Whether ultraviolet B (UVB) radiation can penetrate glass is a common inquiry related to the physics of light and the chemical composition of transparent materials. Light from the sun, including ultraviolet (UV) radiation, is categorized into distinct types, and standard window glass interacts differently with each category. Understanding this requires examining the UV spectrum and the materials used in common architectural glass. The composition of glass determines which energy wavelengths are transmitted or screened out, which has significant implications for health and the indoor environment.
Defining the Ultraviolet Spectrum
The ultraviolet spectrum is a form of electromagnetic radiation with wavelengths shorter than visible light, spanning from 100 to 400 nanometers (nm). This spectrum is subdivided into three classifications based on wavelength, which correlates to the energy level and biological effects of the radiation. The shortest and highest-energy UV is UVC (100 to 280 nm), which is highly damaging but is completely filtered by the Earth’s atmosphere and ozone layer.
The medium-wavelength range is UVB (280 and 315 nm), known as the cause of sunburn and skin reddening. Although only a small percentage of solar UV is UVB, it initiates the biological process of Vitamin D synthesis in the skin. The longest wavelength is UVA (315 to 400 nm), making up approximately 95% of the UV radiation that reaches the Earth’s surface. UVA penetrates more deeply into the skin layers and is associated with long-term aging effects.
Standard Window Glass Blocks UVB
Standard architectural glass, known as float glass, is composed primarily of silica and contains various impurities, notably iron oxides. These impurities are responsible for the glass’s slight green or blue tint and act as a physical barrier to higher-energy, shorter-wavelength radiation, specifically UVB and UVC. The chemical structure of the silicates absorbs the energy from the UVB photons, preventing transmission through the pane.
This mechanism means that ordinary window glass blocks nearly 100% of incoming UVB light. Consequently, a person sitting behind a closed window will not experience sunburn, which is an acute reaction caused by UVB exposure. Furthermore, the synthesis of Vitamin D, which is triggered by UVB radiation, cannot occur indoors behind standard glass because the necessary wavelengths are entirely screened out.
Why UVA Transmission Matters Indoors
While standard glass effectively blocks UVB, it is largely transparent to the longer-wavelength UVA radiation. Approximately 75% to 85% of UVA light can pass through typical clear glass windows, presenting a chronic, low-level exposure risk indoors. Because UVA has a longer wavelength, it penetrates deeper into the dermis layer of the skin, reaching fibroblasts and collagen fibers. This deep penetration causes photoaging, which manifests as wrinkles, sagging, and a loss of skin elasticity.
The damage caused by transmitted UVA is distinct from the acute damage of sunburn, representing a slower, cumulative form of solar injury. Individuals who spend significant time near windows, such as drivers or office workers, can still show signs of sun damage on the exposed side of their face or body. The constant, lower-intensity exposure to UVA contributes to the breakdown of collagen and the formation of reactive oxygen species within the skin. Being indoors behind glass prevents the acute effects of UVB but does not provide complete protection against the chronic, aging effects of UVA.
Exceptions to the Rule: Specialized Glass
Not all glass products exhibit the same UV-blocking properties as standard float glass; specialized materials are designed to either enhance or eliminate UV transmission. Laminated glass, commonly used for car windshields, offers superior UV protection compared to tempered side windows. This enhanced protection is due to a polyvinyl butyral (PVB) plastic interlayer sandwiched between two sheets of glass, which acts as a highly effective UV filter, often blocking up to 99% of both UVA and UVB radiation.
Conversely, certain high-purity glass types are engineered to allow UV light to pass through for scientific or industrial applications. Quartz glass, made of nearly pure silicon dioxide, contains almost none of the iron impurities found in standard glass. This makes it highly transparent to UV radiation, including UVB and UVC, and it is used in laboratory equipment, UV lamps, and optics where short-wavelength light transmission is required.