Researchers say it spurts ozone-causing chemicals into the environment.

The filthy, black grime crusting city buildings and statues isn’t just an eyesore. It’s also an unlikely contributor to lung-searing smog, according to Canadian scientists.

Pollution from cars, factories, and other sources can waft around until settling on surfaces as grime. Locked into this solid-looking state, it’s easy to assume it doesn’t do much after that until somebody power-washes it into oblivion. But according to James Donaldson at the University of Toronto, when sunlight hits grime it kicks off a reaction that releases nitrogen-oxide compounds. These chemicals float back into the air and combine with others to make ozone, the principle ingredient in smog.

“We don’t know yet to what extent this is occurring, but it may be quite a significant, and unaccounted for, contributor to air pollution in cities,” Donaldson says in a press release. Here’s more:

[S]cientists had long suspected that nitrogen oxides become inactive when they are trapped in grime and settle on a surface.

However, Donaldson and his colleagues at the University of Toronto have collected data that are inconsistent with this theory. In previous work, they discovered that nitrate anions disappeared from grime at faster rates than could be explained by wash-off due to rainfall. And, in a subsequent laboratory comparison, they found that nitrate disappeared from grime 10,000 times faster than from a water-based solution when both were exposed to artificial sunlight. ...

“If our suspicions are correct, it means that the current understanding of urban air pollution is missing a big chunk of information,” Donaldson says. “In our work, we are showing that there is the potential for significant recycling of nitrogen oxides into the atmosphere from grime, which could give rise to greater ozone creation.”

Donaldson and his compatriots also found large differences in the amount of grime in two cities they tested, Toronto and Leipzig. The latter showed 20 times the amount of grime, suggesting locals might be breathing in larger amounts of ozone, which can cause chest pain and wheezing and aggravate bronchitis and emphysema.

Anybody interested in learning more can tune into an American Chemical Society YouTube talk today at 1:30 p.m. ET.

Top image: John Lindsay-Smith/

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