The protective capability of glass against ultraviolet (UV) light depends entirely on its composition and the specific type of UV radiation involved. UV radiation is a component of sunlight that can damage skin, eyes, and interior furnishings. While some common glass types offer protection against certain UV wavelengths, they can be ineffective against others. Understanding the differences between UV bands and glass manufacturing processes provides clarity on which materials offer a robust barrier against solar radiation.
Understanding UV Light and Wavelengths
Ultraviolet radiation is electromagnetic energy categorized by its wavelength; shorter wavelengths carry more energy and potential for harm. The UV spectrum is divided into three primary bands: UVA, UVB, and UVC, distinguished by their nanometer (nm) ranges. Glass filtration is not uniform because different materials interact with these varying wavelengths differently.
The shortest band is UVC (100 to 280 nm), but this is almost entirely absorbed by the Earth’s atmosphere before reaching the surface. UVB radiation (280 to 315 nm) is the primary cause of sunburn and contributes to skin cancer risk, affecting superficial layers. The majority of solar UV radiation that reaches the ground is UVA, covering the longest wavelengths (315 to 400 nm).
UVA penetrates deeper into the skin than UVB and is associated with premature aging and long-term DNA damage. This distinction between the biologically active UVB rays and the deeper-penetrating UVA rays is the central factor in assessing glass performance.
The Filtration Properties of Standard Glass
The most common type of glass used in residential and commercial buildings is standard soda-lime glass, often referred to as float glass. This glass is made primarily from silica, soda, and lime. The raw materials used in its manufacture, particularly trace iron oxides, make it effective at filtering the shorter, more energetic UV wavelengths.
Standard window glass blocks virtually 100% of UVC radiation and the majority of UVB radiation, typically filtering more than 90% of rays below 300 nm. This means that sitting behind a common window largely protects you from the UV rays that cause immediate sunburn. This inherent blockage is a passive function of the glass chemistry.
The protection breaks down significantly when considering the longer-wavelength UVA band. Standard soda-lime glass allows a substantial amount of UVA radiation (315 to 400 nm) to pass through. Depending on the glass thickness and iron content, 50% to 75% of UVA can be transmitted. This transmission is why prolonged exposure through a normal window can still lead to skin aging and fading of interior fabrics.
Specialized Glass Designed for UV Protection
Achieving near-total UV protection requires glass engineered with specific additives or structures, moving beyond basic soda-lime formulations. Two main categories of specialized glass offer a higher degree of filtration, addressing UVA transmission and ensuring a robust barrier against the entire UV spectrum.
The first category is laminated glass, created by sandwiching a layer of polyvinyl butyral (PVB) plastic between two panes of glass. The PVB interlayer is a highly effective UV absorber that blocks over 99% of both UVA and UVB radiation. Laminated glass is required for all car windshields for safety reasons, which is why a car’s windshield offers superior UV protection compared to its standard tempered glass side windows.
The second method involves specialized coatings, such as Low-Emissivity (Low-E) coatings. Low-E glass features microscopically thin, transparent metallic layers deposited onto the glass surface. These coatings are primarily designed to reflect long-wave infrared heat, improving energy efficiency. They also significantly reduce UV transmission. Many modern Low-E products block up to 95% of UV rays, offering a practical solution for architectural windows where both energy performance and comprehensive sun protection are desired.