UVA Is the Real Winter Sun Problem (the Damage You Never Feel)

People skip sunscreen in winter because they think about burning. Burning is UVB, and UVB really does fade when the sun sits low in the sky. But burning was never the whole threat. There is a second ray you never feel, and it barely notices the season changed. In winter, UVA is the real problem.

UVA and UVB are not the same ray

Sunlight carries two kinds of ultraviolet, and they behave very differently. The easiest way to keep them straight is what they do: UVB burns, UVA ages.

• UVB hits the surface of the skin and causes sunburn. It is strongest in summer, at midday, and at low latitudes, and it is largely blocked by glass. It rises and falls with the season.
• UVA reaches deeper into the skin. It does not burn, so you never feel it, but it drives photoaging, wrinkles, loss of firmness, and pigment. It passes through glass and clouds, and it stays remarkably steady all year.

Why UVA barely drops in winter

UVB depends heavily on the angle of the sun, which is why it falls off in winter and the farther you get from the equator. UVA is far less picky. Its intensity stays much more constant across seasons and even across the hours of the day. So when winter cuts UVB down, the share of what reaches your skin tilts toward UVA. The ray that ages you does not get the memo that it is cold out.

The damage you never feel

This is the trap. Because UVA does not burn, there is no signal that anything is happening. You can rack up UVA exposure all winter, on the walk to the car, through the office window, on a gray afternoon, with zero sensation and real cumulative effect. No burn does not mean no damage. It just means the damage is the slow kind: collagen breakdown, fine lines, and the patchy pigment behind conditions like melasma, which UVA and visible light both feed.

Glass and clouds are not protecting you

Standard window glass blocks most UVB but lets UVA through. A sunny desk by a window in January, or a long winter commute, is steady UVA exposure. Cloud cover does not close the gap either; UVA passes through it. The overcast, freezing days that feel the safest are exactly the ones where UVB is low and UVA is doing its quiet work unopposed.

Does your sunscreen even block UVA?

Here is the catch that ties it together. The SPF number only rates UVB. A product can be SPF 50 and still be weak on UVA. The word to look for is broad spectrum, and even then, not all broad-spectrum formulas cover UVA equally. We broke down how to judge that in what broad spectrum means. The short version: zinc oxide is the single filter that covers the widest UVA range on its own.

Where Swellies fits

Swellies gets all of its protection from 21 percent non-nano zinc oxide, the only active, which covers the full UVA and UVB range in one ingredient. That means the winter gap, the UVA that other formulas quietly leave open, is covered. And because it wears like a lightweight daily gel, it is easy to keep up in December, which is the whole point. A sunscreen only protects you on the days you put it on. For the everyday case, see should you wear sunscreen every day.

Frequently asked questions

Is UVA present in the winter?

Yes. UVA stays much more constant across the seasons than UVB. While UVB and burn risk drop in winter, UVA intensity barely changes, so a larger share of the UV reaching your skin in winter is UVA, the ray that drives aging and pigment.

What is the difference between UVA and UVB?

UVB causes sunburn, is strongest in summer and at midday, and is mostly blocked by glass. UVA reaches deeper, drives skin aging and pigment, does not cause a burn, and passes through glass and clouds. A simple way to remember: UVA for aging, UVB for burning.

Does UVA come through windows and clouds?

Yes to both. Standard window glass blocks most UVB but lets UVA through, and UVA passes through cloud cover. That is why you can accumulate UVA exposure indoors by a window or on an overcast winter day.

Does SPF protect against UVA?

Not by itself. SPF only measures UVB protection. For UVA you need a broad spectrum formula, and the strength of that UVA coverage varies. Zinc oxide covers the widest UVA range of any single filter.

Full UVA and UVB from one active, easy to wear all winter. Shop Swellies SPF 46 →