As the Northern Hemisphere passes the summer solstice and ultraviolet radiation reaches its annual peak, weather forecasts increasingly feature a single number alongside the temperature: the UV index. Health authorities say it is one of the simplest tools for deciding when to protect your skin.
What the UV index measures
The World Health Organization describes the UV index as a measure of the level of UV radiation reaching the ground, with higher values signalling a greater potential for harm to skin and eyes. The scale starts at 1 and is open-ended at the top, grouped into bands: low (1–2), moderate (3–5), high (6–7), very high (8–10) and extreme (11 and above), according to WHO guidance and the US Environmental Protection Agency.
The key threshold is 3. The WHO recommends taking protective measures when UV levels reach 3 or higher, noting that below an index of 2 the risk of short- and long-term UV damage is limited. The EPA likewise marks index 3 as the point where protection is needed, advising extra caution at index 8 and above.
UVA versus UVB, and how the sun damages skin
Not all ultraviolet light behaves the same way. The American Academy of Dermatology explains that UVB rays are primarily responsible for sunburn and DNA damage, while UVA rays penetrate deeper into the skin, breaking down collagen and contributing to wrinkles, discoloration and skin cancer.
When UV strikes skin, it can damage the DNA inside skin cells. Sunburn is the visible sign of that injury, but harm accumulates over a lifetime even without obvious burning, and repeated damage raises the risk of skin cancer — the reason authorities frame sun protection as a long-term habit rather than a beach-day afterthought.
What SPF numbers actually mean
SPF — sun protection factor — measures protection against UVB only. The UK's NHS advises using a sunscreen of at least factor 30. The American Academy of Dermatology puts the figures in context: SPF 30 blocks about 97 percent of the sun's UVB rays, and while higher numbers block marginally more, no sunscreen can block 100 percent.
Because SPF says nothing about UVA, both the NHS and the academy recommend "broad-spectrum" products, which protect against UVA and UVB together. The NHS also advises looking for at least four-star UVA protection or the circular UVA symbol on labels.
Official sun-safety guidance
The core advice is consistent across authorities. The NHS recommends spending time in the shade during the middle of the day, wearing a wide-brimmed hat, long-sleeved tops and sunglasses. On sunscreen, it advises applying generously — roughly six to eight teaspoons for full-body coverage — and reapplying every two hours, and immediately after swimming or sweating.
Two myths worth correcting
"You can't burn on a cloudy day." You can. The NHS states that you can burn even when it is cloudy, because clouds scatter and reduce UV but do not block it entirely, so the index can still be high under a grey sky.
"Darker skin doesn't need protection." People with naturally brown or black skin are less likely to develop skin cancer, the NHS notes, but people of all skin tones can get skin cancer and should avoid burning. The academy agrees that anyone can get skin cancer regardless of skin tone.
The through-line from every authority is the same: when the UV index hits 3, it is time to seek shade, cover up and apply broad-spectrum sunscreen — and to remember that the number, not the temperature, is the real measure of risk. This article is general information, not individualized medical advice; consult a health professional for personal concerns.
