Glass blocks much of UVB, but UVA protection can vary — particularly through vehicle side windows.

Being behind glass is not identical to being outdoors, but it is not always complete ultraviolet protection either. Standard glass is generally effective at blocking most UVB, the shorter wavelengths strongly associated with sunburn. Some UVA can pass through, and the amount depends on the type, thickness, lamination, coating and tint of the glass.

Vehicle glass is a useful example. In a 2016 cross-sectional study of 29 automobiles, front windscreens blocked an average of 96% of measured UVA. Driver-side windows blocked an average of 71%, with a wide range from 44% to 96%. Only four of the 29 side windows blocked more than 90%. The study was small and measured specific vehicles in Los Angeles, so its numbers should not be applied to every car. Its central finding remains useful: windscreens and side windows do not necessarily provide the same UVA protection.

Exposure also depends on behaviour. A person who drives for many hours, sits beside a bright window every day or has a photosensitive condition may have different needs from someone whose exposure is brief. Distance from the window, orientation, latitude, season and time of day all influence dose. Visible brightness is not a reliable measure of UVA.

For everyday protection, avoid fear-based rules. You do not need to treat every indoor space as direct midday sun. If you spend sustained time beside a sunny side window, practical options include broad-spectrum sunscreen on exposed skin, tested UPF sleeves or gloves, shade and professionally installed UV-filtering film where permitted. Window tint laws vary, and dark-looking tint is not automatically proof of UVA performance.

The same nuance applies to homes and offices. Modern architectural glazing differs widely. If a medical condition makes strict photoprotection necessary, product specifications or professional measurement are more reliable than assumptions based on the colour of the glass.

The accurate message is simple: glass changes the UV spectrum and usually reduces exposure substantially, but some UVA may remain. Long, repeated exposure near certain windows deserves more attention than a few incidental minutes.