No matter what time of day, always wear sunscreen

Dear Doctor: Because I have fair skin and live in an area with a lot of sun, I try to do my outdoor activities early in the day (before 9 a.m.) or late in the afternoon (after 4 p.m.).

Do I still need to wear sunscreen?

Dear Reader: In a word, yes.

If your goal is to protect your skin from sun damage, which includes sunburn, wrinkles and a range of cancers, our advice is to wear sunscreen during the daylight hours.

In fact, research now shows that even a light tan is a sign of sun damage.

This dawn-to-dusk timing for sunscreen application may seem counterintuitive; after all, sunlight looks and feels markedly weaker during the early morning and late afternoon than at midday.

While it’s true that the sun’s rays are strongest between 10 a.m. and 4 p.m., the fact is that from the moment it appears above the horizon to the instant it vanishes from sight, our closest star is sending a full dose of skin-damaging ultraviolet radiation our way.

How can that be?

Sun damage is caused by two types of radiation — ultraviolet A and B, better known as UVA and UVB. These are part of a broad spectrum of light, which includes the light we can see. But because ultraviolet rays are shorter than rays of visible light, they can’t be seen with the naked eye. However, what you can’t see definitely can hurt you.

The so-called “sunburn ray” is UVB.

It has a slightly shorter wavelength than UVA, and is not as concentrated in the early morning and late afternoon.

The highest amounts of UVB radiation reach us between 10 a.m. and 4 p.m. in the months of April through October.

And while UVB accounts for only 5 to 10 percent of all UV radiation that reaches us, it plays a key part in the onset of skin cancer.

By contrast, UVA maintains the same intensity all day, which makes dawn-to-dusk sunscreen use important. UVA is responsible for signs of aging, like wrinkles and dark spots.

It also damages certain cells in the basal layer of the epidermis, which is where most skin cancers form.

In the past, UVA was absolved of a cancer connection.

Now, however, researchers believe that it plays a role in cellular changes that lead to cancer.

The other thing to know is that UVA can penetrate clouds and glass.

When you’re riding in a car, sitting near a window or spending time outdoors in poor weather, you’re still in the path of UVA rays.

Reflective surfaces, such as water, snow, ice and glass, refract up to 80 percent of the UVA and UVB rays that hit them.

That means you’re getting close to a double dose of the sun’s harmful rays. And don’t forget that the sun’s rays become more potent at higher altitudes.

Our final word of advice is to be sure to use sunscreen rated for both UVA and UVB radiation.

It will be clearly marked on the label. Use it generously and reapply according to product instructions. Your skin will thank you.