Controlling particulate matter (PM), the primary ingredient in thick, black, diesel exhaust smoke, remains an important health and aesthetic concern. Due to the small size of particulate matter, it is easily inhaled into the lungs where it may damage tissue. These particles are also known to carry harmful chemicals into the lungs where they can exacerbate conditions such as asthma and emphysema. Some of the chemicals present in diesel exhaust, such as benzo[a]pyrene, have also been determined to be carcinogenic. Soiling and discoloration of buildings and any other surface exposed to deposits of soot is another concern. Please see the Diesel Emission Health Effects page to learn more about health effects.

Since 1984, most Marylanders have had their cars tested regularly for excessive emissions; before July 2000, diesel trucks and buses operating in and driving through Maryland were not subject to any emissions testing. All vehicles are accountable for helping to improve Maryland's air quality.

For information about research activities related to particulate matter see Particulate Matter Research Activities.

If you have additional questions, please contact Tim Shepherd at (410)537-3270 or by email at tim.shepherd@maryland.gov.​