A new study has found that the dirty coating that most people grime often seen coating buildings and streets is extremely hazardous to public health — and it’s the sun that’s unleashing these toxins on the environment.

The study, which was presented at the American Chemical Society’s meeting in Boston, found that urban grime that is exposed to sunlight releases nitrogen oxides, which results in smog as well as ground-level ozone, according to a Discovery News report.

Nitrogen oxides are formed when nitrogen mixes with oxygen, forming either nitric oxide, nitrogen oxide, or nitrous oxide. This is a common chemical process that can happen naturally due to combustion from forest fires or biological decay, but the burning of fossil fuels also unleashes a large amount of it into the atmosphere.

And when it comes from humans in the form of vehicle and power plant emissions, it is part of a reaction cycle that results in the creation of ground-level ozone, not to be confused with the ozone layer in the atmosphere.

As the study found, some of this pollution departs the atmosphere when other chemical reactions produce nitrates, which are compounds that have a nitrogen and three oxygen molecules, as well as another element, which ends up being a part of urban grime.

Scientists thought these chemicals were merely stored within the grime, but the worrying new research by the University of Toronto found that the sunlight actually unleashes these chemicals on the unsuspecting public, potentially contributing to breathing problems and other health effects.

D. James Donaldson, the lead researcher on the project, found that trays of glass set up on rooftops had lower levels of nitrates if they were exposed to less sunlight. And using a photoreactor to simulate sunlight released nitrogen oxides when the grime-covered glass was exposed.

More research will be needed to determine exactly how much nitrogen oxides are being released from the grime into the atmosphere.

For the experiment, a tower above the city of Leipzig in Germany was stacked with two large shelves containing beads of window glass, and both sets had the same air flow, allowing them to become coated in grime. However, only one of them was placed in direct sunlight, with the other remaining in the shade.

The results? Those that sat in the sun had 10 percent less nitrate that the ones that were in the shade, indicating that there was a loss of nitrogen that was similar to what was seen in the lab, according to a BBC report.

Although 10 percent may not seem like much, it indicates that there is a constant release of nitrogen, and constant deposits to replace it.

To find out exactly where the nitrogen goes after being exposed to sunlight, the team went back to Toronto to examine it in the lab, and found that two gases — nitrogen dioxide and nitrous acid — is what comes out of the urban grime. Nitrous dioxide is toxic, and nitrous acid can result in the production of ozone.