Daylight has been used for centuries as the main source of light in interiors and has always been an implicit part of architecture ever since buildings exist. Daylight is an irreplaceable environmental factor and is a prerequisite for a healthy stay in buildings. Ensuring daylight access to the interior is a challenge when designing buildings. The aim is to provide enough daylight for the visual work that indoor building users do. The deposition of air pollutants on the glazing can significantly affect the transmission of daylight into buildings located in urban areas. This study shows the impact of air pollution on glazing permeability in three cities in the Czech Republic. The throughput was measured on a total of 9 windows for half a year. Air purity, exposure to rain, the slope of glazing and use of rooms are factors influencing the final permeability of the glazing. Where glazing is exposed to rain, dirt on the outside of the windows tends to accumulate. The final reduction in throughput is usually 4-8%, with some exceptions.