Can we tan through a window?

Theskin is the largest organ of the human body; it is attributed with numerous functions, among which are thermal insulation and protection from external aggressions. Therefore, it partially protects us from the harmful effects of the sun's UV rays.

As a reminder, these are classified into three categories according to their wavelengths: UVA (400-315 nm), UVB (315-280 nm) and UVC (280-100 nm). UV-C rays are the most harmful, but fortunately, they are filtered by the ozone layer and do not reach the Earth's surface. However, UVA and UVB do come into contact with the epidermis. When exposed to the sun, to protect the skin, melanocytes then produce melanin (the pigment responsible for the brown colour) which migrates to the surface of the epidermis and colours it: this is known as tanning. If the exposure is too long and/or the UVB rays too intense, the tan gives way to a sunburn.

UVB rays prove to be particularly aggressive for the skin and are responsible for sunburns. UVA rays penetrate further than UVB rays and reach the dermis, the deep layer of the skin. They are, in turn, the cause of "photo-ageing".

According to a recent study conducted by American researchers, typical windows in homes, offices and cars block most UVB rays, which are responsible for tanning and sunburn. Therefore, tanning through a window remains possible but greatly limited. Similarly, getting sunburnt behind a window remains possible, although the risks are minimal.

Beware, while windows indeed have the ability to absorb a significant portion of UVB rays, they are less effective at blocking the passage of UVA rays. These rays penetrate into the dermis, accelerating skin ageing. A study perfectly illustrates this case. It reveals a photo of a 69-year-old milkman who spent 28 years behind his steering wheel. The left side of his face has aged rapidly due to the effects of the sun (photo-ageing), while his right profile appears much less marked.

Thus, if you spend a lot of time behind a window, do not forget to apply a broad-spectrum sun protection suitable for your skin type. Also, make sure to use a moisturising cream that meets your skin's needs to combat dehydration, wrinkle formation, and protect it from external aggressions.