The sun is a star located about 150 million kilometres from Earth whose light takes 8 minutes and 18 seconds to arrive here. Without it, there would be no life on Earth; it would be pitch-black and very cold.
The nuclear reaction in the sun’s core creates large amounts of radiation. This energy, also called the electromagnetic spectrum, occurs in the form of radio waves, microwaves, infrared radiation, visible light, ultraviolet rays, X-rays, and gamma radiation.
Depending on the wavelength, these different kinds of solar radiation have different impacts:

• Infrared waves create heat
• Visible light allows us to see
• Ultraviolet rays have effects on our health

The level of ultraviolet rays that reach the Earth’s surface depends on the distance that they travel in the atmosphere. Thus, the shorter the path, the more UVB there will be. This is why the level of ultraviolet radiation that reaches the Earth’s surface depends on a number of factors:

• The ozone layer
• The seasons
• Time of the day
• Latitude
• Altitude
• The albedo (the level of radiation reaching the Earth’s surface that is then reflected (sand, snow)
• Meteorological conditions

The different types of UV

Ultraviolet radiation is invisible and has a wavelength from 200 to 400 nanometres. Its wavelength is shorter than visible light and contains more energy. Depending on the wavelength, it can travel through the ozone layer and has different effects on health. The shorter the wavelength, the more harmful it is; however, it will also have greater difficulty penetrating the skin. Ultraviolet radiation is divided into three bands:

1/ UVC rays

UVC rays have a wavelength from 200 to 280 nm and are the UV rays with the most energy. They are extremely dangerous to all forms of life (even in very weak doses). The percentage of UVC rays reaching the Earth’s surface is minimal: as soon as they arrive in the atmosphere, a photochemical reaction (that requires the presence of light) occurs with the ozone. In this way, 99% of UVC rays are absorbed by the atmosphere, thanks to the ozone layer. The minuscule portion of these UV rays that get through the atmosphere have difficulty penetrating the skin, as they are stopped by the epidermis.

2/ UVB rays

UVB has a wavelength from 280 to 315 nm and is 100 times less common than UVA. A large portion of UVB is absorbed by the ozone layer. Only 2 to 5% of UV rays reach the surface of the Earth, although they remain highly energetic. They primarily penetrate the horny layer of the epidermis. Only 10% reach the dermis.

Impact on the skin:

They are responsible for long-term tanning and sunburn, as well as for most skin cancers.

• Anti-rickets action: The synthesis of vitamin D takes place in the deepest part of the epidermis under the influence of UVB rays. Vitamin D3 increases the intestinal absorption of calcium, improves the mobilization of calcium and phosphorus in the bones, and suppresses the secretion of parathyroid hormones. Fibroblasts and keratinocytes possess vitamin D3 receptors. Vitamin D3 inhibits the proliferation of keratinocytes and participates in their differentiation and cornification.
• Tanning or delayed pigmentation: This pigmentation occurs two or three days after exposure, with a maximum range of three weeks after exposure. Then, it fades progressively in the absence of new exposure through natural desquamation of the superficial layers of the epidermis. This pigmentation is linked to new synthesis of melanin by melanocytes and an increased transfer of melanin to the keratinocytes. Tanning plays an essential role in natural photoprotection.
• Influence of certain kinds of dermatoses (acne): Exposure to solar radiation leads to a thickening of the epidermis, which tends to block the pilosebaceous follicles and therefore to prevent the removal of sebum; furthermore, the anti-inflammatory action of UV rays dries visible pustules. This phenomenon gives the impression that acne has improved. However, this effect, called summer hyperkeratosis, is only temporary. The sebum that cannot be removed stimulates the formation of comedones and micro-cysts, which will appear once UV exposure stops.

3/ UVA rays

UVA has a wavelength from 315 to 380 nm. We distinguish between short UVA wavelengths (between 315 and 340 nm) and long UVA wavelengths (between 340 and 380 nm). It is not easily absorbed by the ozone layer, and thus almost 95% of this radiation gets through. It easily penetrates the skin, with about 20% to 30% reaching the medium dermis.

Impact on the skin:

• Immediate pigmentation or the Meirowsky phenomenon: UVA is responsible for the Meirowsky phenomenon, an immediate and temporary grayish-brown skin discoloration that appears a few minutes after prolonged exposure to the sun. This discoloration is due to photo-oxidation of the pre-existing melanin under the influence of UVA; it peaks in less than an hour and disappears progressively in a few hours.
• Generation of free radicals: UVA is also able to indirectly modify DNA by provoking the generation of free radicals. These are responsible for deterioration of lipids, proteins, and DNA in chromosomal and cellular DNA, as well as the destruction of collagen and elastin fibers. Free Radicals are, among other things, responsible for skin aging.

4/ UVA and UVB

UVA and UVB radiation has combined effects and acts synergistically on the skin.

• Sunburn: Sunburn occurs several hours after intense exposure to the sun and can last several days. Its appearance is easily recognizable by all, and it presents four levels of severity depending on the amount of sun received and the phototype: roseate, bright red, cyanic, edematous and painful with the appearance of blisters. In the repair phase, the skin peels profusely to eliminate dead keratinocytes. During sunburn, it is UVB that is most aggressive, but UVA, while less active, is more numerous and, in the end, plays a part in the attack as well.
• Heliodermy or photo-induced cutaneous aging: The sun ages the skin differently than natural aging; it does not simply accelerate the aging process, it makes different cells (keratinocytes and melanocytes) able to evolve toward carcinogenesis, which does not simply provoke intrinsic aging. UVB stimulates cell proliferation in the skin’s superficial layer, which thickens the epidermis.
  The deep penetration of UVA into dermal tissue destroys the elastin and collagen fibers that provide tissue with resistance and elasticity, leading to a relaxing of these tissues, a loss of firmness of the skin, and the appearance of wrinkles.
  In terms of the cells, skin that has aged naturally shows an overall decrease in the activity and number of fibroblasts, which are responsible for making collagen and elastin. The sun’s action, on the other hand, maintains the overall population of fibroblasts at a high level, but these fibroblasts are altered and manufacture poor-quality elastic fibers.
  The skin becomes wrinkled, develops spots, and loses its smoothness. It can also thicken, particularly on the neck, or become thinner on the forearms and the backs of the hands, or the number of sebaceous glands may increase on the face. The appearance of early wrinkles is attributable to sun exposure. It occurs primarily on the exposed areas of the face and neck.

• DNA damage: Cellular DNA is the ultimate target of attack for UVB rays. Nucleic acid absorbs UVB radiation, which creates specific bonds. UVA alters DNA via the intermediary of reactive oxygen species that are responsible for chain ruptures, protein-nucleobase crosslinking and oxidative damage to base pairs. These abnormal links then lead to a modification of the DNA’s structure, which alters the normal operation of DNA polymerase, an enzymatic complex that is involved in replication. This consists of the replication of genetic information through the replication of the DNA molecule (a parent strand with a newly synthesized strand). In the long term, if this occurs repeatedly, these abnormal links will overwhelm the DNA repair system. Indeed, these pairing errors are normally detected by enzymes that cut off the short fragment of DNA containing the error and eliminate it. DNA polymerase ensures the synthesis of the missing portion, which repairs the DNA. If these anomalies are not detected, they can lead to potentially carcinogenic cell mutations, such as melanoma or carcinoma.

  

• Skin cancers

Sunscreen in Australia

Sunscreen provide one of the best ways to protect yourself from the sun’s ultraviolet (UV) rays. Australia has one of the highest rates of skin cancer in the world.

Sunscreens contain substances that either absorb or reflect UV rays. They prevent most UV rays from penetrating the skin and damaging skin cells. Many Australians use sunscreens daily. This is why sunscreens need to be regulated in Australia to ensure they are safe, efficacious and of good quality.

Only approved ingredients can be included in sunscreens, and each of these ingredients has been assessed for safety and quality.

Sunscreen products come in many forms including creams and aerosol sprays. The amount of a sunscreen you need to use is explained on the label and when used as intended should provide the appropriate protection.

The TGA requires the efficacy of each sunscreen product to be tested to determine the sun protection factor (SPF). The SPF value for the product is printed on the label.

TGA regulates sunscreens in Australia that are classified as therapeutic goods (therapeutic sunscreens) to make sure they are safe, efficacious and high quality. This means that therapeutic sunscreens need to meet all the legal requirements for therapeutic goods before they can be supplied in Australia. Sunscreens in the Australian Register of Therapeutic Goods (ARTG) are approved for supply in Australia. Most sunscreens in the ARTG are listed (lower risk) medicines.

Therapeutic sunscreens include:

• Primary sunscreens: Products that are used primarily for protection from UV radiation.
• Some secondary sunscreens: Products with a primary purpose other than sun protection, that also contain sun screening agents and are not excluded from therapeutic goods legislation. E.g. sunbathing and moisturising skin care products with an SPF of over 15.

(tga.gov.au)