Selecting a Sunscreen: A guide to current options

Sunlight is an integral, almost inescapable part of the external environment. It is a form of radiation, which is emitted from the sun as light of various forms or wavelengths. One is visible light, which is reflected by most surfaces, thus allowing them to be seen by human eyes. Other types include ultraviolet radiation, which is invisible to the human eye, but exerts profound effects on living things nonetheless. It radiates in great amounts onto the Earth's surface on a daily basis.

Some exposure to this UV radiation in moderation may have some health benefits - for example, it causes the release of Vitamin D in human skin, which benefits the health and condition of bones and other tissues. However, prolonged or sustained UV radiation exposure can have negative health effects, particularly on the skin.

There are a number of subtypes of this radiation, two of which make up the greater proportion of the UV light we absorb every day.

UVA

UV radiation type A makes up approximately 90% of the UV light delivered to the planet's surface. It has a relatively long wavelength, (from 320nm to 400nm) although shorter than that of visible light (up to ~700nm). It has the ability to penetrate deeply into skin, down to the deeper layers where new skin cells are produced.

UVA radiation, like most UV light types, can cause skin damage.

They do this by direct damage, i.e. simple burning of the skin, and through DNA damage.

When DNA is exposed to UV light, it results in the conversion to DNA molecules carrying specific types of damage, known as photoproducts. These may affect genes that control the natural cell mechanisms that control the replication of damaged DNA.

This is the basis of cancer development, in which DNA mutations contribute to the growth of cells that are abnormal, resistant to normal cell death and/or malignant.

These cells may divide, proliferate and form a discrete type of tissue called a tumour, which may cause mechanical or chemical damage to surrounding 'normal' tissue, and significantly affect overall health if they become enlarged.

UVA radiation may not cause cancer directly in all cases, but may elicit DNA damage that contributes to the development of cancer cells over time.

Carcinoma. UVA is associated with a type of tumour known as a carcinoma, which can arise from two types of cell found in the base of the epidermis (the uppermost layer of the skin); basal and squamous.

Melanoma. UVA is also associated with the development of melanomas, a malignant tumour type arising from melanocytes. Melanocytes produce the protein melanin, which plays a role in protecting the tissue from UV exposure.

Tanning. UVA radiation is associated with tanning (i.e. increases of skin pigmentation, which is associated with melanin).

Aging. UVA radiation is associated with photoaging, or increased deterioration of the skin due to UV radiation, which may appear as premature aging.

Deaths resulting from melanoma, basal cell carcinoma and squamous cell carcinoma growth have reduced considerably over the last few decades, but treatment for these cancers still represents a sizeable proportion of healthcare burden worldwide.

UVB

Ultraviolet radiation type B is much less abundant than UVA. It penetrates the upper layers of skin (i.e. the epidermis and dermis) and is associated with skin burning and redness.

It also pervades clouds, other gases and glass more efficiently than UVA. Therefore, UVB exposure is a valid concern even on days without direct sunlight.

UVB radiation is also associated with melanomas and carcinomas, and is considered by some researchers to be significantly more effective in cancer promotion than UVA. Like UVA, it damages genes that control DNA replication. It is possible that UVA 'kick-starts' tumour development by inducing DNA damage, which is then exacerbated by UVB, or vice versa. UVB radiation has a wavelength of between 290 and 320nm.

Factors in Sunscreen Selection

Sun Protection Factor: Sun protection factor is a measure of how well the sunscreen blocks, or deflects, UV light from the skin. The factor is a measure of how long it can do this for; for example, if the radiation needs twenty minutes to elicit skin reddening, an SPF of 15 indicates that the amount of time to do this will need to be multiplied by fifteen, i.e. it would take five hours for skin to become red with this sunscreen applied. However, SPF does not account for a full spectrum of UV protection. Therefore, it is important when selecting a sunscreen to find an option with both UVA and UVB protection, in addition to an SPF appropriate to how long you intend to spend in the sun. Protection against the subtypes of UV radiation is also measured in factors, i.e. UVA protection factor (PFA) and UVB protection (PFB). The spectrum of UV protection may be affected by the ingredients that make up the sunscreen, and their radiation-deflecting properties. These ingredients include:

Titanium Dioxide: This is included in many sunscreens both currently on the market and in development. It physically 'blocks' UV radiation, and is effective against UVB radiation and UVA radiation of between 320 and 340nm (also known as UVA type 1). Titanium dioxide is inorganic, as opposed to more traditional sunscreen ingredients, which are organic chemicals.

Zinc oxide: This is another inorganic (or 'mineral') sun-blocker, but unlike titanium dioxide, it can deflect UVB, UVA type 1 and UVA type 2. An in-vitro trial (a standard procedure that tests human skin analogues against radiation that may be natural or artificial) found that it was significantly more effective in blocking UVA in comparison with titanium dioxide.

Avobenzone: This is an organic sunscreen ingredient with UVA-blocking properties. A recent in vitro test showed that avobenzone was significantly more effective than titanium dioxide, increasing UVA protection factors threefold. Unlike inorganic sunscreen components, avobenzone absorbs UV radiation, and may break down over time.

Oxybenzone: Oxybenzone is an organic ingredient with similar UV protection properties to those of titanium dioxide It has been associated with some undesirable effects, such as toxicity, pollution and the increased risk of male infertility. However, recent research has found no evidence of an association between oxybenzone and hormonal disruption or dangerous levels of absorption through skin.

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