# What are UV levels and how can you protect yourself?
Ultraviolet radiation comes from the sun and reaches Earth in two main forms: UVA and UVB rays. Both penetrate the skin and cause damage at the cellular level, though they work differently. UVA rays age skin and contribute to melanoma risk, while UVB rays burn the surface and drive squamous cell carcinoma development. Together, they account for nearly all skin cancers.
The UV Index, which runs from 0 to 11 or higher, tells you how strong the sun's rays are on any given day. The World Health Organization developed this scale to help people understand their exposure risk. A reading of 0 to 2 means low risk; 3 to 5 indicates moderate risk; 6 to 7 is high; 8 to 10 is very high; and 11 or above is extreme. The index changes with time of day, season, latitude, and altitude. Midday sun produces the strongest rays, particularly between 10 a.m. and 4 p.m.
Paradoxically, some UV exposure benefits health. Sunlight triggers vitamin D production in the skin, essential for bone health, immune function, and mood regulation. Most experts recommend 10 to 30 minutes of midday sun exposure several times weekly for adequate vitamin D synthesis, though this varies by skin tone and latitude.
Protecting yourself requires layered strategies. Sunscreen with SPF 30 or higher blocks 97 percent of UVB rays; SPF 50 blocks 98 percent. Apply generously about 15 minutes before going outside, and reapply every two hours or after swimming. Physical barriers work too. Wear lightweight, tightly woven clothing, wide-brimmed hats, and