Electrostatic filtering is not recommended, because electrostatic precipitators off-gas the pollutant ozone into the air as they filter particulate matter. All wavelengths of light are damaging to library and archival materials. Ultraviolet light, which is light at wavelengths less than 400 nanometers, can not be seen by the naked eye and causes the most damage. Sunlight has the highest proportion of UV radiation, followed by fluorescent and incandescent lighting. The damage caused by light is dependent upon the intensity, the type of light, and the duration of exposure.
Light damage is cumulative.
Exposure levels are monitored by calculating total exposure, which is the product of the intensity of the source multiplied by the length of exposure. All light is damaging, so lights should be turned off as much as possible in the stacks. In a situation where study carrels or desks are located in the stacks, carrels should be illuminated by individual lamps. The stack ranges should have timer switches or, at least, switches with signs that remind people to turn off the lights when they are not needed. Direct sunlight and skylights should be avoided since they emit so much UV radiation.