At home, in the car, or behind your sunglasses... glass appears to protect from the sun's rays. This common belief is partly true as glass does block the passage of UVB rays responsible for tanning. However, this is not the case for UVA rays. Let's explain.
Misconception: I am protected from the sun behind a window.
Published on February 14, 2024, by Stéphanie, PhD, Doctorate in Life and Health Sciences — 4 min of reading
Themes:
Can ultraviolet rays penetrate glass?
There are three types of UV rays: UVA, UVB, and UVC. However, UVC rays, which are the most dangerous, are blocked by the ozone layer and therefore do not reach the Earth. As for UVB rays, they are the cause of sunburn and are thus particularly aggressive to the skin. On the other hand, UVA rays are more responsible for photoaging as they can reach the deeper layers of the skin. However, UVA rays can also result in a tanned complexion after a certain amount of exposure. Yet, while windows indeed have the capacity to absorb UVB rays, they do not absorb UVA rays.
A study perfectly illustrates this case. It reveals the portrait of a 69-year-old milkman who spent 28 years behind his wheel. The left side of his face has aged at an accelerated rate due to the effects of the sun (photo-ageing). Indeed, this part of his face was regularly exposed to UV rays through the window of his truck. While his right profile appears much less marked, smoother and healthier, his left face is more damaged, wrinkled, the skin is thicker and sagging, and the grooves are accentuated. These marks were caused by UVA rays, which can penetrate deeply into the skin beyond the epidermis where they are capable of destroying elastin and collagen fibres, and altering DNA. This is why it is important to apply a sunscreen every day and to renew it regularly (every two hours) for optimal skin protection against the harmful rays of the sun, even behind a window.
Ultraviolet Rays and Sunglasses.
In this context, it is important to understand the sensitivity of eyeglass lenses to ultraviolet rays. Do they protect against tanning? The answer is yes. These lenses perform an essential function in terms of protection against the sun's rays. They are particularly useful in blindingly bright places such as on the beach, at sea or in the mountains. They also protect against ultraviolet rays, reducing the risks of corneal ageing and the onset of certain retinal pathologies. Therefore, choosing a dark tint is not enough when it comes to sunglasses. It is essential to also take into account the percentage of light filtered. Indeed, dark accessories can shield from glare, but remain permeable to UV rays. In all cases, the use of sun care is still recommended.
Sources
FUSARO R. M. & al. Broad-spectrum photoprotection: the roles of tinted auto windows, sunscreens and browning agents in the diagnosis and treatment of photosensitivity. Dermatology (1992).
PARISI A. V. & al. Ultraviolet radiation penetrating vehicle glass: a field based comparative study. Physics in Medicine and Biology (1999).
TURNBULL D. & al. Comparison of the solar spectral ultraviolet irradiance in motor vehicles with windows in an open and closed position. International Journal of Biometeorology (2002).
KORN M. & al. UV exposure in cars. Photodermatology, Photoimmunology & Photomedicine (2003).
EDLIC E. C. & al. Use of UV-protective windows and window films to aid in the prevention of skin cancer. Journal of Long-Term Effects of Medical Implants (2004).
LIM H. K. & al. Current status of photoprotection by window glass, automobile glass, window films, and sunglasses. Photodermatology, Photoimmunology & Photomedicine (2013).
