Should sunscreen also be used during the winter period?

The Earth experiences seasonal changes due to two phenomena: its movement around the Sun and its tilt relative to its axis, an imaginary line that runs from the North Pole to the South Pole. When the Earth is further from the sun, this alters the angle and amount of sunlight that reaches the ground. A lower and longer angle of sunlight means it has to filter through a larger portion of the atmosphere, which weakens it, resulting in cooler temperatures and a reduction in UV rays that can damage the skin. This tilt, combined with the Earth's rotation, means that the sun's path in the sky is shorter, which reduces daylight. Less and weaker sunlight results in colder weather, hence the existence of winter.

Other meteorological and behavioural changes also influence UV exposure during the winter months.

• Clouds: Colder weather leads to the formation of a greater number of clouds, which scatter UV rays and alter the amount that reaches the ground. Dense clouds can obstruct up to 50% of UV rays, while thick clouds on sunny days can increase them. All of this varies quite significantly depending on the thickness, height, and type of clouds.

• Snow: Snow can reflect UV rays. Fresh snow in summer (it happens) can increase UV levels by about 30%, but it's likely much less in winter, due to the angle of the sun.

• Altitude: The closer you get to the sun, the stronger the sun will be. UV levels increase by approximately 2 to 5% for every 1,000 metres increase in altitude, likely due to the thinning of the atmosphere.

• Time spent outdoors: People tend to spend less time outdoors in cold weather. In fact, it is estimated that winter only accounts for 5% of your annual UV exposure.

• Winter Clothing: Not only do you spend less time outdoors, but the time you do spend outside, you're wearing thicker fabrics and more layers of clothing than in the summer, which significantly limits the potential for UV exposure.

One argument in favour of using sunscreen in winter is that the level of UVA reaching the Earth's surface remains constant throughout the year, whether it be summer or winter, but this appears not to be the case. Apparently, the levels of UVA and UVB exhibit temporal variations over the course of the year, these evidently decrease during the winter months (figure 1). Indeed, the elongated angle and reduced duration of sunlight in non-equatorial regions result in a decrease in UV and their weakening in winter, this is also true for UVA.

Even though exposure to UV and visible light is significantly reduced in non-equatorial regions during winter, the low levels still present can in fact exacerbate photosensitive conditions, such as melanoma, acne andhyperpigmentation. Given that visible light and UV can worsen these issues, it may be wise to use a sunscreen with protection against visible light all year round to prevent pigmentation from darkening.

After taking into account your place of residence, your personal levels of UV exposure, your skin type, and whether you are photosensitive... you can make an informed decision as to whether or not you need a sunscreen in winter. Some will need it, others will not. When in doubt, it may be wise to apply it, just in case, and also to maintain this habit. People inaccurately associate two weather phenomena with UV, namely temperature and cloud cover.

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