SPF Degradation and The Two Hour Rule

SPF Degradation and The Two Hour Rule

With an ever growing focus on skin cancer issues, health, beauty and anti-aging, there are few of us who have not incorporated sun protection into our skin care regimens. But what exactly do these SPF’s mean to us? And how long are we protected after each application?

Well perhaps a good starting point would be to understand what kind of protection each SPF is supposed to provide. According to the Environmental Working Group, an SPF is a measurement of protection primarily based on UVB rays. So, if your skin would normally burn after 10 minutes of sun exposure, an SPF of 15 would theoretically allow you to stay in the sun 15 times longer, or up to 150 minutes. Of course, all this is based on the types of activities you do while in the sun, i.e. how much you sweat or spend time in the water. The intensity of sunlight is also a factor.

Also, to get the maximum results from sunblock, a thick layer should be applied 20 minutes before sun exposure. This will allow time for the lotion to bond to the skin.

So knowing that sunscreen does not last all day, how often do we reapply? “The general principle is to reapply every two to four hours,” says dermatologist James Spencer, MD of St. Petersburg, FL. He also warns that in addition to covering up exposed skin every couple of hours, you should try to cover up as much as possible with hats and long sleeves and avoid being under the sun between the hours of 10 AM and 2 PM.

Here are some other tips to be aware of in order to stay protected on those sunny days:

  • Use an SPF of 30 or higher, preferably water resistant. This will block 96 percent of the harmful incoming rays and the water resistant properties will keep it from washing away. Higher SPFs will provide only a minimal boost of protection but for exceptionally fair skinned people, that slight amount may prove beneficial as it adds up over a lifetime.
  • To get a good idea of how much sunblock one should use, Jennifer Stein of New York University Langone Medical Center says, “To cover your whole body, you would need to fill a shot glass.”  She also warns to apply protection to the tips of the ear and use a lip balm with a high SPF.
  • One should also be aware that overcast conditions do not mean you are safe from the sun’s rays. A cloudy sky may only block as little as 20% of inbound radiation. In fact, geographic features like snow, sand and water can reflect as much as 70% of sunlight and increase UV exposure.
  • Finally, be aware of sunscreens that are old, or have been left in your car over the winter. Although sunscreens have a shelf life of about 3 years, they are unstable at high temperatures. Ideally, sunscreens should be kept at a temperature of no higher than 77 degrees whereas the inside of your car can easily reach 120 when parked at a beach parking lot. When you’re on the beach, you may want to toss that sunscreen in your cooler to keep it from breaking down. It will also feel great on your skin when you reapply!
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