SPF: How Much Protection Does Your Sunscreen Really Offer?

Sunscreen (or sunblock) is commonly rated in terms of its sun protection factor, or SPF. This number is a measure of a sunscreen's ability to prevent damage to your skin from a type of ultraviolet radiation-UVB. According to the Skin Cancer Foundation, if it takes 20 minutes for your skin to redden, then an SPF 15 sunscreen should delaying your turning red for 15 times longer-roughly five hours.

There are two major problems with this measurement system.

Firstly, no sunscreen, regardless of whether it is SPF 15 or 50, is going to remain effective for 5 or more hours. Over time and with exposure to sunlight, sunscreen will break-down. It can also rub off on clothing, beach towels, etc.. In order to stay adequately protected, you need to reapply sunscreen roughly every two hours. If you have been swimming or sweating, you should reapply even more frequently. It is also important to apply enough sunscreen-1 oz (a shot glass sized quantity) each time you apply.

Secondly, reddening of your skin is a result of overexposure to UVB rays but is not a complete measure of overexposure to the sun. Most of the UVB radiation that reaches Earth is filtered out by the ozone layer before it reaches us. UVA rays on the other hand, pass through the ozone layer and are therefore much more common down here on the surface. These rays don't give you a classic sunburn but they are even more dangerous than UVB rays for a few reasons. For one, without the burn there is no visible warning of overexposure to UVA radiation, so it is easy to overexpose yourself without knowing it. Secondly, unlike UVB rays, UVA rays can affect your skin through glass, possibly damaging your skin even if you are indoors or inside a car. Finally, UVA rays penetrate more deeply into your skin, potentially affecting the DNA in skin cells and thereby increasing your risk of developing skin cancer.

IS A HIGHER SPF BETTER?

The amount of protection offered by a particular sunscreen does not increase proportionally to its SPF rating. The American Academy of Dermatology explains that an SPF 2 screens 50 percent of UVB rays, an SPF 15 screens 93 percent of UVB rays, and an SPF 30 screens 97 percent of UVB rays. No sunscreen can offer full protection from UVB radiation though.

The majority of sunscreens on the market with SPFs over 10 are broad-spectrum, that is they offer protection against both UVA and UVB radiation. However, few people realize that a product's SPF rating is not connected to the amount of protection it provides against UVA radiation.

When selecting a sunscreen, it is important to remember that some UVB exposure will occur, even with a high SPF product. Be aware that the SPF number on the bottle has no relation to the amount of protection the product offers against more dangerous UVA radiation. If you do not apply sunscreen frequently enough or if you apply too little, you will not receive the full amount of protection offered by your sunscreen.

SOURCES

Skin Cancer Foundation. "Sunscreens Explained". Accessed September 4, 2009. http://www.skincancer.org/sunscreens-explained.html

American Association of Dermatologists. "Facts about Sunscreens". Accessed September 4, 2009. http://www.aad.org/media/background/factsheets/fact_sunscreen.htm

Canadian Broadcasting Corporation. "Sunscreen makers' SPF ratings 'misleading,' lawsuit alleges". August 31, 2009. http://www.cbc.ca/consumer/story/2009/08/31/sunscreen-class-action-lawsuit-uva.html