National Sunscreen Day is marked annually on May 27 to make people aware of the need to use sunscreens while stepping out in the sun or hitting the beach.

With the summers setting in, many love visiting traditional sunbathing spots, considered harmless in the past but now carry a yellow flag of warning against skin cancer and other ill effects of the sun's ultraviolet (UV) rays.

On this day, remind yourself to rub the sunscreen lotion, cream or spray on your body before enjoying the warmth and light of summer.

SPF and sunscreen ratings

A sunscreen's SPF or "sun protection factor" is the lotion/cream's ability to shield the sun's ultraviolet rays. Dermatologists highly recommend reapplying sunscreen every two to four hours, as it can rub off or get washed off in the water.

"SPF is not a consumer-friendly number," Florida dermatologist James Spencer told WebMD. "It is logical for someone to think that an SPF of 30 is twice as good as an SPF of 15 and so on. But that is not how it works."

It refers to how much of the sunlight it can block. A sunscreen with an SPF 15 blocks about 94% of the sun's dangerous rays. SPF 30 products block about 97% of rays and SPF 45 shields about 98% of rays.

Ultraviolet radiation

Ultraviolet or UV rays of the sun are invisible, but the damage they can cause to your skin and health is a big concern. UV radiation is just one of the forms of energy that reach the earth's surface. UV radiation is a combination of UVA, UVB and UVC rays. The rays are strongest during the spring and summer. Some 80% of the sun's UV rays can pass through clouds. Ozone depletion has resulted in an increase in the number of skin cancer cases from across the world.

Always put on sunscreen.
Always put on sunscreen. Armin Rimoldi from Pexels
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