Glass is a ubiquitous material, commonly found in home and car windows. Standard glass blocks most, but not all, forms of ultraviolet (UV) radiation. UV radiation is an invisible form of energy emitted by the sun, categorized into three types based on wavelength. Understanding how these UV types are filtered by common materials is important for protecting skin and interior furnishings.
How Standard Window Glass Filters UV Light
Standard window glass is primarily composed of soda-lime silicate. Its structure inherently filters ultraviolet light unevenly because the composition includes metal oxides and impurities. These components act as natural absorbers for shorter, higher-energy wavelengths of UV radiation, making the glass highly effective against two of the three main UV types.
The highest energy UV light, UVC, is entirely blocked by the Earth’s atmosphere. The second type, UVB, is the primary cause of sunburn and most skin cancers. Common window glass almost entirely absorbs UVB, typically preventing more than 90% from passing through. This is why sitting by a window usually prevents sunburn.
The glass’s effectiveness changes dramatically with UVA, the longest wavelength UV light. UVA rays are lower in energy but penetrate deeper into the skin, causing tanning and photoaging. Standard soda-lime glass allows a significant portion of this radiation to transmit through. Studies show that up to 70% or more of UVA can pass unhindered. Therefore, standard windows offer excellent protection against UVB but only partial protection against the deeper-penetrating UVA.
Specialized Materials for Complete UV Blocking
When complete UV resistance is required, specialized glass compositions and treatments must be employed. Laminated glass is one of the most effective solutions. It is created by sandwiching a polymer interlayer, such as polyvinyl butyral (PVB), between two panes of glass. This interlayer acts as a powerful absorber, effectively blocking over 99% of both UVA and UVB radiation. This material is used for car windshields and certain architectural applications.
Another specialized material is glass treated with low-emissivity (Low-E) coatings. These are thin, metallic layers applied to the glass surface. While primarily designed to reflect infrared heat and improve energy efficiency, many are also engineered to reflect UV light. Some Low-E coatings offer excellent UV reduction, but they may not provide the near-total blockage of laminated glass, often allowing a small percentage of UVA to pass through.
A notable exception is quartz glass, which is made from high-purity silicon dioxide and lacks the impurities found in common glass. Due to this purity, quartz glass is highly transparent and allows UVC and UVB to pass through efficiently. This property makes it unsuitable for general window use. However, it is indispensable for scientific and medical applications, such as UV disinfection lamps, where maximum UV transmission is necessary.
Real-World Effects of UV Transmission Through Glass
The UVA radiation that penetrates standard glass has measurable long-term consequences on human health. Chronic, low-level exposure to UVA rays through windows contributes significantly to photoaging, resulting in wrinkles and a breakdown of collagen. While protected from immediate sunburn, this cumulative exposure also increases the risk of developing skin cancer over time.
This phenomenon is particularly evident in high-exposure environments like driving. Side windows are often made of tempered glass that offers minimal UVA filtration. Studies show that people who spend extensive time driving, such as truck drivers, exhibit significantly more sun damage and a higher incidence of skin cancer on the exposed side of their body. While a car’s laminated windshield blocks most UVA, the side windows typically allow a substantial amount to enter the cabin.
The penetrating UVA rays are also responsible for the degradation of materials inside homes and offices. When textiles, wood furniture, artwork, and flooring are repeatedly exposed to sunlight, the UVA radiation breaks down chemical bonds. This process leads to irreversible fading, discoloration, and structural weakening of the materials. UV filtration is a major consideration for preserving valuable interior items. Applying a UV-blocking film or using specialized glass is often the only way to halt this material breakdown.