Smoke is made up of microscopic particles of soot called particulate matter. Many particles are 2.5 microns in size - about the thickness of a human red blood cell. Too small to be seen individually with the unaided eye, dense concentrations appear as smoke. Various gases and toxins are also in the smoke.

Because particulate matter is so tiny, it bypasses respiratory defenses to penetrate deep inside lung tissue. It can remain imbedded for years to cause coughing, wheezing, chest pain and shortness of breath--symptoms that might not occur until several days after exposure.

Leaf and trash smoke can irritate lung tissue in healthy adults. But it can be more harmful to small children, the elderly, and people with lung and heart diseases. Smoke is a known trigger for asthma attacks. For many people, smoke is a nuisance as well.

Some people believe smoke just "disappears" when released. That is not the whole story. Particulate matter can remain airborne for a month. These scattered particles contribute to haze and reduced visibility in Iowa and across the nation, even in remote wilderness areas and national parks. Eventually, particles are deposited onto land and water, hundreds and even thousands of miles from where they originated.