Ceiling tiles reflect UV germicidal irradiation towards occupants

The reflectances of ceiling tiles for UV germicidal irradiation have been measured by US scientists to estimate their contribution to the exposure of human occupants. Low-pressure mercury discharge lamps irradiating at a wavelength 254 nm are commonly used to irradiate rooms to kill of any airborne germs and there is renewed interest in the technology due to threats like bioterrorism and pandemic influenza.

UV irradiation at this wavelength can be harmful to humans and the US authorities have set an exposure limit of 6 mJ•cm²/day. This is a low limit but it takes into account the damage that can be caused by skin inflammation and photokeratitis of the eyes. The irradiation is normally directed towards the upper portions of the rooms, away from human contact, but it can be reflected from surfaces towards any occupants. Reflection from walls has been studied previously and now that from ceilings has also been assessed, as reported in Photochemistry and Photobiology.

Scientists used a double monochromator spectroradiometer fitted with an integrating sphere reflectance attachment to measure the reflectances of 37 types of ceiling tile at 200-400 nm. They found that reflectances ranged from 0.020 to 0.822, and more specifically from 0.035 to 0.459 at 254 nm, which is the principal emission wavelength.

These levels would not pose a risk to normal room occupants who come and go but they might be harmful to sedentary people like hospital patients who lie in bed all day facing the ceiling. Then the exposure might reach or even exceed the official threshold limit. In extreme circumstances, standing people might also exceed the official limit but only if they repeatedly looked at the ceiling during the day. The figures can be added to models for calculating the irradiances of rooms of different dimensions.