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/

About the Author

Most Popular

  1. photo: An elderly resident of a village in Japan's Gunma Prefecture.

    In Japan’s Vanishing Rural Towns, Newcomers Are Wanted

    Facing declining birthrates and rural depopulation, hundreds of “marginal villages” could vanish in a few decades. But some small towns are fighting back.

  2. photo: Swedish journalist Per Grankvist, AKA the "Scandinavian Malcolm Gladwell."

    To Survive Climate Change, We’ll Need a Better Story

    Per Grankvist is “chief storyteller” for Sweden’s Viable Cities program. His job: communicate the realities of day-to-day living in a carbon-neutral world.

  3. Life

    Tailored Place-Based Policies Are Key to Reducing Regional Inequality

    Economist Timothy Bartik details the need for place-based policy to combat regional inequality and help distressed places—strategies outlined in his new book.

  4. a photo illustration of a map from "Treasure Island"

    The Treasure Map That Led Me to the Bottom of the Sea

    It wasn’t always easy being a black woman in my early days as an oceanographer. But a fictional pirate and a pioneering ocean explorer helped chart my course.

  5. Design

    How Advertising Conquered Urban Space

    In cities around the world, advertising is everywhere. We may try to shut it out, but it reflects who we are (or want to be) and connects us to the urban past.