You may not be able to see them, but they’re there: tiny particles of dust, dirt, soot, smoke, and liquid droplets hanging in the air. These particles — the ones under 2.5 micrometers in diameter — are too small for our respiratory systems to filter out, and end up getting trapped in our lungs where they may adversely affect our health. The very smallest pass through the lungs into the blood stream and can damage the heart.

Particles enter the air from a variety of sources and may be either directly emitted or may form under a chemical process much like the way ground level ozone forms. Many enter the air directly from power plants, factories, automobiles, construction vehicles, unpaved roads, wood burning, and agriculture sites. Others come from a reaction between gases from burning fuels, sunlight, and water vapor.

Unlike ground level ozone, particles are not a seasonal pollutant; high levels can occur any time of the year. Unhealthy levels of particle pollution in the air can cause or trigger significant health problems. These range from coughing and difficult or painful breathing to the possibility of an emergency room visit or even premature death. Exposure to particles can decrease lung function, weaken the heart, and possibly bring on a heart attack. The environment also suffers from particle pollution. Particles are the major source of haze, and can harm the environment by changing the nutrient and chemical balance in soil and water.

A good air day and a bad air day from the Washington, DC air quality web cam