Screening Sunscreens

Screening Sunscreens, an article in the Washington Post

They discussed the importance of skin care in the sun and different methods of sun protection. I've always been a little unclear on sunscreen and what to look for, so I decided to do some research myself.

Important definitions:
1. UVA
-long- wave solar rays of 320-400 nanometers
-penetrate skin more deeply
-less likely to cause sunburn
-more likely to cause wrinkling and leathering

2. UVB
-short-wave solar rays of 290-320 nm
-more likely to cause sunburn
-main rays associated with basal cell and squamous cell carcinoma

3. Sunscreens: chemically absorb UV rays
-octylcrylene, benzophenones, avobenzone indicate increased UVA protection

4. Sunblocks: physically deflect UV rays
-titanium dioxide and zinc oxide indicate increased UVA blocking
-micronized preparations are less conspicuous

5. SPF (sun protection factor): measures the length of time a product protects against skin reddening from UVB, compared to how long the skin takes to redden without protection. If it takes 10 minutes without protection to begin reddening, using an SPF 15 sunscreen theoretically prevents reddening 15 times longer -- about 2.5 hours.
-this is only a measure of UVB protection, there is no standard for UVA protection
yet
-reapply every two hours to maintain the SPF and immediately after
sweating/swimming/toweling off
-the dermatologist I am working with recommends SPF 30 or higher, which blocks 97%
of UVB rays

6. Broad-spectrum protection: indicates that a product has UVA and UVB protection
-does not indicate spectrum of UVA coverage
-avobenzone, titanium dioxide, and zinc oxide indicate full UVA spectrum coverage

Both UVA and UVB rays are linked to skin cancer. The main mechanisms include:
1. altered cell signaling
2. decreased immune responese
3. increased DNA damage

Most studies I found could not identify which of these factors were greater predilections for skin cancer, making it difficult to blame one or the other for skin cancer. The study below used an animal model to show that UVB caused melanoma, but the mechanism of action remains unclear. Sunscreen companies and dermatologists continue to act under the impression that UVA and UVB are equal contributors to skin cancer. An interesting note... tanning bed manufacturers have acted on this evidence, however, and their newer beds emit a higher fraction of UVA rays.


De Fabo EC, Noonan FP, Fears T, Merlino G. Ultraviolet B but not ultraviolet A radiation initiates melanoma. Cancer Res. 2004 Sep 15;64(18):6372-6.



A number of articles address different methods of assessing a sunscreen's ability to block UVA rays. Although the following article seemed to have a promising method, it was published in 2000 without any forthcoming results. The in vitro assessment seemed to be ideal, as human and animal testing could be avoided. Similar studies assessing different methods of quantifying UVA coverage have been conducted over the last ten years, with no conclusive results. One thing that can be agreed upon is that SPF is not an indicator of UVA spectrum coverage.

Diffey BL, Tanner PR, Matts PJ, Nash JF. In vitro assessment of the broad-spectrum ultraviolet protection of sunscreen products. J Am Acad Dermatol. 2000 Dec;43(6):1024-35.

Something else to consider is the interaction between all of these chemicals, their shelf lives, and their photostability. More studies are beginning to address these issues, and a good example of a chemical stability and photostability study is belo They directly tested photostability, but indirectly tested chemical stability by including different UV-filtering combinations.

Gaspar LR, Maia Campos PM. Evaluation of the photostability of different UV filter combinations in a sunscreen. Int J Pharm. 2006 Jan 13;307(2):123-8.

Look here for a comprehensive analysis of the current sunscreens/blocks out there. I liked this analysis because they looked for active ingredients as well as photostability.