Particulate matter is the term used for a mixture of solid particles and liquid droplets found in the air. PM2.5 refers to particulate matter that is 2.5 micrometers or smaller in size. The sources of PM2.5 include fuel combustion from automobiles, power plants, wood burning, industrial processes, and diesel powered vehicles such as buses and trucks.
Fine particles are of concern because they are risk to both human health and the environment.
Because these particles are so small they are able to penetrate to the deepest parts of the lungs. Scientific studies have suggested links between fine particulate matter and numerous health problems including asthma, bronchitis, acute and chronic respiratory symptoms such as shortness of breath and painful breathing, and premature deaths. Most of these premature deaths are the elderly who's immune systems are weaker due to age or other health problems such as cardiopulmonary diseases. Children are more susceptible to the health risks of PM2.5 because their immune and respiratory systems are still developing. The average adult breaths 13,000 liters of air per day and children breath up to 50 percent more air per pound of body weight than adults.