The UV rays that reach your skin
Sunlight includes ultraviolet radiation, which is split into three bands by wavelength: UVA, UVB and UVC. UVC is absorbed by the atmosphere and does not normally reach the ground. The two that matter for everyday sun safety are UVA and UVB.
Both can harm skin, but they do it in somewhat different ways, which is why sunscreens are expected to address both.
UVB: the burning rays
UVB has a shorter wavelength and is the main cause of sunburn. It mostly affects the outer layers of skin. Its intensity changes a lot with time of day, season and latitude: it is typically stronger around midday and in warmer months, and weaker in winter or early morning. UVB is also largely blocked by ordinary window glass.
UVB damages the DNA in skin cells directly, and this is one reason repeated burns are linked to a higher risk of skin cancer. The role of that cell damage is discussed more in the piece on skin DNA.
UVA: the deeper, steadier rays
UVA has a longer wavelength and penetrates deeper into the skin. It does not usually cause the obvious redness of a burn, so you can be exposed without feeling anything. It is commonly associated with tanning, long-term changes such as wrinkles and uneven tone, and it also contributes to skin cancer risk.
Some practical points that are often noted about UVA:
- It is present at fairly similar levels through much of the day and year, compared with UVB.
- A good share of it passes through clouds.
- Much of it passes through window glass, so long hours near a sunny window or in a car may add up.
For more on how cumulative exposure shows up over time, see the explanation of skin photoaging.
Why a burn-only measure is not enough
The SPF number on a bottle is mainly a measure of protection against UVB. A high SPF therefore says a lot about burn protection but not, by itself, about UVA. In the past a person could use a high-SPF product, avoid burning, and stay outdoors much longer than they otherwise would while still getting plenty of UVA. That gap is why a second claim exists.
What “broad spectrum” means
Broad spectrum is a label claim indicating that a sunscreen has been shown to protect across both UVB and a meaningful part of UVA. In the U.S., products must pass a specified test to carry the wording. Rules differ in other countries and use different symbols or ratings, such as a UVA seal or a star system, so labels may look different when you travel.
Some things to keep in mind:
- Broad spectrum does not mean equal protection against UVA and UVB. In general, UVA protection is a smaller fraction of what the SPF suggests.
- Under U.S. rules, a broad spectrum product with a higher SPF is generally expected to offer greater UVA protection as well, but this is a general pattern, not a precise promise.
- A product without the wording may protect only against burning and is not meant for long-term protection by itself.
Understanding the SPF side of the label is easier once you read about SPF numbers, which is a separate topic from the UVA question.
How to use this when shopping
A simple approach is to look for three things on the front of the pack: broad spectrum wording (or your country’s equivalent), an SPF that suits your plans for the day, and a texture you will wear. The different filter families can also vary in how they cover UVA, which is part of the story in the comparison of mineral sunscreen and chemical products. You do not need to memorize filter names to choose well.
Habits that follow from this
Because UVA is fairly constant and passes through glass, many people choose to apply sunscreen on days they mostly stay indoors near windows, or drive a lot. How much that matters depends on the person, the place and the time spent. Applying enough product and reapplying matter just as much as the label. The idea of making this a regular habit is covered under daily sunscreen.
If you notice a mole or spot that changes, itches, bleeds or looks different from others, have it looked at by a qualified health professional. This article is general information and not a substitute for medical advice.