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New research shows that when sunlight hits the grime accumulated on buildings, toxic pollutants are released into the atmosphere.

The discovery was presented on Monday, during the final day of an American Chemical Society (ACS) conference in Boston. Lead researcher James Donaldson explained that "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." Great.

Donaldson explained to the BBC that lab analysis shows the grime pollution is made up of the toxic nitrogen dioxide (NO2) and nitrous acid (HONO), which can increase ozone production. Ozone, in turn, can exacerbate health problems and lead to more smog in cities.

To test the theory, Donaldson and his team put grime collectors on sunny and shady buildings in Leipzig, Germany, and Toronto. After six weeks, the researchers found that grime collectors in sunny spots in Leipzig had 10% more nitrates than their shaded counterparts, suggesting that the sunlight prompted NO2 and HONO pollution from the grime. The Toronto experiment has not yet been completed.

The ACS notes in a release that Donaldson describes urban grime as a combination of pollutants—like the chemicals that come out of cars and factories—and other air pollutants. Grime has been a constant source of strife for big cities. Back in 1992, the New York Times detailed the lengthy and expensive process of trying to rid the city of grime:

For a small building or older elevator apartment house, a competent cleaning generally runs from $5,000 to $50,000. For larger or more complicated projects the cost can be far greater. The cleaning of the Flatiron Building last year cost about $80,000, that of Trinity Church this year ran to more than $750,000 and the cost to clean the Custom House was $600,000, although that also includes repointing.

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No wonder this city is so dirty.

Danielle Wiener-Bronner is a news reporter.