Coating the entire bodies of mice that are susceptible to developing melanoma skin cancers with potent sunscreens (SPF50) does not protect them from developing this lethal skin cancer (Nature, July 2014;511, 478–482). Shining ultra violet light on their sunscreen-coated skin directly damaged the DNA in their skin pigment cells, to cause melanoma skin cancer. In the same way, sunlight would damage the genes that protect a person from developing melanomas. Strong sunscreens can reduce the damage by 30 percent, but they do not prevent skin cancers.
What You Can Learn From This Study Sunscreens offer only partial protection from the sun. It is dangerous to expose large areas of your sun screen-coated skin to sunlight. If you want to really protect your skin from sun damage, get under a roof when the sun is out in full force. Clothes are next best, and sunscreens are a poor third.
• Since cumulative exposure to sunlight over a lifetime causes skin cancers, always cover the areas that have had the most sun exposure: face, tops of the ears, hands and arms. Avoid getting sunscreens in your eyes.
• Some sunscreens contain stronger UVA filters (avobenzone, mexoryl, titanium dioxide or zinc) that are less likely to be absorbed into the skin. Check the list of ingredients.
• Reapply sunscreens frequently.
• Windows do not block all ultraviolet rays. Glass blocks most of the short UVB rays, but only 30 percent of the UVA rays, so windows prevent sunburns but they do not prevent skin aging or wrinkling. Since only UVB can cause your skin to make vitamin D, you get no vitamin D from the sun’s rays that pass through glass.
• Dark red, blue or black materials block more of the sun's rays than lighter colors.
• Most shirts allow some UVA rays to reach your skin. Wearing a shirt markedly decreases your chances of being burnt, but if you plan to spend long periods in the sun, look for special sunblock materials or shirts with very dense weaves.
• To meet your daily vitamin D requirements from sunlight, expose your legs or other areas of your body that have received little cumulative sun exposure over your lifetime. Take care to avoid sunburn.
Safety of Sunscreens We do not know how safe sunscreens are because they have never been tested systematically. Oxybenzone in sunscreens has been shown to be absorbed into the bloodstream in humans, and to disrupt hormones in animals. Zinc oxide and titanium dioxide appear to be safe, except that manufacturers often convert them to nanoparticles that can be inhaled. The zinc oxide and titanium dioxide-based sunscreens that do not contain nanoparticles are generally thicker and whiter.
Review and critique of more than 1700 sunscreens conducted by the Environmental Working Group More on sunscreens
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