Monday, July 20, 2015
Digging into the Facts on Sunscreen
By: Gregory Bourgeois, MD
Shelby Dermatology, PC
What is the point of sunscreen? It is to help prevent the public health problem that is skin cancer. Recently, the US Surgeon General had a Call to Action to Prevent Skin Cancer to put skin cancer on the radar of millions of Americans. There are nearly four million melanomas, basal cell carcinomas, squamous cell carcinomas, and other skin cancers – all linked to ultraviolet radiation damage – diagnosed annually. We can do a better job of prevention, yet many Americans are not using sunscreen regularly.
Could it be that we don’t trust sunscreen? We have all had the experience of putting on sunscreen, yet still experiencing a burn or a tan. Maybe we didn’t put on enough. Maybe the sunscreen had expired. Maybe we forgot to reapply. Maybe we really just wanted to tan and didn’t care about whether the sunscreen worked; we just knew we needed to put some on because that’s “what you do.” Since recent consumer studies in Consumer Reports and similar consumer organizations in Europe show that labeled SPF may not be accurate (by their own testing, not by FDA tests), maybe we doubt sunscreen altogether.
I want to shed some light on the development of sunscreen and its inherent strengths and weaknesses. Hopefully, this helps build confidence in sun protection using sunscreen yet serves as a warning that sunscreen can only do so much.
A sunscreen should really be considered a “UV-screen” because this is the portion of the electromagnetic spectrum that it blocks. UVA and UVB rays are the most common damaging electromagnetic radiation in terms of causing skin cancer. UVA rays tend to cause a persistent pigment darkening of the skin that one notices within 24 hours of exposure; this is why tanning beds use UVA lamps so that tanning is seen soon after a session. UVB rays tend to cause sunburn which usually occurs about six hours or more after exposure. There is some overlap between the two sets of radiation, and they both lead to what is the desired look of many beachgoers – the delayed tan, which is persistent skin darkening for weeks.
A tan is the result of your body’s imperfect attempt to protect itself from the UV damage it already received by increasing the amount of pigment (specifically melanin) to act as a UV cover at the cellular level. Sunscreen does not protect against the reactive oxygen species (ROS) that are formed by UV radiation. It can prevent their formation by blocking the UV radiation to some extent, but once ROS are formed, sunscreen is helpless and an antioxidant is needed to stop their havoc.
The Sun Protection Factor (SPF) is a measure of the sunscreen’s ability to prevent sunburn, so it is mainly a measure of how well it blocks UVB radiation. When testing sunscreen, a paid volunteer has two mg/cm2 of sunscreen evenly applied to test sites and then is exposed to various doses from a high intensity solar simulator (which has all the spectrum of electromagnetic radiation emitted by sunlight). A minimal erythema dose (MED) is the smallest UV dose needed to produce perceptible redness in the skin 16to 24 hours after exposure. The SPF is the ratio of the MED’s of sunscreen-treated skin to untreated skin. UVA protection does not have a quantifiable number similar to SPF in the US. Here, it is measured with in-vitro critical wavelength testing (mandated by the FDA to be considered broad-spectrum) that measures the amount of UV radiation transmitted through transparent plastic smeared with the test sunscreen. A critical wavelength is the wavelength of radiation measured in nanometers (nm) where a sunscreen absorbs 90 percent of the UV radiation transmitted through it. Since the UVB-UVA spectrum is 290 to 400 nm on the electromagnetic spectrum, a critical wavelength that is at least 370 nm is considered a broad spectrum sunscreen.
Sunscreen is made of organic and inorganic ultraviolet filters. The organic filters are aromatic chemical compounds that work by absorbing UV photons. The inorganic filters are minerals zinc and titanium that also work by absorbing UV photons as well as scattering them. Each have their own absorption spectra within the UV range with some protecting very well against UVA while most protect very well against UVB. Some absorb UV photons better than other filters while some filters are more photostable (the ablilty to withstand the onslaught of UV radiation).
Because of these various properties, sunscreen is often of mix of organic and inorganic filters in order to gain the most UV protection while remaining stable out in the sunlight and maintaining safe concentrations of each filter. An ideal sunscreen forms a film on the skin that uniformly distributes the ingredients, so certain emollients and other photostabilizing boosters are included to help with UV protection as well as enhance the proper application of the product. In the US we are limited by the amount of broad spectrum ingredients, particularly those that block UVA radiation. This is due to the FDA regulation of sunscreen as OTC drugs, and the application process is arduous for new products to be introduced to market. Since 1999 there has only been one new approved UVA blocking ingredient. I will avoid discussing the politics of the situation, but hopefully, this area of regulation will improve as the FDA standstill on sunscreen has been exposed recently and ostensible Congressional action has been taken.
Remember that sunscreen is only part of a complete photoprotection package that also includes clothing, wide-brimmed hats, shade, avoiding peak hours of sunlight, and sunglasses. Most of us don’t apply enough sunscreen (it has been shown we get about 1/3 of the labeled SPF based on our typical application) and don’t reapply every two hours as we should, or after being submerged in water for a while. If you towel off after getting out of the water, you are wiping your sunscreen off, so reapply. When it comes to recommending sunscreen for your patients, tell them to choose an SPF 30 or greater and one labeled broad-spectrum. Most of all, use common sense and enjoy the sun responsibly.