Example: the smokestack of a coal plant ejects coal smoke into the air. Water vapor droplets in the clouds can pick up these smoke particles and drop them later in the form of acid rain.
How is particulate air pollution measured?
Due to the different sizes, shapes and chemical compositions of these microscopic particulate air pollution agents, the task of measuring the potential damage that they can do is often an arduous undertaking. Most agencies involved in pollution research and prevention classify these particles by size: fine particles are less than 2.5 microns (10^-6 m) across and inhalable coarse particles are between 2.5 and 10 microns across. To put these measurements into perspective, a human hair is usually between 70 and 100 microns thick; a red blood cell is about 7 microns across.
What are some other sources of particulate air pollution?
According to a study carried out by the US Environmental Protection Agency (EPA), one of the leading sources of fine particulate air pollution is also the oldest: fires. During 2002, EPA estimates put the quantity of fine particulate air pollution originating from fires at well over one million tons. Since man first learned to create a spark and build fires for light, warmth, and comfort, he has also sent untold tons of untreated, unfiltered smoke into the air. While the concern over particulate air pollution may be a recent occurrence, the sources and the issue itself are both as old as civilization, whether the source is a small campfire or a raging forest blaze.
According to the same study, road dust generated over eight hundred thousand tons of fine particulate air pollution, followed by electricity generation at five hundred thousand. Surprisingly, fossil fuel use (coal, oil, kerosene, gasoline) and automobile usage combined for less than four hundred thousand tons, less than a third of the total for fires.
Of course, air pollution is not limited to outdoor sources. Indoor air pollution can also be a major source of particulate air pollution. Dust, sheet rock particles, cigarette smoke and dirty ventilation systems can create fine particles that can contribute to an increase in indoor particulate air pollution.