Sun and UV Exposure
The primary environmental cause of aging of the skin and most skin cancers is ultraviolet (UV) light from the sun. Its
damaging effects are cumulative, so daily protection throughout life is important. In the US, most people will get < 25% of
their lifetime UV dose by 18 years of age. By the age of 60, they have absorbed 80% of their lifetime dose.1 The predominant
acute biologic damage, as well as chronic damage risks, such as elastosis and squamous cell carcinoma, are associated with
the UVB portion of the solar spectrum over UVA in a ratio of 4:1.2
Chronic UVA exposure can occur by several scenarios:
- Tanning beds (some promote UVA-only radiation)
- Exposure to the sun using a UVB-absorbing sunscreen
- UVA exposure through windows (only UVB rays are blocked)
People may be unaware that they are unprotected from the damaging effects of UVA under these circumstances.
A 2003 survey by the American Academy of Dermatology reported regular sunscreen use by 47% of women and 33%
of men.3 There is general consensus among experts that daily, year-round, broad-spectrum photoprotection of at least sun
protection factor (SPF) 15 is a key component of a sun-safe strategy to reduce cumulative lifetime exposure to UV light.
Sun Protection Factor
- The SPF number indicates how much longer one can stay out in the sun before burning compared with no protection.
- An SPF of 15 means that theoretically one can stay out without burning, 15 times longer than with no protection. However, this should not be used to encourage prolonged sun exposure.
- The level of UV filtration is not proportional to the SPF. The amount of UV transmission is 1/SPF so with an SPF of 2, 50% of UVB light is transmitted. An SPF of 30, will block out 97% of UVB light (1/30 transmitted).
- SPF relates to UVB protection only. However, higher SPF sunscreens tend to provide more UVA protection.
- While UVA rays don’t cause sunburn, they penetrate deeper into skin and cause photoaging, cutaneous immunosuppression, and can cause some skin cancers.
- There is no universally accepted rating to determine how good a sunscreen is at blocking UVA rays, although several measures have been proposed.4
Sunscreens are divided into two broad categories:
1. Inorganic or physical blockers, which:
- reflect back and scatter the UV and visible light.
- are regarded as nontoxic and stable and do not penetrate below the stratum corneum.
- are esthetically less acceptable.
2. Organic or chemical agents, which:
- work through electron excitation and grounding, thus changing light energy into heat energy.
- are divided into UVA, UVB, and broadband absorbers.
The ideal sunscreen would:
- provide superior efficacy with broad-spectrum coverage, photostability, and high substantivity.
- have an appealing feel and smell and apply easily, uniformly coating the skin surface.
- not cause irritant or allergic contact dermatitis.
- be sold at a reasonable cost.5
New Generation Sunscreens
- Newer sunscreens combine ingredients that will protect the skin from both UVA and UVB rays and are available in creams, gels, lotions, sprays, and sticks.
- New generation sunscreens are more esthetically pleasing.
- The need for broad-spectrum, photostable filters has led to the development of new agents, including ecamsule and drometrizole trisiloxane, which provide both UVB and UVA protection.
- Degradation of some UVA filters by sunlight requires chemical stabilization to prevent loss of efficacy.
- An advanced formulation that incorporates the photolabile filter avobenzone, provides protection throughout the UVA range, and is combined with diethylhexyl 2,6-naphthalate (DEHN) and oxybenzone to achieve a photostable product. DEHN is a non-UV filter that stabilizes avobenzone by accepting energy absorbed by avobenzone during UVA exposure, while oxybenzone both enhances photostability and provides additional UVA protection.
- This technology yields protection that is reported to be comparable to that of drometrizole trisiloxane, which provides efficient UVA coverage and photostability.6
- Experts recommend that sunscreen be applied 15-30 minutes before going outdoors and reapplied every 2 hours or after swimming/ heavy exertion.
- The recommended dose is 2mg/cm2 of skin, or 30ml (2 tablespoons) for the whole body. However, the actual dose being used is believed to be much lower.
- Application of an adequate amount of sunscreen is by far the most important factor influencing efficacy.
Sunscreens and Cancer
- Sunscreens, at sufficient SPF levels, are effective in protecting the skin from actinic keratoses and squamous cell carcinomas.7
- They have not been shown to reduce the incidence of a first basal cell carcinoma, but they may prolong the time to develop a second lesion.
- Dennis, et al., looked at sunscreen use and the risk for melanomas, and found no association between these two factors.8
The Vitamin D Controversy
- The UV action spectra for DNA damage leading to skin cancer and for vitamin D photosynthesis are virtually identical.9
- Nash, et al., analyzed the risks/ benefits of sunscreens on vitamin D by estimating its production, based on measures of sunlight exposure and determining the impact of an SPF 15 sunscreen on vitamin D levels in humans. The study found that the combination of diet and sunlight, even with daily use of an SPF 15 sunscreen, provides adequate intake of vitamin D.10
The new generation of sunscreens serve an important role in overall skin protection from the sun. However, they should be
used in conjunction with other sun protection measures such as sun protective clothing, hats, and sunglasses.
- Godar DE, et al. Photochem Photobiol 77(4):453-7 (2003 Apr).
- Cole C. Photodermatol Photoimmunol Photomed. 17(1):2-10 (2001 Feb).
- American Academy of Dermatology. 2005 Skin Cancer Survey Fact Sheet. URL: www.aad.org.
- Chen T, et al. Comprehensive evaluation of sunscreen protection beyond SPF and PF values. Presented at the 66th Annual Meeting of the American Academy of Dermatology, February 2008, San Antonio, TX. P2404.
- Palm MD, et al. Derm Ther 20(5):360-76 (2007 Sep-Oct).
- Rigel D, et al. Photostability of UVA/UVB sunscreens under extreme tropical sun exposure. Presented at the 64th Annual Meeting of the American Academy of Dermatology, March 2006, San Francisco, CA. P2623.
- Naylor MF, et al. Arch Dermatol 131(2):170-5 (1995 Feb).
- Dennis LK, et al. Ann Intern Med 139(12):966-78 (2003 Dec 16).
- Wolpowitz D, et al. J Am Acad Dermatol 54(2):301-17 (2006 Feb).
- Nash JF, et al. J Am Acad Dermatol 52(3):161 (2005 Mar).
- Drealos Z. Am J Clin Dermatol 3(5):317-8 (2002).