Whether we are driving to work, walking to class or on vacation on a remote beach, we are all exposed to the sun. Therefore, the proper sun protection with the use of sunscreen or avoidance of excessive sun exposure is very important. Here is an overview of the effects of ultraviolet radiation (UVR), explanation of sun protection factor (SPF), how sunscreens work, the different sunscreens that are available, and how to choose sunscreens. 

What is UVR?

Ultraviolet radiation (UVR) is one of the major risk factors for skin cancers and other sun induced skin conditions such as premature aging. Its exposure is cumulative over a lifetime and can cause serious long term problems. The most common skin problem caused by UVR is sunburn, experienced by many people at least once in their lifetime. There are varies sunscreens on the market that help prevent UVR exposure and minimize the risk of sunburns and other skin disorders.

It is important to understand the different UV bands to properly select effective sunscreens that will provide enough coverage. The UV (ultraviolet) spectrum is divided into UVA, UVB, and UVC. The wavelength of UVA band ranges between 320 to 400 nm. UVA radiation penetrates deeper into the skin compared with UVB and can cause damage to DNA. Damage from UVA is less noticeable than UVB since it is between the visible layer of the skin. UVA may increase the effects of UVB and contribute to further premature aging such as wrinkles and can play a role in some cancers. Additionally, UVA is involved in photosensitivity (sun sensitivity) reactions caused by medications (for example, certain antibiotics) that can cause photosensitivity.

The wavelength of the UVB band is between 290 and 320 nm. It is often referred to as the sunburn radiation since it is the most active UVR wavelength for causing redness of the skin (erythema). The intensity of the radiation is highest from 10 am to 4 pm. It is also the primary contributor of skin cancer which may be increased by UVA exposure. UVB is also mainly responsible for wrinkling of the skin and collagen damage. However, it is important for vitamin D₃ synthesis in the skin and the amount of UVB needed depends on the individual and environmental factors. Currently, there are a wide range of Vitamin D supplements available and the need for excessive UVB exposure in order to increase production of Vitamin D is not needed.

The last wavelength is UVC band between 200 to 290 nm. Therefore, very little of it reaches earth since most of it is screened out by the ozone layer. However, the very little UVC that does pass through is absorbed by the dead cell layer of our skin and is normally not harmful or the cause of skin cancer.

What is SPF?

The sun protection factor (SPF) is a measure of the effectiveness of a sunscreen. It is the amount of UVR that is required in order to cause sunburn after applying the sunscreen to the skin compared to the amount required on unprotected skin. It is important to note that the SPF is specific to the sunscreen formulation. Although a higher SPF provides better sun protection there is a limit to the benefit. For instance, an SPF of 15 blocks 93% of UVB, an SPF of 30 blocks 96.7%, and an SPF of 40 blocks 97.5%. Sunscreens with SPFs higher than 45 do not offer much additional benefit and no sunscreen provides 100% protection. 

What are the different types of sunscreens?

There are two different types of sunscreens. The first and most popular are chemical sunscreens. They work by absorbing UVR and thus blocking its transmission into the skin. There are a large variety of chemical sunscreens. The other type of sunscreen is a physical sunscreen, which is usually opaque and work by reflecting UVR instead of absorbing it.

Chemical Sunscreens

Aminobenzoic Acid and Derivatives

Aminobenzoic acid is an effective UVB sunscreen. It penetrates deep into the skin, provides lasting protection, and can last long on sweaty or wet skin. However, since it is an alcoholic solutions, it can be irritating to the skin and can cause redness, stinging, rashes or drying of the skin. Therefore, it is usually used with a combination of other ingredients to limit irritation.

Anthranilates

These are weak UV sunscreens used in combination with other sunscreen agents.  They provide the highest absorption of UVA and are usually combined with products that provide high UVB coverage.

Benzophenones

The benzophenone group includes three sunscreen agents: dioxybenzone, oxybenzone (benzophenone-3), and sulisobenzone (benzophenone-4). They provide the best UVB protection but also provide UVA protection. They are popular because they do not cause allergic reactions seen with aminobenzoic acid.

Cinnamates

The Cinnamates group includes cinoxate, octinoxate, and octocrylene. Cinoxate and octinoxate are effective for the same UVB range while octocrylene absorbs higher into the UVA range.

Dibenzoylmethane Derivatives (Avobenzone)

Avobenzone offers the best protection in the UVA range. It is often added to other sunscreen products to increase their UVA coverage and to increase the stability avobenzone because it degrades easily in the sun.

Salicylates

Salicylic acid derivatives are fairly weak sunscreens. They only absorb in the UVB range and provide no UVA coverage. In addition, they also have to use in large concentrations. They are not commonly used alone because they do not adhere well to the skin and are easily removed through excessive sweat or water.  

Physical Sunscreens

Zinc oxide and titanium dioxide

These are the only two approved physical sunscreens that are on the market today. They provide both UVB and UVA coverage. Newer formulations of these agents are transparent and therefore preferred in cosmetic products. Sunscreens with zinc oxide or titanium dioxide are popular because of their high UVB and UVA coverage. 

How to choose a sunscreen

Many sunscreens on the market today use a combination of agents to achieve broad spectrum coverage of UVA and UVB. Therefore, when choosing a sunscreen it is important to choose a sunscreen with high enough SPF and coverage well into both the UVA and UVB range to obtain maximum protection.

References

American Cancer Society. Skin Cancer Prevention and Early Detection. July 6, 2010.

US Food and Drug Administration. Over-the-counter human drugs; labeling requirements; delay of implementation date. Final rule: delay of implementation date of certain provisions. Fed Regist. 2004;69: 53801-04.

Diehl JW, Chiu MW. Effects of ambient sunlight and photoprotection on vitamin D status. Dermatol Ther. 2010;23:48-60. PubMed

Davidow LW. Self-Care and nonprescription pharmacotherapy. In: Berardi RR, ed in chief. Handbook of Nonprescription Drugs: An Interactive Approach to Self-Care. 17th ed. Washington DC: American Pharmacists Association; 2012.

Gonzaga ER. Role of UV light in photodamage, skin aging, and skin cancer. Am J Clin Dermatol. 2009;10:19-24. PubMed

Kullavanijaya P, Lim HW. Photoprotection. J Am Acad Dermatol. 2005;52:937-58. PubMed

Reichrath J. Skin cancer prevention and UV-protection: how to avoid vitamin D-deficiency? Br J Dermatol. 2009;161:54-60. PubMed

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