A study in The British Journal of Dermatology reports that sunscreen application plays a crucial role in the percentage of UV rays blocked.
Studies over the years have shown that sunscreen with an SPF, or sun protection factor, of 30 blocks about 90% of UV rays. A rating of 15 means 93% of UV rays are blocked, and anything higher than 30 remains in the 97% or 98% range.
Researchers found that applying less than 2 ounces of sunscreen over the entire body in one session can make the sunscreen far less effective in protecting against UV rays. It is important that dermatologists speak to their patients about the importance of using sunscreens with avobenzone, zinc oxide or titanium dioxide, all of which block UVA.
Past studies indicate that most people only apply 10% of the recommended amount of sunscreen.
Source: The New York Times; August 7