UV exposure during winter drops towards the poles, but does that mean we should use less SPF?Jon Bower/AlamyThis article is part of a special issue investigating key questions about skincare. Find the full series here.Even if we live far from the equator, we are often told to apply sun protection factor (SPF) lotions year-round to protect our skin from premature ageing and wrinkles. But is this backed up by solid science?Just 13.5 per cent of US adults use SPF creams as part of their daily routine, according to one survey. That is clearly a problem in the summer, when the sun is brightest. While we may not show signs of burning, UV rays can penetrate the deeper layers of skin and damage it. Besides contributing to wrinkles, this can lead to skin cancer and people who wear sunscreen are at a much lower risk of this.Photoageing is a cumulative effect that adds up over the decadesHowever, these rays are much weaker in winter at more northerly or southerly latitudes, and the benefits of SPF must be balanced against the costs. Brian Diffey, an emeritus professor of photobiology at Newcastle University in the UK, points out that wintertime use of SPF may contribute to vitamin D deficiency, for example. SPFs can also have an environmental cost, with some studies showing that they can wash into the oceans, where their component chemicals can be absorbed by corals and other marine life, harming them.Diffey has used historical climate records to calculate how