Still, there’s a lot we don’t know about the mechanisms behind the effects of air pollution. It’s a slurry of different types of particles, of different sizes and from different sources. Some of the largest are about a tenth of the width of a human hair—big, on a microscopic scale, but still small enough to travel into the lungs. Others, known as ultrafine particles, are on the nanoscale. Pollution is made of nitrogen dioxide and ozone, and it might also have microscopic pieces of metals like zinc, tin, or even lead. The composition can change neighborhood to neighborhood, block to block, and hour to hour.