Do you need sunscreen in winter?

Every autumn the same advice goes round again: wear sunscreen every day, all year, whatever the weather. It is repeated so often and so confidently that it has started to sound like medical fact. The truth is more useful, and it depends on where you are standing.

Whether you need sunscreen in winter comes down to how strong the sun’s ultraviolet light is where you are, how long you spend in it, and whether anything about your skin or your medication changes the usual rules. A January morning in Toronto, a January afternoon in Sydney and a January week on a ski slope in the Alps are three different answers.

The number that decides it: the UV index

The strength of ultraviolet light is measured on the UV index, an international scale developed with the World Health Organization and shown on most weather apps. It runs from 0 to 11 and above: 0 to 2 is low, 3 to 5 moderate, 6 to 7 high, 8 to 10 very high, and 11 or more extreme. The WHO’s advice is that sun protection is generally needed once the index reaches 3.

So the real question is not “is it winter?” but “what is the UV index today, here?”

What changes winter UV

Several things push the winter index up or down, and they matter more than the date on the calendar.

• Latitude. The further you are from the equator, the lower the sun sits in winter and the weaker its UV. In northern Europe, Canada and the northern United States, winter levels usually stay in the low band for weeks. Nearer the tropics, the sun can be strong all year.
• Altitude. UV rises as you climb, by roughly 10% for every 1,000 metres, according to the WHO. A mountain town can have a much higher index than the valley below it.
• Snow. Fresh snow reflects up to about 80% of UV back at you, so on a bright day in the mountains you are hit from above and below. A ski slope in February is a different proposition from a city street in February.
• Cloud and weather. Thick, dark cloud cuts UV a great deal. Thin or broken cloud cuts much less than it seems to, and can even scatter UV. A bright, hazy day can reach the moderate band when it does not feel sunny at all.
• Time outdoors. Someone who works outside all day, every day, is in a different position from someone who walks to the station and back.

What dermatologists at high latitudes say

In countries far from the equator, specialists are often clearer on this than the skincare industry. In the UK, for example, the British Association of Dermatologists says in its sunscreen fact sheet that year-round sunscreen use is unnecessary there for most people, because in the winter months the UV index does not rise high enough for it to be of benefit; the exception it names is people with certain medical conditions.

That is advice for one latitude, not for the world. The same logic, applied in Brisbane or Mumbai, gives the opposite answer, because the winter index there can sit at moderate or higher. The daily, all-year rule is sound where the sun is strong all year. Where it is weak for months, it is, for most people, a habit rather than a necessity.

When winter sunscreen does matter

• Snow and mountains, for the reasons above.
• Winter sun somewhere else. A week closer to the equator, or in the other hemisphere’s summer, can mean a high UV index in December. Check the forecast for where you are, not where you live.
• Some medical conditions and medicines. Certain conditions, and some medicines, make skin more sensitive to light. If a doctor, pharmacist or dermatologist has told you to protect your skin all year, that advice is about you, and it outranks any general rule.
• The edges of the season. A clear day in early spring can reach the moderate band sooner than people expect.
• Long hours outdoors, for work, sport or travel.

UVA, windows and the ageing argument

The strongest case for winter SPF at high latitudes is cosmetic rather than medical. Ultraviolet light has two parts that matter for skin: UVB, which does most of the burning, and UVA, which penetrates deeper and is linked with skin ageing. UVA is present all year, and it passes through cloud and ordinary window glass. It is weaker in winter too, but it does not disappear.

This is where I think the conversation goes wrong. It is perfectly reasonable to wear a moisturiser with SPF in January because you want to, because you are thinking about lines and pigmentation in twenty years, or because the ritual of a good makeup routine gives you pleasure. What is not reasonable is to be told that skipping it on a dark February morning in a northern city is a health risk. On the evidence and the specialists’ own advice, for most people there, it is not.

The vitamin D question

Winter brings a separate piece of advice that is often muddled up with sunscreen. At higher latitudes the winter sun is too weak for the skin to make much vitamin D, and several countries advise supplements through the darker months. In the UK, for example, the NHS advises everyone to consider a daily 10-microgram supplement in autumn and winter. What is right for you depends on where you live and on your health, so ask your doctor or check your own country’s guidance.

Sunscreen is not the reason for that winter shortfall; the weak sun is. So the vitamin D question stands whether or not you wear SPF, and it may be the more useful thing to act on if looking after your health is the aim.

A simple way to decide

Look at the UV index where you are today. At 3 or above, protect your skin with shade, clothing, a hat and sunglasses, and a broad-spectrum sunscreen on skin you cannot cover (many health bodies recommend SPF 30 or higher), reapplied through the day. Below 3, sunscreen is a skincare choice rather than a medical one, unless you are on snow, at altitude, somewhere nearer the equator, or have been told otherwise by someone who knows your skin.

That is less tidy than “every day, forever”. It is also closer to what the people who study UV and treat skin for a living actually say.

Sources

• World Health Organization. Radiation: the ultraviolet (UV) index.
• World Health Organization. Global solar UV index: a practical guide, 2002.
• British Association of Dermatologists. The sunscreen fact sheet (UK).
• NHS. Vitamin D (UK).