The grime that accumulates on old buildings and city surfaces gives off an alarming amount of air pollution, scientists say. According to a report from the BBC, urban grime has been known to absorb pollutants from the air, but a new study shows just how much nitrogen-based pollutants are making their way back out into the atmosphere.

Researchers examined the contents of urban grime in both sunlight and in the shade during rooftop experiments in Germany. They discovered that when exposed to sunlight, urban grime emits nitrogen in two different forms; the toxic gas nitrogen oxide, and nitrous acid, one of the primary components of smog.

The study’s findings were presented at a conference of the American Chemical Society in Boston, and could help explain where the infamous smog that hangs over London originates.

“Rather than being a permanent sink for nitrogen oxide gases, grime exposed to sunlight can re-release some of these gases into the urban atmosphere,” says James Donaldson, the study’s head author and chemistry professor at the University of Toronto.

The sun had a significant effect on the concentrations of nitrogen gases released. Grime samples on a roof exposed to the sun had 10 percent less nitrogen than samples covered by the shade, suggesting that the nitrogen gases were escaping at an alarming rate.

The study may explain why photochemical smog forms in some cities. As nitrous acid seeps into the air from the urban grime, it contributes to the heavy haze that shrouds over London, Los Angeles, and Shanghai on certain days. Scientists will continue to tweak their measurements and try to figure out exactly where these gases are coming from.