What is particle pollution?

This is quoted from http://www.stateoftheair.org/2008/health-risks/health-risks-pm.html

What Is Particle Pollution?
Particle pollution refers to a mix of very tiny solid and liquid particles that are in the air we breathe. But nothing about particle pollution is simple. First of all, the particles themselves are different sizes. Some are one-tenth the diameter of a strand of hair. Many are even tinier; some are so small they can only be seen with an electron microscope. Because of their size, you can’t see the individual particles. You can only see the haze that forms when millions of particles blur the spread of sunlight. You may not be able to tell when you’re breathing particle pollution. Yet it is so dangerous it can shorten your life.

The differences in size make a big difference in how they affect us. Our natural defenses help us to cough or sneeze larger particles out of our bodies. But those defenses don’t keep out smaller particles, those that are smaller than 10 microns (or micrometers) in diameter, or about one-seventh the diameter of a single human hair. These particles get trapped in the lungs, while the smallest are so minutethat they can pass through the lungs into the blood stream, just like the essential oxygen molecules we need to survive.

Researchers categorize particles according to size, grouping them as coarse, fine and ultrafine. Coarse particles fall between 2.5 microns and 10 microns in diameter and are called PM10-2.5. Fine particles are 2.5 microns in diameter or smaller and are called PM2.5. Ultrafine particles are smaller than 0.1 micron in diameter¹¹ and are small enough to pass through the lung tissue into the blood stream, circulating like the oxygen molecules themselves. No matter what the size, particles can be harmful to your health.

Because particles are formed in so many different ways, they also can be composed of many different compounds. Although we often think of particles as solids, not all are. Some are completely liquid; some are solids suspended in liquids. As the U.S. Environmental Protection Agency puts it, particles are really “a mixture of mixtures.”¹² The mixtures differ between the eastern and western United States. For example, the eastern states have more sulfate particles than the west, largely due to the high levels of sulfur dioxide emitted by large, coal-fired power plants. By contrast, in Southern California, nitrate particles from motor vehicle exhaust form a larger proportion of the unhealthful mix.¹³