Mexico City’s horrendous air pollution problem is being exacerbated by dog doo. More than 2 million dogs live in Mexico City’s inner Federal District and deposit at least 353 tons of waste a day, the vast majority of which is not scooped up by pet owners. The waste dries into dust that combines with particles from factories, erosion from dried lake beds, and hydrocarbons from car exhaust to form noxious particulate pollution. The dog-doo dust and other particulates settle on food sold at the city’s 13,000 open-air food stands. Ozone is still the worst air pollution threat in the city, but particulates are gaining.