The complexity of getting a suntan through glass rests on how standard window glass filters the sun’s radiation. Standard glass effectively blocks the ultraviolet rays primarily responsible for sunburn, making a traditional tan difficult to acquire. However, this common glass allows the majority of the sun’s longer-wavelength ultraviolet radiation to pass through. This means a person is still exposed to the rays that cause tanning and photoaging.
The Science of UV Light and Skin Response
Ultraviolet radiation from the sun is categorized into three types, but UVA and UVB are most relevant to skin health. These rays differ in their wavelength and how deeply they penetrate the skin’s layers. UVA rays are the longer-wave radiation, spanning approximately 320 to 400 nanometers.
These longer wavelengths allow UVA to penetrate deep into the dermis, the skin’s middle layer, where they primarily cause tanning and photoaging. This deep penetration damages collagen and elastin fibers, leading to the formation of wrinkles, fine lines, and skin laxity. Chronic exposure to UVA rays is responsible for a significant percentage of visible skin aging.
UVB rays have shorter wavelengths, typically falling between 280 and 320 nanometers, affecting the outermost layer of the skin, the epidermis. This radiation triggers the classic signs of sunburn and is the primary driver of DNA damage and Vitamin D synthesis. Because UVB has a higher energy level, it is the more potent cause of acute skin damage and is strongly associated with the risk of skin cancer.
Standard Glass: Which UV Rays Pass Through
The composition of common glass, often called soda-lime silica glass, explains why sunburn rarely occurs through a closed window. This type of glass is an excellent filter for the shorter, more energetic UVB rays. Window glass absorbs nearly all UVB radiation, preventing the skin’s acute burning response.
However, the longer-wavelength UVA rays behave differently when encountering the same glass structure. Standard window glass is largely transparent to the majority of UVA radiation, allowing a significant amount to pass through unimpeded. This difference in filtration is directly related to the rays’ wavelengths.
The glass structure halts the shorter, high-energy UVB photons, but the longer-wave UVA photons pass through the molecular matrix more easily. Consequently, sitting by a window for extended periods still leads to cumulative UVA exposure. This exposure promotes tanning and accelerates the process of photoaging.
Practical Exposure: Vehicle and Building Windows
The degree of UV protection depends heavily on the specific type of glass used. The tempered glass typically used for the side and rear windows of vehicles, and in most buildings, allows substantial UVA transmission. Studies show these side windows may block only about 65% of total UV radiation, leaving occupants vulnerable to the remaining UVA rays.
This explains why many long-haul drivers exhibit noticeable photoaging, such as sunspots and deeper wrinkles, predominantly on the side of their face and arm nearest the window. Chronic, low-level exposure to UVA through the side glass accumulates over years, resulting in asymmetrical skin damage.
Vehicle windshields offer a much higher level of protection because they use laminated glass. This design features a layer of plastic, typically polyvinyl butyral, sandwiched between two sheets of glass. This plastic interlayer acts as a highly effective UV blocker, filtering out nearly 97% to 99% of both UVA and UVB rays. To mitigate the risk from side windows, UV-blocking films can be applied to existing glass, or sunscreen can be used on exposed skin during long periods of exposure.