50 percent of its pace in youth. That means that older skin renews itself much more slowly, forming a rough surface of cells, rather than a smooth surface.

Substances released by the cells in the middle of the epidermis form a protective film made of lipids (fats) that surround skin cells and helps keep the skin hydrated. You fingers and toes contain fewer lipids are therefore not as "watertight" as your legs, which is why your fingers and toes look shriveled after immersion in water, but your legs do not. Your skin cracks in cold weather because the chilled lipids become stiff and less able to adjust to movement. The goal of the best moisturizers is to increase the amount of these important lipids.

Dryness and oiliness depend primarily on the condition of the skin barrier and oil production. The barrier is like a brick wall, with each brick (or cell) held in place by mortar (fats called lipids). The main components of the barrier are different kinds of lipids: ceramides_, `fatty acids`_, and cholesterol_. These must be present in the right proportion to keep skin watertight. An impaired barrier will tend toward both dryness and sensitivity. Dryness results when skin moisture evaporates. Sensitivity results when a deficient barrier permits the entry of outside irritants.

The skin has many oil (sebaceous) glands, which secrete oil that contains wax esters, triglycerides_, and squalene. These lipids form a film that helps keep moisture in the skin. Oil production can be affected by diet, stress, hormones, and genetics.

Baumann describes four different types of sensitive skin:

Acne subtype
Develops acne, blackheads, or whiteheads
Rosacea subtype
Develops recurring flushing, facial redness, and hot sensation
Stinging subtype
Develops stinging or burning of skin
Allergic subtype
Develops redness, itching, and flaking skin

Each subtype has inflammation in common.

Select a product with a minimum SPF of 15 for daily protection, when you will not be receiving prolonged sun exposure. You can also derive sun protection from a variet of skin care products, such as moisturizers, facial foundations, and facial powders that contain SPF. Make sure to apply it your face, neck, hands, and chest.

Clothes provide less protection than most people realize. A normal T-shirt has an SPF of only 5. Tighter weave fabrics offers more protection.


The majority of the epidermis consists of keratinocytes. [10] A keratinocyte (= comeocyte) is a cell that consists mostly of keratin. [10]

The body creates keratinocytes at the dermal-epidermal junction (DEJ). They are produced by `stem cells`_, which are also called basal cells because they reside at the base, basal layer, of the epidermis. [10] When the stem cells divide, they crease "daughter cells", which slowly migrate to the top of the epidermis. This process of daughter cells maturing and moving to the top is called keratinization.

There are two types of keratin: acidic (type I, K10-20) and basic (type II, K 1-10).

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Stem cells divide and produce amplifying cells that greatly increase the number of keratinocytes. These in turn become the mature, terminal, and differentiated cells. The numbers indicate the cell generation.

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Structure of the skin barrier.

Desmosomes attach keratinocytes together. Keratinocytes are surrounded by lipids.

As keratinocytes mature, they develop different characteristics. The layers of the epidermis are named for these characteristic traits. The layers are the following, from most proximal to most distal:

Basal layer

Located at the base of the epidermis. Basal cells are cube-like in shape.

These basal keratinocytes contain keratins 5 and 14. Kertains 5 and 14 are presumed to established a cytoskeleton_ that permits flexibility of the cells. This flexibility allows cells to proceed out of the basal layer and migrate superficially, thus undergoing the keratinization process. [10]

Basal cells are responsible for maintaining the epidermis by continually renewing the cell population. Of the basal layer, 10% of cells are stem cells, 50% are amplifying cells, and 40% are post-mitotic cells. Normally stem cells are slowly dividing cells, but under certain conditions such as wound healing or exposure to growth factors, they divide faster. [10]

Stem cells give rise to transient amplifying cells. Transient amplifying cells are responsible for most of the cell division in the basal layer and produce post-mitotic cells, which undergo terminal differentiation and move superficially to be become suprabasal cells that continue their upward migration to become granular cells, and ultimately part of the SC. [10]

Spinous layer

The cells in the layer have prominent spiny attachments called desmosomes. Desmosomes are structures composed of adhesion molecules and other proteins and important in cell adhesion and cell transport. [10]

Keratins 1 and 10 are first seen in this layer. These keratins form a more rigid cytoskeleton that confers greater mechanical strength to the cell. [10]

Lamellar granules, which are considered the first sign of keratinization, first appear in this layer. They contain lipids such as ceramides_, cholesterol_, and `fatty acids`_, as well as enzymes_ such as porteases_, `acid phosphatase`_, lipases_, and glycosidases_. These granules migrate to the surface and expel their contents by exocytosis_. The released lipids coat the surface, imparting barrier-like properties. [10]

Granular layer

Named so because these cells contain visible keratohyaline granules, which contain profilagrin_, the precursor to filaggrin_. The protein filaggrin cross-links keratin filaments providing strength and structure. [10]

Calcium is known to be an inducer of differentiation and a suppressor of of proliferation in epidermal keratinocytes. [10]

Granular cells exhibit anabolic properties such as synthesis of filaggrin, cornified cell envelope proteins, and high molecular weight keratins. In addition, they show catabolic events such as dissolution of the nucleus and organelles. [10]

Stratum corneum

Condensated mass of cells that have lost their nuclei and granules. The SC is covered by a protein material called the "cell envelope", which aids in providing a barrier to water loss and absorption of unwanted materials. [10]

The SC is on average, approximately 15-cell layers thick. [10] The keratinocytes that resides in this layer are the most mature and have completed the keratinization process. These keratinocytes contain no organelles and their arrangement resembles a brick wall. [10]

The SC is composed of protein-rich corneocytes_ embedded in a bilayer lipid matrix assembled in a "brick and mortar" fashion. The "bricks" are composed of keratinocytes, and the "mortar" is made up of the contents extruded from the lamellar granules. [10]

Cells of the midcornified layer have the most amino acid content and therefore have the highest capability for binding to water. Deeper layers have less water-binding capacity. [10]

The SC is described as the "dead layer" of cells because these cells do not exhibit protein synthesis and are unresponsive to cellular signaling. [10]

The SC functions as a protective barrier. One of its protective functions is to prevent transepiderrmal water loss (TEWL). Amino acids and their metabolites, which are by-products formed from the breakdown of filaggrin, comprise a substance known as the natural moisturizing factor (NMF). [10] Intracellularly-located NMF and lipids released by the lamellar granules, located extracellularly, play an important role in skin hydration, suppleness, and flexibility. [10]

It is known that keratinocyte activity, such as the release of cytokines_, can be affected by topical products administered to the skin.

4.1.1   Layers

The epidermis consist of multiple layers:

  1. Stratum corneum
  2. Stratum lucidum
  3. Straturm granulosum
  4. Stratum spinosum
  5. Stratum basale

The stratum corneum (from Latin strautum "thing spread out" + cornu "horn") is the outermost layer of the epidermis, consisting of dead cells. The SC forms a barrier to protect underling tissue from infection, dehydration, chemicals, and mechanical stress. The stratum corneum is composed of flattened keratin-ized cells,each of which is about 0.5 micron thick and 40 microns in diameter. [3] Dry stratum corneum consists of about 75% protein, 25% lipid, and small amounts of low-molecular-weight substances. [3] The stratum corneum cell is enclosed in a chemically resistant membrane. The barrier properties of the stratum corneum to penetration of small molecules and steroids are dependent upon the level of hydration of stratum corneum, as are its mechanical properties (e.g. flexibility). [3]

The epidermis is composed of thousands of cells known as keratinocytes. They begin life deep in the stratum basale, where they slowly make their way upwards, propelled by the growth of new cells below. As these cells get closer to the surface, they flatten down, harden, die, and eventually flake away. Some skin does this well, some not so well (dry skin, acne skin, etc.). [9]

This process is generally referred to as skin cell turnover, and as we get older, this process gets slower and slower. In children, this only takes about two weeks, and in teens it is three to four weeks. Once you're into your adult years, this slows down to a month, and by the time you're over 50, it's closer to 45 to 90 days. [9]

The water that keeps these skin cells happy and healthy comes from deep within the epidermal layers and works its way upwards, until it is eventually lost to evaporation, but just like any organ of the body, it functions best with moisture. When skin is broken or it's natural processes disrupted by wounds, burns, exposure to surfactants (cleansers), or extreme dryness (winter), it loses water quicker. That's why doctors tell you to keep wounds moist and covered and you feel the desire to use richer moisturizers in the winter. When you keep even the upper most layers of the skin hydrated, the cells aid in elasticity and the enzymes within the skin are able to function more efficiently. [9]

`Ultraviolet light`_ and ozone_ deplete vitamin E from the skin. [2]

4.1.2   Cell cycle

The cell cycle refers to the keratinization process. [10] The normal cell cycle of the epidermis is from 26 to 42 days. [10] This series of events, known also as "desquamation", normally occurs invisibly with shedding of individual cells or small clumps of cells. [10]

The cell cycle lengthens in time as humans age. [10] A longer cell cycle causes slower wound healing and a dull skin appearance. [10]

Many cosmetic products, such as retinol_ and alpha hydroxy acids, are believed to quicken the pace of the cell cycle, thus imparting a more youthful appearance to the skin. [10]

4.2   Dermis

The dermis (1830, a Latinized form of Greek derma "skin, leather") is the layer of skin beneath the epidermis and above the flesh.

The dermis is a type of connective tissue. It consists mainly of densely interwoven fibrous tissue made of collagen which cushion the body from stress and strain.

Between the fibre bundles are inter-fibrillary proteins. These proteins can cause problems during leather production. Unlike collagen, when they dry out, they form hard glues that clog up the leather, making it hard and inflexible. These are proteins similar to egg white which, if not removed, would harden like glue and make the leather as stiff as a board.

The dermis contains hair follicles, sweat glands, sebaceous glands, apocrine glands, lymphatic vessels and blood vessels.

  • Erector pili muscles raise the hair on living animals and cause goose pimples.
  • The sudoriferous glands exude sweat which is important in regulating the temperature of the skin and hence the animal.

The dermis consists of three layers: the grain, the junction, and the corium.

4.2.1   Grain

The grain is the layer of the dermis closest to the epidermis.

The grain consists of densely woven fibre bundles, which have a much finer construction. These fibers are so so tight that they effectively resist moisture. [1] The grain is the strongest and most durable part of the skin.

4.2.2   Junction

The junction is the area between the corium and the grain layer.

In certain animals the junction can cause problems. For example, in sheep (basil) splitting can occur along the junction area.

4.2.3   Corium

The corium consists of large and strong fibre bundles and fat cells, which vary depending on the type of animal, the breed, the foodstuffs used, and the time of year it was slaughtered.

The fibre bundles lie at at angle to the grain layer above (called the "angle of weave") which affects the physical properties of the leather; a lower angle of weave produces a softer, weaker and less elastic leather. To some extent, the angle can be altered during the leather manufacturing process.

The fats here are easy to remove, but in animals with a high percentage of fat (e.g. sheep), this can leave large empty space which can make the leather feel empty.

5   Burns

First degree burns only affect the epidermal layer, and when it is regenerated by the underlying dermis it will have essentially the same whorls and ridges—unless there was severe mechanical deformation—like severe blistering and tearing.

Second degree burns affect both the epidermis and the dermis, but the later not completely. The dermis first has to regenerate from whatever is left of it, and then it regenerates the epidermis. Barring severe, extensive or very deep damage (destruction or tearing), the epidermis recovers partially its whorls and ridges, but usually with some deformation.

But third degree burns destroy both dermal layers, and even some deep tissue. With enough medical care it is possible to recover the dermal layer—even if that requires a skin transplant—and the dermal layer will usually (with some help) regenerate the epidermal layer.

6   Classification

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Different parts of the skin have different names: [1]

  1. The butt is the thickest and strongest part of the animal and normally provides the best quality leather.
  2. The neck is thick and strong but provides a poorer cut due to creases and growth marks.
  3. The shoulder is thinner and weaker than the butt or neck.
  4. The backbone tends to be thinner than the butt area.
  5. The belly is thin and has a less dense structure.
  6. The flanks are the thinnest and weakest part of the skin.
  7. The shanks
  8. The tail end

6.1   Skin types

Helena Rubinstein, a cosmetics entrepreneur, first identified four fundamental skin types in 1915: "dry", "oily", "combination", and "sensitive". [19] Since then, there have been few advances in understanding or classifying skin types. This is a problem because the existing classification are insufficient for guiding physicians and consumer toward identifying the most appropriate products. Rubenstein's skin types do not address several other features of skin that have been clinically observed, such as oiliness, resistance, or propensities toward pigmentation or wrinkling. [19]

Leslie Baumann classifies skins according to four dimensions: oily versus dry, sensitive versus resistant, pigmented versus non-pigmented, and wrinkled versus tight. [19] A person's skin type is determined from a questionnaire as well as assessments are significantly life changes. (The questionnaire and a complete description of each skin type can be found in the book "The Skin Type Solution".) [19]

7   Maintenance

7.1   Moisturizers

A moisturizer is a ... that is intended to trap or replenish to moisture in the epidermis.

The three most common types of moisturizers are occlusives, humectants, and emollients. Most products have a combination of all or some of these.

Occlusive

Hydrophobic compound that prevents water from evaporating. Forms a barrier over the skin that water can't penetrate, stopping evaporation. The long carbon chains of the molecules that make up occlusives repulse water. For example, petroleum jelly (sold as Vaseline).

Can feel heavy or greasy, but the they are the most effective at present trans-epidermal water loss.

The most common and most effective ingredient is petrolatum. It reduces water loss by 99%. The second most common ingredient is dimethicone.

Occlusives should be applied last in a skin care routine.

Humectant

Hydroscopic compound that attract and hold onto water molecules. However, they can also pull water away from the dermis and cause excessive water loss. Therefore, it's best to combine with occlusives.

The most common humectants are glycierin, or glycerol and hyaluronic acid.

Emollient
Fill in the cracks between skin cells. Many times, occlusives can be function as emollients.
Rejuvenator
Replenish the protein skin. Include ingredients such as collagen, keratin, and elastin. These ingredients have limited permeability due to their large molecular size, but they can fill a similar role as emollients.

7.2   Mineral oil

A mineral oil is any of various colorless, odorless, light mixtures of higher alkanes from a mineral source, particularly a distillate of petroleum.

Mineral oil is a substance of relatively low value, and it is produced in very large quantities. Mineral oil is available in light and heavy grades, and can often be found in drug stores.

Wood and bamboo oils are food grade mineral oils. You should use wood oil in your kitchen instead of vegetable oil because mineral oil does not break down or become rancid. Mineral oil gives sheen and water and stain resistance to wood items.

Snow River does have added vitamin E (which makes it thicker).

7.3   Mint julep

A mint julep mask is a good clarifying mask which pulls a lot of excess oil from the pores. You can tell by examining the T-zone. It's similar to when you use a blotting sheet. The mask dries kind of chalky, so all those spots appeared when oil got pulled out of the pores

It's a little drying since it's a clay mask but it should be no problem as long as your moisturize after cleaning it off.

It should be put on pretty thick.

8   Applications

Animal skin can be tanned to produce leather.

9   Sun exposure

UV radiation is a carcinogen_ responsible for most of the estimated 1.5 million skin cancers and the 8,000 deaths due to metastatic melanoma_ that occur annually in the United States. [5] Lifetime cumulative UV damage to skin is also largely responsible for some age-associated dryness and other cosmetic changes. [5]


Sunburn is caused by the DNA of your cells being damaged and the cells then killing themselves so they don't into cancer. And the reason we get cancer is because they don't always succeed.


On why 4 continuous hours in the sun results in a sunburn, but 4 hours broken up into 15 minutes chunks does not

The explanations so far are not correct. 4 continuous hours of sunlight will give your damaged cells less time to repair DNA and less time to apoptose (safely die without causing inflammation).

4 hours spread out over increments gives your cells time to repair DNA and, if need be, apoptose irreparable cells in-between exposures.

If you do not have this break time in-between exposures, cells will become damaged so badly that they cannot die in a safe, organized manner (apoptosis). Instead, they die quicker and release their proteins into the space between cells. Nearby cells sense this, freak out and release chemicals that cause inflammation and pain.-

When you feel heat, when you are out in the sun, it’s from infrared light. A lot of people have the misconception that they aren’t feeling the heat from the sun, so they aren’t getting burned. The “heat” (really just pain) from a sunburn will come on much later, and is produced by inflammation and hypersensitive nerve endings.

Snow and standing on a boat or near a pool will make this worse because they reflect the light.

Something I think folks are missing is how much the sun's power depends depends on time of day- if that four hour window includes noon-2PM, you are getting way more light intensity than 15 minutes at 8 AM or 10 AM. There's a classic Australian sun-safety jingle that says "From 11 to 3, sit under a tree."

10   Disease

10.1   Acne

Acne is a skin disease that occurs when `hair follicles`_ become clogged with dead skin cells and oil from the skin.

Acne is caused by excess andogens (testosterone or estrogen).

Acne can take weeks to to form below the surface

10.2   Eczema

Eczema a disease of the skin that removes the oil which sits between dead skin cells and living ones.

Eczema is the most common chronic skin disorder in infants and children. It often occurs in patient with asthma. It is genetic. It affects 15-20% of children in industrialized countries. It can be treated, but not cured.

There are multiple kinds of eczema:

  • Atopic dermatitis
  • Contact dermatitis

10.2.1   Causes

The cause of eczema is not understood. It is thought to involve the immune system and the ability of the skin to stay hydrated naturally. The skin of people with eczema seems to allow irritants into the body which results in the immune system reacting and resulting in rashes and irritation. Wind, low humidity, cold temperatures, excessive washing without the use of moisturizers, and harsh soaps can all cause additional drying.

## Food

The first comprehensive study that proved that food can cause eczema was done by David Atherton, in London, in the 1970s. He studied 36 children who had eczema. It was a thorough study with a design called a double-blind controlled crossover trial . The purpose was to see a diet excluding dairy and egg products would influence their eczema: 20 children completed all parts of the study. His results were startling: 14 children (70%) responded more favourably to the antigen-avoidance diet than to the control diet, whereas only one (5%) in the control diet group had a good response. Of interest, there was no correlation between a positive skin-prick test (SPT) to egg and cow milk antigen and response to the trial diet (see an explanation for this in the next chapter).

In later study, Guillet (1992) assessed over 250 children with eczema. He noted that increased severity of eczema in the younger patients was directly correlated with the presence of food allergy.


The clear liquid usually appears when an area of broken skin is infected. In my own case a course of flucloxacillian (sp) antibiotics is normally prescribed. Infected eczema I know to be called atopic eczema, and is very uncomfortable. A steroid cream such as betnovate and emollients such as double bass and aveeno come highly recmomended. Personally I had a bad reaction to hydrocotisone so it may be best to seek advice before using it.

10.2.2   Treatment

  • Often associated with hay fever and asthma
  • Can be treated in a variety of ways, but one is to replace the oil with oil
    • Note to self: Keep a stock of olive oil
    • Moisturizer can fill gaps

Use gentle cleansers (soapless cleansers) such as Aquanil, Cetaphil, Aveenon, VAnicream, Purpose, or Dove senstive skin liquid wash. Avoid harsh soaps such as Coast, Irish Spring, Jergens, Caress, Ivory, or other anti-bacterial soaps. Soaps need only bue used onf ace, hands, and skin folds. Do not use loofahs or body scrubs.

Apply appropriate moisturizers. Ointments and creams are the recommended moisturizers. The more greasy the moisturizer is, the better is it able to protect your skin from water loss. Lotions are not recommended, as they are water based and tend to leave the skin dryer than prior to application. For the face, some examples of good moisturizers are: Aveeno, Dove, Eucerin, Neutrogena, Cetaphil, Oil of Olay. For the body, DML Forte cream, Aveeno, Cetaphil, Dove, Eucerin, or Neutrogena. Petroleum jelly is a cheap and excellent moisturizer as it reduces water loss by 99%.

Do not use softeners or dryer sheets in the dryer. They tend to put more chemicals in the clothing that can then be depositing on the surface of the skin.

Wear loose fitting cotton clothing. Avoid woolens and acrylic.

The doctor said I should be moisturizing 15-20x a day.

11   Care

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A 69-year-old man who drove a delivery truck for 28 years shows damaged skin on the left side of his face.

11.1   Suncare

[51]

The sun emits non-ionizing `electromagnetic radiation`_ composed of ultraviolet (100-400 nm), visible (400-780 nm), and infrared (780-500) radiation_. Ultraviolet radiation is composed of wavelengths between 100 and 400 nm that further divide into UVC (100 - 290nm), UVB (290 - 320 nm) and UVA (320-400nm). Wavelengths below 290 nm are absorbed by atmospheric ozone and do not reach the earth's surface (and therefore are not of concern).

The amount of UVR reaching a given location of earth varies seasonally, geographically, diurnally. For example, UVR intensity is highest at the equator and high altitudes. The intensity of UVB is considered highest during the summer and on a daily basis between 110 and 1500 h. UVA intensity is more consistent throughout the day and from season to season compared to UVB. Atmospheric conditions including cloud cover, pollution, humidity, and temperature modify the spectrum and intensity of terrestrial sunlight.

For most individuals in developed countries, exposure to solar UVR comes in short, multiple episodes to the face, neck, and hands that are a consequence of everyday life. This incidental exposure can account for as much as 80-90% of an estimated yearly exposure to UVR and over 60% of non-melanoma_ skin cancers appear at these sites.

Exposure to UVR has pronounced acute, chronic, or delayed effects on the skin. The UVR include skin effects manifest as acute responses such as inflammation, sunburn, pigmentation, and Vitamin D synthesis. Chronic effects include photo-carcinogenesis and photo-aging.

Exposure to UVR from sunlight probably cause nonmelanoma skin cancer (NMSC) based in part on the following evidence:

  • People with zeroderma pigmentosum, a genetic disease with defective DNA repair, are exquisitely sensitive to UVR and develop NMSC at an early age predominant on sun-exposed parts of the body.
  • The incidence of NMSC is inversely related to latitude in populations of mainly European origin and is greater in outdoor compared to indoor workers.
  • The NMSC is most common on the head, necks, arms, and hands, areas of the body that receive the largest dose of UVR
  • Persons that easily sunburn are more susceptible to the development of NMSC

The case for the role of sunlight exposure as a risk factor for development of `malignant melanoma`_.

Sun sensitivity refers to pigmentation traits such as color of eyes, hair, and skin, and skin reaction to sun exposure, that is, inability to tan.

Chronic exposure to solar UVR is thought to accelerate aging of human skin. The skin photo-aging is characterized by dryness, roughness, irregular pigmentation such as freckling/lentigenes, actinic keratoses, winkling, elastosis, inelasticity, and sebaceous hyperplasia. The incidence and severity of skin photo-agin are believed to be a function of cumulative UVR exposure, based on human and animal studies. For example, Caucasian women with excessive sun exposure have a higher incidence of photagain than women with a low UVR exposure history. In addition, signs of photo-damage are absent in unexposed skin, e.g. inner portion of the farm on the same on individual.

SUNSCREENS AS PART OF A PHOTOPROTECTION STRATEGY

Sunscreen-mediated photoprotection is concerned with the with reduction of exposure to UVR, specifically UVB and UVA primarily from the sun.

DISCUSSION

Clinical study results all support the human safety of the currently used organic and inorganic sunscreens.

The attenuation spectrum of that sunscreen defines the spectrum of UVR to which underlying cells in the skin are subjected. Sunscreen alters the light spectrum to which the skin is exposed. The sunscreen-protected spectrum (SPS) will depend on the kind of sunscreen used. With the majority of sunscreen used, longer UVA wavelengths will comprise this SPS.

Some sunscreen also contain UVA-absorbing chemicals. However, except for butylmethoxydibenzoylmethane and the metal oxides of titanium and zinc, currently used sunscreens are not impervious to all UVA wavelengths.

The term "broad spectrum" can be used for sunscreen products in the United States containing UVAII filters, even though the majority of marked products do not contain UVAI filters. Thus, cells in the skin, even if protected with a currently labeled broad-spectrum sunscreen, can be exposed to biologically relevant doses of longer UVA wave lengths.

Although UVB is the most damaging component of sunlight, UVA is responsible for numerous morphological, molecular, and biochemical events that may contribute to photodamage of skin.

Chronic exposure to UA appears to produce changes in human skin indicative of photaging.


[52]

The US government has officially identified ultraviolet radiation (UVR) both from the sun and from tanning machines as a known cause of cancer in humans. UVR produces DNA damage that may lead to mutations (abnormalities) in genes involved in the development of skin cancer. Therefore, along with other sun safety strategies, sunscreens that absorb or block UVR serve an important protective function.

The US Environmental Protection Agency (EPA) estimates that the sun causes 90 percent of all nonmelanoma skin cancers,1 and other research links it to 65 percent of all melanomas.2 Each year, an estimated 3.5 million or more new cases occur in the US of the nonmelanoma skin cancers basal and squamous cell carcinoma (BCC and SCC).3 An estimated 76,250 new cases of invasive melanoma, the most dangerous form of skin cancer, will be diagnosed in the US in 2012, with nearly 9,180 resulting in death, according to the American Cancer Society.4

Photoaging, or UV-induced skin aging, is another long-term result of sun exposure. While not threatening to life, it is threatening to quality of life. Excessive unprotected time in the sun leads to premature wrinkling, sagging, a leathery texture and hyperpigmentation (so-called “aging spots” or “liver spots” that are really the result of sun damage).

Sunburn, the most immediate, obvious example of UVR damage, results from sun-induced inflammation and/or blistering of the skin. When immune cells called mast cells race to the injured skin site in response to the damage, they dilate the blood vessels and produce erythema (reddening), edema (swelling), and burning and stinging sensations as part of the healing process. This DNA damage can be the first step towards skin cancer. Intermittent, intense UVR exposure, often producing sunburn, is believed to be more closely associated with melanoma than is chronic sun exposure. One blistering sunburn in childhood or adolescence doubles the risk of melanoma later in life; five sunburns by any age doubles the risk as well.5

Sunscreen is one vital tool that can help prevent all of these UVR-induced assaults on the body as part of a comprehensive photoprotection program, along with sun avoidance or use of shade during peak sunlight hours (10 AM to 4 PM), and protective clothing, including a wide-brimmed hat and UVR-blocking sunglasses.

Finally, both intermittent, intense exposure (the kind you get on vacation on a sunny isle, often leading to sunburn) and chronic lifetime exposure add to skin cancer risk. Studies have shown that chronic sun exposure is most associated with the development of squamous cell carcinoma, the second most common skin cancer, while both chronic and intense, intermittent exposure are believed to play a role in basal cell carcinoma,7,8 the most common skin cancer.

Sunscreen should not be neglected on overcast days, as 70-80 percent of the sun's rays – above all, long-spectrum UVA rays – go through clouds and fog. In addition, according to the World Health Organization, UVR levels rise by about 4 to 5 percent for every 1000 feet of altitude, and reflection from sand, water, snow or concrete magnifies their effects by up to 80 percent.

  1. Is unprotected sun exposure necessary to prevent vitamin D deficiency?

A. This is a highly controversial issue.25,26 Vitamin D is synthesized when the skin is exposed to UVR. Its active form, 1,25dihydroxy Vit D 3, regulates calcium metabolism, essential for bone and muscle health, and recent epidemiologic studies have suggested that individuals with adequate serum vitamin D levels had a lower incidence of internal cancers and multiple sclerosis. However, in 2010, after extensive review, the Institute of Medicine determined that the benefits of vitamin D beyond bone health have not been conclusively proven.

In any event, the action spectrum for cutaneous vitamin D synthesis is in the UVB range, known to be carcinogenic; staying out in the sun long enough to produce sufficient vitamin D exposes you to potentially harmful amounts of the sun's UVB as well as UVA rays.

Therefore, for individuals at risk for vitamin D insufficiency, such as those who are elderly, homebound, or dark-skinned, a balanced diet with adequate intake of food rich in vitamin D (e.g., salmon and fortified milk) is the most appropriate way to maintain a good vitamin D level, and vitamin D supplements can be added as necessary. The IOM recommends 600 International Units (IU) of vitamin D daily for all age groups between 1 and 70 years old.


Exposure to UV radiation (UVR) from the sun plays a causal role in acute and chronic skin damage including skin cancers. As such, the medical community have advocated a strategy of limiting sunlight exposure between midday hours of 1110 and 1500, wearing protective clothing, and using sunscreens. [51]

  • UV-A radiation is responsible for immediate tanning or darkening of the skin due to excess production of melanin in the epidermis
  • UV-B radiation are 1000 times more capable of causing sunburn than UV-A. UV-B rays act mainly on the epidermal basal cell layer of the skin.

11.1.1   Tanning

  1. Is there any truth to the claim that a "base tan" helps prevent skin cancer?

A. No. Tanning is the skin's response to DNA damage, pure and simple. This damage may permanently affect the skin cells (melanocytes and keratinocytes).18 The activation and proliferation of these cells increase the risk of skin cancer and photoaging. UV tanning lamps can be as or more dangerous than the sun; in fact, their use has now been documented to be associated with all three major skin cancers. [52]

11.1.2   Sunscreen

11.1.2.1   Risks


[52]

Q. Sunscreen still has its detractors. Some have claimed that it encourages excessive sun exposure by making people think they are totally protected from UVB and UVA, and thus increases their skin cancer risk. Is this valid?

A. Most photobiologists and dermatologists strongly disagree. Thorough analysis of all available data has failed to show any correlation between the use of sunscreen and an increase in melanoma or any other skin cancer.23,24 What’s more, combined UVA and UVB protection has vastly improved in recent years in sunscreens, extending the range of protection. However, it must always be remembered that broad-spectrum sunscreen is just one of several vital measures that need to be practiced in anyone's photoprotection program, along with seeking the shade during the sun’s most intense hours (generally between 10 AM and 4 PM) and wearing sun-protective clothing, including a wide-brimmed hat and UV-blocking sunglasses.

11.1.2.2   Ingredients

[51]

There are two categories of sunscreen agents: organic and inorganic. The organic sunscreens are referred to as soluble or chemical sunscreens. The inorganic sunscreens are commonly known as physical, mineral, insoluble, natural or non-chemical.

Organic sunscreens have been the mainstay of sunscreens formulation for decades, and remain more popular. Organic sunscreens are often classified as derivatives of anthranilates, benzophenones, camphors, cinnamates, dibenzoylmethanes, p=aminobenzoates, or salicylates. These aromatic compounds absorb a specific portion of the UVR spectrum that is generally re-emitted at a less enerigc longer wave length, i.e. heat or light, or used in a photochemical reaction.

There are 23 organic sunscreen agents currently available in the United States for use in over-the-counter sunscreen products. Of these, nine are ordinarily used in sunscreen products, and the remaining are rarely if ever present in sunscreen products marketed today. Of the nin sunscreens that are used, five of these comprise the majority of sunscreen products used in the world. The organic sunscreens are almost always used in combination because no signale organic sunscreen agent used at level currently allowed by the U.S. Food and Drug Administration can provide a high sun protection factor (SPF). Further, individual organic sunscreens have a relatively narrow absorption spectrum that can be broadened by combinations. Specific combinations are used depending on the frequency on the intended product usage, recreational or daily photoprotection, and the desired attributes such as waterproof or sweatproof.

The combination of organic and inorganic sunscreens has become increasingly popular in sunscreen products.

Inorganic sunscreens are gaining popularity in beach and daily use photo-protection products. This has been driven by their safety and effectiveness, particularly in block UVA and the concern regarding potential adverse effects of organic sunscreens. Inorganic sunscreens are generally viewed as harmless pigments that cannot enter the skin.

The two most commonly used inorganic sunscreens are titanium dioxide (\(TiO_2\)) and zinc oxide (\(ZnO\)). ZInc oxide and TiO2 exist as odorless white powders comprised of a Gaussian distribution of particles sizes. Microfine powders, used in sunscreen products, have an average particle size of approximately 0.2 microns or less with a distribution that is narrow and controlled.

Each particulate has a size at which it maximally scatters visible light. This is the ideal size for use as a white or colored pigment. As a sunscreen, however, any color rendered to the product by an ingredient is undesirable. Thus, the average particle size of a metal oxide is reduced below the optimal light scattering size, allowing visible light to be transmitted, and therefore appearing invisible on the skin.

Zinc oxide or TiO2 used in sunscreen preparations is often coated with other materials such as silicones, fatty acids, or oxides of aluminum, silicon, or zirconium to aid in dispersion. The coating were developed by the paint industry to reduce particle agglomeration, which improves the distribution of particles when applied as a thin film on a surface.


[52]

  1. What other ingredients may be included in sunscreens?

A. Antioxidants, which to some degree can neutralize damaging “free radicals” (unstable, highly reactive oxidized molecules believed to cause tissue damage at the cellular level, harming our DNA), are sometimes added to sunscreens, though their impact is not universally accepted. Vitamin E (alpha tocopherol) gives some protection against UV-induced DNA damage, and vitamin C (L-ascorbic acid) helps protect against sunburn. The free radicals are produced by melanin and other skin tissues exposed to light. A number of sunscreens today list "free radical protection" on the package.

Cosmetics neither increase nor decrease the photoprotective value of the sunscreen. They have the great advantage that most women apply them every day. Some products for men also combine moisturizer and sunscreen. It's important, however, to use a separate sunscreen or reapply the cosmetics every two hours when outside.

"Micronized" versions of zinc oxide and titanium dioxide that render them invisible have greatly increased their popularity.


In the United States, the FDA has approved 17 `active ingredients`_ for sunscreens. [52] The FDA provides the approved concentration of each ingredient, and this often is included on the label of the product.

Active Ingredient Maximum FDA-approved concentration Range of protection
Aminobenzoic acid 15 UVB
Avobenzone 3 UVA1
Cinoxate 3 UVB
Dioxybenzone 3 UVB, UVA2
Ecamsule (Mexoryl SX) 3 UVA2
Ensulizole (Phenylbenzimidazole Sulfonic Acid) 4 UVB
Homosalate 15 UVB
Meradimate (Menthyl Anthranilate) 5 UVA2
Octocrylene 10 UVB
Octinoxate (Octyl Methoxycinnamate) 7.5 UVB
Octisalate (Octyl Salicylate) 5 UVB
Oxybenzone 6 UVB, UVA2
Padimate O 8 UVB
Sulisobenzone 10 UVB, UVA2
Titanium dioxide 25 Inorganic/Physical (UVB, UVA2)
Trolamine salicylate 12 UVB
Zinc oxide 25 Inorganic/Physical (UVB, UVA2, UVA1)

Sunscreen ingredients fall into two categories: physical screens and chemical absorbers of UVR. Titanium dioxide and zinc oxide, commonly used today in micronized form for a more inconspicuous cosmetic look (these physical sunscreens formerly tended to be stark white on the skin), are used singly or combined with other active ingredients to protect against both UVB and UVA (titanium dioxide across the UVA2 spectrum, 320–340 nm; zinc oxide against both UVA2 and UVA1 — 340–380nm).

Chemical sunscreen ingredients that provide only UVB absorption are usually combined with others that cover some UVA wavelengths. UVB-absorbing organics include the cinnamates, with octinoxate (octyl methoxycinnamate) being the best known and most widely used; and the salicylates, most often octisalate (octyl salicylate) and homosalate. The benzophenone group of organics, including dioxybenzone, oxybenzone, and sulisobenzone, brought coverage into the UVA range, specifically offering protection against UVA2 as well as UVB.

Avobenzone, introduced in the early 1990s, extended chemical sunscreen protection into the UVA1 range (340–400 nm); it is currently the best UVA1 filter available in the US. It also offers some absorption across the UVA2 range. However, it does not provide adequate UVB protection, so it must be used in combination with UVB-absorbing ingredients. Until recently, the effectiveness of avobenzone was limited by its being photounstable, degrading by over 50 percent after one hour of sun exposure. Now, it has been photostablilized by combining it with other, photostable UV filters, such as octocrylene and/or oxybenzone.

11.1.2.3   Effectiveness

Sun protection factor (SPF) is the ratio between the time it takes for sunburn to start (the minimal erythemal dose) in skin protected with sunscreen versus in unprotected skin.

a measure of how well a sunscreen protects against ultraviolet B radiation. The relationship between SPF and UVB protection is nonlinear: SPF 15 filters 93 percent of UVB rays, while SPF 30 filters 97 percent and SPF 50 filters 98 percent. And nothing blocks 100 percent. [53]


SPF is a useful assessment of primarily UVB filters. The SPF test, however, does not adequately assess the complete photoprotective profile of sunscreens specifically against long wavelength UVAI (350-400nm). To date, there is no singular agreed upon method for evaluating UVA efficacy despite the immediate and seemingly urgent consumer need to develop sunscreen products that provide broad spectrum UVB and UVA photoprotection. [51]


[52]

Q.What does Sun Protection Factor (SPF) measure, and what are its limitations?

A. The SPF rating is a reliable measurement of protection against UVB (short-spectrum) wavelengths (290-320 nanometers; 1 nm is a billionth of a meter). SPF is the comparative ratio between the minimal erythemal dose (MED) – the time it takes for reddening or sunburn to start – in skin protected with sunscreen and the MED in unprotected skin. For example, if it takes 20 minutes without protection to produce erythema, an SPF 15 sunscreen might prevent reddening 15 times longer—about five hours. That figure is theoretical, however, and sun damage can occur even without reddening, so dermatologists normally advise reapplying after approximately two hours.

The Skin Cancer Foundation considers SPFs of 15 or higher acceptable UVB protection for normal everyday activity, and SPFs of 30 or higher acceptable for extended or intense outdoor exposures. Such sunscreens also provide some protection against UVA wavelengths (320-400 nm), though the SPF rating refers only to UVB protection. Until 2011, no FDA-approved measurement standard existed for UVA protection in the US, even though such standards already existed abroad, and even though UVA penetrates more deeply into the skin than UVB, reaching the dermis. In the past, experts believed that UVB caused burning and skin cancer, while UVA caused photoaging, but the truth has proven more complex. In addition to producing sunburn, UVB can contribute to photoaging, and both UVA and UVB exposure can lead to skin cancer.

In June of 2011, the FDA issued its long-awaited new regulations for sunscreen labeling, including, for the first time, testing and labeling requirements for protection against UVA. These regulations will go into effect in December 2012, but products grossing under $25,000 in sales will have until December 2013 to comply with the new rules.

Sunscreens that meet FDA standards for both UVB and UVA protection may be termed “broad-spectrum,” a term that, until now, was frequently used but had no official meaning. Sunscreens may be labeled broad-spectrum if they provide “proportional” protection against both ultraviolet A (UVA) and ultraviolet B (UVB) radiation. In other words, a product with an SPF of 15 must have a comparable level of protection against UVA to be considered broad-spectrum.

Newly standardized methods for measuring UVA protection made these improvements possible. Broad-spectrum sunscreens combine UVB- and UVA-absorbing chemicals and/or physical screens, and thus give the widest range of protection. As of December, 2012, broad-spectrum sunscreens with an SPF of 15 or higher will be able to state that they reduce the risk of skin cancer and early skin aging caused by the sun, if used as directed with other sun protection measures. Sunscreens that are not broad-spectrum or that have an SPF of 2-14 will be required to have a warning stating that the product has been shown only to help prevent sunburn, not skin cancer or early skin aging.

In vitro tests have shown that SPF 15 sunscreens filter out 93 percent of UVB rays, while SPF 30 protects against 97 percent and SPF 50 98 percent. But remember, it is important for the sunscreen to include broad-spectrum protection that also shields adequately against UVA.

In January 2011, definitive clinical research from Australia showed for the first time that sunscreen can drastically reduce melanoma incidence. Researchers found that daily application of an SPF 16 sunscreen to the head, neck, arms, and hands reduced melanoma incidence by 50 percent in subjects studied for more than a decade. Only 11 melanomas developed in the daily sunscreen users, vs. 22 in the control group. Even more impressively, invasive melanomas (those that penetrate beyond the skin surface) were reduced by 73 percent (3 tumors vs. 11), and those that were found in the sunscreen group were smaller on average and more readily curable.

When the FDA announced its new regulations on sunscreen labeling in June 2011, it also proposed a regulation that sunscreen manufacturers not be allowed to claim an SPF rating above 50; the highest rating would be 50+”. The Skin Cancer Foundation's Photobiology Committee agrees with this proposal, which remains under FDA consideration. High-risk individuals can, however, benefit from higher SPFs up to 45 or 50.

Labeling of UVA protection in sunscreen is routine in a number of countries, including Australia, Japan and Germany, and in June, 2011, the FDA issued its first official guidelines on UVA protection for products sold in the US.14 The FDA has instituted a pass/fail test based on the critical wavelength value of 370 nm as its standard for acceptable broad-spectrum protection. In this widely used in vitro (lab-based) test, the UV absorption spectrum of the sunscreen is plotted against wavelength; the wavelength where 90 percent of absorption occurs is defined as the critical wavelength. Therefore, the more potent and broad the UVA protection, the longer the critical wavelength. Most consider a critical wavelength of 370 nm or longer as being good UVA protection.

In Europe and Asia, the most widely used in vivo UVA testing method (a test performed on live human subjects) is the Persistent Pigment Darkening test (PPD).15 The subjects are exposed to a UVA light source, with and without sunscreen. After two to three hours, darkening of the exposed skin is used as a biological endpoint. In treated subjects, the sunscreen should reduce darkening.


[51]

The SPF for a sunscreen is defined as the ratio of sun exposure that skin can tolerate before burning with and without sunscreen protection.

Because the action spectrum for UVR-induced sunburn is similar to that for a specific measure of DNA damage, it often has been inferred that protection against sunburn is the same as protection against DNA damage. However, it is now clear that each biological response has a unique action spectrum. Thus, though SPF provides a measure of sunburn protection, its value for other endpoints is limited and could be viewed as misleading.

When the SPF system originated, it was commonly accepted that the action spectrum for UVR-related skin changes or damage was similar to that for erytherma (reddening) in human skin. We now know that the action spectra for other endpoints such as photo-again and perhaps melanoma are not the same as erythema.

Because UVA is only mildly erythrogenic, SPF does not adequately describe a sunscreen's protective profile. The ideal test for UVA protection should use some biological event known to be mediated by these wavelengths. To date, an endpoint use as a representativity surrogate for UVA events has not been agreed upon.

11.1.2.4   Safety

The toxological evaluation of any chemical where human exposure is likely often includes short-term in vitro studies that are believed to be predictive of long-term or delayed toxicity. [51]

11.1.2.5   Selection

[52]

  1. What is the easiest way to know if a sunscreen provides proper protection?

A. Besides checking the ingredient label, consumers can look for The Skin Cancer Foundation's Seal of Recommendation on the label or in the packaging. The Seal is awarded to sun protection products that meet the strict requirements of the Foundation's Photobiology Committee.

In summer 2012, the Foundation implemented new standards for sunscreens in the Seal program. These new standards include effective UVA protection requirements. Sunscreens will also be divided into two categories based on their intended use – one called “Daily Use” and one called “Active.”

"Daily Use" products are intended to protect consumers from incidental sun exposure that occurs in short periods of time. Examples of these products might include: daily moisturizers, color cosmetics, foundations, eye creams, lip balms, etc. Requirements for the “Daily Use” Seal include:

  • UVB protection of SPF 15 or higher
  • UVA protection with a Critical Wavelength of 370
  • Testing for contact irritancy and phototoxic reactions

"Active" products should protect consumers from extended sun exposure, or during recreational activities. Examples might include: high-SPF products, sport sunscreens, zinc/titanium sticks, baby products, etc. Requirements for the "Active" Seal include:

  • UVB protection of SPF 30 or higher
  • UVA protection with a Critical Wavelength of 370
  • Proof of water resistance (following the FDA guidelines, the product must specify whether it maintains its SPF after 40 or 80 minutes of water immersion)
  • Testing for contact irritancy and phototoxic reactions.

[53]

You might consider going with an SPF significantly above 30 for another reason. While the FDA requires that companies produce test results to prove their sunscreens match the SPF stated on the label, Consumer Reports’s independent lab testing (subscription required) claims that the listed SPF is often higher than the effective SPF. To add to the confusion, sunscreen formulas may change year to year, meaning that whatever did a fine job of protecting you in the summer might be less effective if you rebuy it for a trip to the Caribbean in April. Your chances of getting an effective SPF above 30 regardless of what the label says are better if you buy a sunscreen labeled SPF 50 or even 70. However, our basic argument still stands: Even an SPF 15 lotion that you apply liberally and often can protect you better than an SPF 50 or 70 lotion you put on only once in eight hours.

Higher SPF doesn’t pick up the slack for poor application. Some studies have found a nonlinear correlation between the amount of sunscreen people apply and the effective SPF, possibly in part because higher SPF sometimes means thicker sunscreen, which is more difficult to apply.

Developing good habits will do more to protect most people from the sun than worrying about having a precise amount of SPF. When it comes to blocking sunrays, consider that SPF 30 blocks an additional 4 percentage points' worth compared with SPF 15, and SPF 50 blocks an additional single percentage point compared with SPF 30. Although that difference can affect your skin, and can do so to a greater degree when you’re nearer the equator, it’s far smaller than the typical error in application: Wang estimates that most people apply a third of the sunscreen they need.

SPF number aside, it is critical to have a sunscreen that is labeled “broad spectrum.” This label means the sunscreen has ingredients that protect against both kinds of UV rays, UVA and UVB. UVB causes sunburn, and UVA leads to wrinkles. Both types of radiation can lead to cancer.

There’s no SPF-rating equivalent for UVA rays, but a sunscreen that passes the FDA’s broad-spectrum test has UVA coverage proportional to its UVB coverage. If a sunscreen has materials in it to protect from UVA rays, it will say “broad spectrum” somewhere on the bottle.

https://thewirecutter.com/reviews/best-sunscreen/

11.1.2.6   Application

[52]

  1. What is the protocol for applying sunscreen?

A. It should be applied one-half hour before going outside, giving the skin time to absorb it. Because sunscreen tends to be broken down over time by the sun, and rubbed or washed off with sweating and water exposure, it should be reapplied at least every two hours outdoors, and immediately after swimming or heavy sweating. At least one ounce (two tablespoons) is needed to cover the entire body surface.

As facial skin is thin and highly exposed, it is particularly important to apply sunscreen there liberally. Many sunscreens are now incorporated in facial moisturizing creams. In routine daily activity with no extended outdoor exposure, if the face is untouched and there is no sweating, it is acceptable to apply the sunscreen just once at the beginning of the day.

There is no need to throw away last year's left-over sunscreens. Shelf life is typically two to three years. Many sunscreens now have an expiration date stamped on the container. Store the sunscreen in a cool place, since heat can gradually break it down.

11.3   Skin types

Not sure about the validity of these concept.

11.4   Formulations

A formulation is ... Formulations have the following properties:

Targets
Format (Anhydrous Solution, Emulsion)
Conflicts
Time of use (AM/PM)

Many formulas contain substances that can irritate the skin.

Concerns:

  • pH balance

11.5   Ingredients

Squalane

The human body produces its own version, known as squalene, but the amount made and retained in the skin decreases over time. It peaks in our teens. Squalane is found in a variety of plants and in shark liver.

No matter where it comes from, squalene is extremely unstable. When squalene is hydrogenated it converts to skin-friendlier and safer squalane.

11.6   Types of formulations

Types of formulations:

Vitamin C

Antioxidants

Hydrators and Oils

Suncare

Benzoyl Peroxide
Chemical exfoliator. Reduce the bacteria P. acne and cats as an anti-microbial.

11.6.3   Direct Acids

Alpha Hydroxy Acid
Water soluble chemical exfoliator. For example, lactic acid.
Beta Hydroxy Acid
Lipid soluble chemical exfoliator with anti-inflammatory properties. For example, aslicyclic acid.
Azelaic Acid
?
Salicylic Acid
?

11.6.4   Retinoid

A retinoid is a form of vitamin A.

11.6.4.1   Tretinoin

Tretinoin is a gel or cream that is used to treat acne vulgaris, sold under the name Retin-A. Although the exact mode of action is unknown, current evidence suggests that topical tretinoin decreases cohesiveness of follicular epithelial cells with decreased microcomedo formation. Additionally, tretinoin stimulates mitotic activity and increased turnover of follicular epithelial cells causing extrusion of the comedones. [4]

Instructions:

  • Apply once daily before bedtime or as directed by your physician. Your physician may advise that you start your therapy by applying retin-a every other night, especially if your skin is sensitive. [4] First, wash with a mild soap and dry your skin gently. Wait 20 to 30 minutes before applying medication; it is important for skin to be completely dry in order to minimize possible irritation. [4]
  • Do not apply more than the amount suggested by your physician or more than frequently than instructed. Too much may irritate the skin and won't give faster or better results.
  • Keep the medication away from the corners of the nose, mouth, eyes, and open wounds. [4]

11.7   Regimens

A regimen is a combination of a morning routine and nighttime routine. The Ordinary recommends using a maximum of three serum formulation in a single regimen.

A routine is a sequence of formulations to be applied in order. Routines have the following steps:

  1. Cleaning (washing your face)
  2. Toning (balancing the skin)
  3. Prescription topical
  4. Treatments (BHA/AHA)
  5. Serums
  6. Moisturizing - (hydrating and softening the skin)
  7. Occlusives
  8. Sunscreen

A simple routine might just consist of cleansing and moisturizing, and applying sunscreen during the day.

Water formulations should be applied before oil formulations because ...

Advice:

  • Spot test new products before adding them to your routine. Skin may take up to 28 days to react

11.8   Dermatologist

See a dermatologist if your skin does not responded to gentle at-home treatments.

12   Market

The modern cosmetic and skin care product market began to take shape in 1915 amidst the intense rivalry between the burgeoning cosmetics entrepreneurs Helena Rubinstein and Elizabeth Arden, both of whom opened salons that year that would grow into powerful business empires. Since that period, the categories "dry", "oily", "combination" and "sensitive" have been used to characterize what Helena Rubinstein identified as the four fundamental skin types. [19]

A cosmeceutical is an unregulated cosmetic formulation that may impart biologic function of skin. [19]

13   Further reading

14   Resources

15   Products to investigate

16   References

[1](1, 2, 3) Lanning, David. 1996. The Manufacture of Leather - part 2. http://www.hewit.com/skin_deep/?volume=2&article=2#article
[2]Thiele etal. 1999. The Society for Investigate Dermatology. Sebaceous Gland Secretion is a Major Physiologic Route of Vitamin E Delivery to Skin.
[3](1, 2, 3) Anderson etal. 1973. Hydration of Straum Corneum.
[4](1, 2, 3, 4) Retin-A (tretinoin) patient instructions.
[5](1, 2) Vitamin D. Fact Sheet for Health Professionals. https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/
[7]Gail Jenkins, etal. Anatomy and Physiology: From Science to Life. Second editoin.
[8]The ScA Routine. https://www.reddit.com/r/SkincareAddiction/wiki/sca_routine
[9](1, 2, 3) JoanOfSarcasm. [Research] Evaluating Your Routine: Moisturizers. https://www.reddit.com/r/SkincareAddiction/comments/8k0mb0/research_evaluating_your_routine_moisturizers/
[10](1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27) Leslie Baumann, Sogol Saghari. Cosmetic Dermatology Principles. Chapter 1: Basic Science of the Epidermis.
[19](1, 2, 3, 4, 5, 6) Leslie Baumann, Edmund Weisberg. Cosmetic Dermatology Principles. Chapter 9: The Baumann Skin Typing System.
[50]Leslie Baumann. The Skin Type Solution. Chapter 2: Understanding Skin Type Categories.
[51](1, 2, 3, 4, 5, 6) Francis Gasparro, etal. 1998. A Review of Sunscreen Safety and Efficacy.
[52](1, 2, 3, 4, 5, 6, 7, 8) Henry W. Lim, MD, and Steven Q. Wang, MD. July 3, 2012. The Skin Cancer Foundation's Guide to Sunscreens. https://www.skincancer.org/prevention/sun-protection/sunscreen/the-skin-cancer-foundations-guide-to-sunscreens
[53](1, 2) https://thewirecutter.com/reviews/best-sunscreen/

Blisters form when the top layer of skin gets detached from the lower layers, separating "plasma" sacs in the process. The plasma is completely sterile and serves to protect the lower layers of skin from further harm, basically natures band-aid with a bit of pain thrown in to keep you from doing more damage.


todo: get a video of a mosquito finding blood? kind of interesting visual


Tay bought me "Laneige - Water Sleeping Mask", a Korean skincare product.