Ultraviolet (UV) radiation is a form of solar energy that travels in waves and is invisible to the human eye. Many people assume that being inside a home or office completely shields them from the sun’s rays, but this is a misconception concerning the full spectrum of UV light. The answer to whether UV passes through windows depends entirely on the type of UV ray being discussed and the specific type of glass in question.
Understanding Ultraviolet Light
Solar UV radiation is categorized into three main types based on wavelength, which dictates their energy level and ability to penetrate the atmosphere and skin. UVC is the highest-energy and most damaging type, but it is entirely absorbed by the Earth’s ozone layer and does not reach the surface naturally. UVB rays have a medium wavelength, are responsible for causing sunburn, and trigger the production of Vitamin D in the skin. UVA rays possess the longest wavelength, penetrating deeper into the skin’s layers, and account for approximately 95% of the UV radiation that reaches the Earth’s surface.
How Standard Residential Glass Filters UV
Standard residential and architectural glass, typically made from untreated float glass, provides a highly effective barrier against UVB rays. These common windows block about 97% to 98% of the UVB radiation, which is why a person rarely gets a sunburn while sitting inside. This blocking effect is beneficial for avoiding painful burns but prevents the skin from synthesizing Vitamin D.
The situation is significantly different for UVA rays, which are not effectively stopped by standard glass. Untreated glass allows a substantial amount of UVA radiation to pass through, with studies indicating that between 50% and 75% of these rays can be transmitted indoors.
Specialized Windows and Enhanced UV Protection
The level of protection changes dramatically with specialized glass and treatments. Laminated glass, commonly used for car windshields and in certain residential applications, offers superior UV defense due to its construction. This glass consists of a clear plastic interlayer, usually polyvinyl butyral (PVB), sandwiched between two panes of glass. The PVB interlayer is highly effective at absorbing UV light, blocking approximately 99% of both UVA and UVB radiation.
Conversely, the standard side and rear windows in most cars are made from tempered glass, which behaves more like residential glass, blocking UVB but allowing a significant amount of UVA to pass through. Other options, such as low-emissivity (low-e) coatings or aftermarket window films, are designed to reflect or absorb UV rays, reducing the transmission of UVA and UVB to a much greater degree than clear glass alone.
Health Consequences of Indoor UV Exposure
The cumulative exposure to UVA radiation that passes through standard windows poses a long-term health concern, primarily by contributing to photoaging. This process involves the deep penetration of UVA rays into the skin’s dermis, leading to the breakdown of collagen and elastin fibers. The result is the gradual development of wrinkles, sagging, and leathery texture on skin frequently exposed to the sun through a window.
Since standard windows block the UVB rays necessary for biological conversion, sitting indoors by a sunny window will not lead to Vitamin D production. This continuous, low-level UVA exposure highlights the need for sun-protective measures, such as applying broad-spectrum sunscreen, even when spending extended periods indoors near glass.