Do you need to wear sunscreen at all?

In a word: yes. No matter where you live or what the season is, the best thing you can do for your skin is wear sunscreen every day. Unfortunately, this is something very few of us actually do.

Here's Dr Howard Murad, dermatologist and founder of Murad Skincare, with a timely reminder: "No matter where you are, UV rays are present all year round, even on cold, cloudy days. UVA rays are the most damaging and account for 95 percent of UV that reaches earth. They are the same strength year-round and can penetrate your skin through windows, even on cloudy days - so if you're sitting by a window indoors, you should still apply your SPF. They contribute to premature ageing, collagen degradation and even skin cancer. When you go outside, it takes just 10 minutes for UV rays to kickstart the breakdown of collagen in your skin."

What's the difference between UVA and UVB?

We all know UV rays can damage our skin and lead to cancer, but not everyone's so clear on the difference between UVA and UVB. Dr Mieran Sethi, specialist dermatology registrar with the NHS, puts it simply: "UVA causes ageing and UVB causes burning." This is the crucial part: "SPF sunscreen filters UVB, so it is important to select a sunscreen that has both SPF and UVA filters."

"UVA passes through windows, so damage to skin can occur if you're sitting indoors next to a window, or when sitting in a car. UVB does not pass through windows," Sethi explains. Murad agrees with this analysis, saying: "UVA rays are the most damaging. They penetrate deeply into the dermis layer of the skin, and can even penetrate through clouds and windows."

When buying sunscreen, make sure you're getting something which has both UVA and B protection. "Sunscreen advertised as SPF 50 only filters UVB radiation," says Sethi. "For a sunscreen to filter UVA radiation, it must also have the UVA filter symbol (UVA in a circle or UVA in a circle with star rating)."

How often should you apply?

Sethi says you should reapply "if you go outside to exercise, or if you are repeatedly touching your face. In general, reapplication is more important if you are outside, as it can be removed by effects of ultraviolet radiation exposure and atmosphere".

One of the biggest mistakes Sethi sees people making with sunscreen is not putting enough on, usually due to consistency of the product and the undesired cosmetic effect of a white residue. To make sure you're wearing enough, she adds: "I usually recommend people apply sunscreen twice on all exposed sites."

Murad has a visual way of working out how much product to use. "For each sunscreen application, apply one ounce of sunscreen (equal to a shot glass) to the entire body and face, and continuously reapply when out in the sun for long periods of time," he says.