Sun and UV

What UV does

CDC says too much UV from the sun or from tanning beds causes most skin cancers.

UV harms skin cells. First comes a sunburn. Then, over years, the skin changes texture, ages early, and can grow spots that turn out to be cancer. Those cancers start in one of three kinds of skin cell: squamous cells, basal cells, or the cells that make pigment.

What CDC recommends

Here is CDC's list, for any time the UV index is 3 or higher:

• Stay in the shade
• Wear clothing that covers your arms and legs
• Wear a hat with a wide brim, to shade your face, head, ears, and neck
• Wear sunglasses that wrap around and block both UVA and UVB rays
• Use a broad-spectrum sunscreen with SPF 15 or higher

Look at the order. Sunscreen comes last on CDC's own list. Shade, clothes, a hat, and sunglasses all come first.

CDC adds one note. If clothing and a wide hat are not practical, put plenty of sunscreen on all bare skin. That includes the ears and the back of the neck, which people miss.

Sunscreen, and what NCI will not claim

In the United States, FDA treats sunscreen as a drug, not a cosmetic. So products have to meet standards for safety and for how well they work.

FDA's practical advice:

• Choose broad spectrum, SPF 15 or higher
• SPF measures sunburn protection. It tracks how much sun you get, not how many hours you are out
• No sunscreen is waterproof. FDA says there is no such thing
• Water-resistant products are tested for either 40 or 80 minutes of swimming or sweating
• Put more on at least every two hours, and sooner if you sweat or swim

Now the part that gets left out.

NCI says sunscreen can prevent sunburn. It can also prevent the rough patches that sometimes turn into one kind of skin cancer. But NCI also says this: it has not been proven that using sunscreen to prevent sunburn protects against melanoma. And NCI says it is not known whether staying out of the sun, using sunscreen, or covering up lowers the risk of the other skin cancers.

That is awkward, and it is what the source says.

The sensible reading is not to stop using sunscreen. It is to stop treating sunscreen as permission to stay out longer. Use shade and clothes as well.

Tanning beds

CDC says UV from indoor tanning can cause skin cancers, including melanoma, the deadliest kind. CDC also says people who start indoor tanning as teens or young adults have a higher risk of melanoma. Both IARC and the US Department of Health and Human Services classify UV as a cause of cancer in humans.

CDC cites a study from 2016. Women under 30 who used indoor tanning were about six times more likely to have a melanoma diagnosis than women who did not. For ages 30 to 39 it was about four times.

A 2014 estimate put more than 400,000 US skin cancer cases a year as possibly linked to indoor tanning. Both studies are older now. And that 400,000 is a model, not a head count.

FDA treats tanning beds as medical devices. It requires this label on them: "Attention: This sunlamp product should not be used on persons under the age of 18 years." FDA links these devices to over 3,000 emergency room visits a year, based on data from 2003 to 2012. It has also proposed banning their use by anyone under 18.

Who is at higher risk

CDC says anyone can get skin cancer. Risk is higher with:

• Lighter natural skin color
• Skin that burns, freckles, or reddens easily
• Blue or green eyes
• Blond or red hair
• Many moles, or moles of certain types
• A history of sunburns, or of tanning
• Skin cancer in you or your family
• Older age

NCI adds the point that matters for anyone reading that list and counting themselves out. People of all skin colors can get skin cancer.