UV light exposure is a common concern for people spending time indoors, leading many to question the protective capability of their windows. Most common window glass does filter UV light, but this filtration is highly selective and incomplete, particularly against the type of UV that contributes to skin aging. Understanding the answer requires differentiating between the various types of UV radiation and the unique properties of different glass materials. Glass composition reveals why some windows offer near-total protection, while others leave occupants vulnerable to long-term sun damage.
Understanding UV Radiation Types
Ultraviolet radiation is electromagnetic energy from the sun, categorized into three types based on wavelength. The shortest wavelength, UVC, is the most energetic, but it is completely absorbed by the Earth’s ozone layer and atmosphere before reaching the surface.
Medium-wavelength UVB radiation is responsible for sunburn and plays a role in skin cancer development. While most UVB is absorbed by the atmosphere, a small amount still reaches the ground. Long-wavelength UVA radiation makes up the vast majority of the UV light that penetrates the atmosphere, reaching the Earth’s surface largely unimpeded. UVA penetrates deeper into the skin than UVB, contributing to premature aging, wrinkling, and cancer risk.
How Standard Glass Filters UV Light
Common residential and commercial windows, typically made from untreated soda-lime glass, exhibit a distinct filtering profile. This glass effectively blocks nearly all UVC and UVB radiation, with over 90% of UV light below 300 nanometers being stopped. The glass composition inherently absorbs these shorter, higher-energy wavelengths. This explains why sitting near a window will not result in a sunburn, as the UVB rays are largely kept out.
The problem lies with the longer-wavelength UVA radiation, which is not absorbed as effectively. Standard window glass is transparent enough to allow a significant portion of UVA to pass through. A large percentage of UVA light can still penetrate common, single-pane windows. While the immediate risk of sunburn is eliminated, chronic exposure to the aging and deeply penetrating UVA rays remains a concern for people who spend long periods near windows.
Specialized Windows and Enhanced Protection
Enhanced UV protection requires specialized glass technologies that actively block UVA rays. One effective solution is laminated glass, commonly used for vehicle windshields. Laminated glass is made by sandwiching a layer of polyvinyl butyral (PVB) plastic between two sheets of glass. This PVB interlayer is highly effective at absorbing UV light, resulting in laminated glass blocking up to 99% of all UV radiation, including UVA.
Conversely, tempered glass, often used for side and rear car windows and some residential applications, does not offer this same level of protection. Tempered glass provides increased strength and breaks safely into small pieces, but it does not inherently block a significant amount of UVA unless it has an additional coating. Sitting next to a car’s side window offers less protection than sitting behind the laminated windshield.
Many modern residential windows feature a low-emissivity (Low-E) coating, which is a microscopic, metallic oxide layer applied to the glass surface. While primarily designed to reflect infrared heat and improve energy efficiency, Low-E coatings also provide substantial UV protection. Low-E windows can reduce UV transmission by a high percentage, often up to 95% or more. This minimizes both solar heat gain and the damaging effects of UV light on skin and interior furnishings.