Defining Bad Air

Ground-level Ozone

Ground-level ozone is formed when nitrogen oxides and volatile organic compounds react in the presence of heat and sunlight.

• Nitrogen Oxides (NOx) come primarily from combustion of fuels in automobiles, coal-fired power plants, industrial boilers and gas-powered engines such as lawnmowers and leaf blowers.
• Volatile Organic Compounds (VOCs) are vapors emitted by paint and print shops, gas stations, dry cleaners, lawn chemicals and from combustion engines, such as those in cars and trucks, boats and diesel locomotives. Trees also emit VOCs, especially pine trees.

NOx and VOCs combine to form ground-level ozone on hot sunny days, which is why most exceedances are seen in the summer. Ozone exceedances are most likely to occur between May 1 and September 30. 

While ground-level ozone is a health and environmental problem, ozone in the stratosphere (six to 30 miles above the earth) is beneficial. It shields the Earth from the sun’s harmful ultraviolet radiation.

Particle Pollution

Particle pollution, or particulate matter (PM2.5), comes from some of the same sources as NOx and VOCs, including power plants and factories, motor vehicles (especially older diesel vehicles) and others. Particle pollution is also created by wood burning, construction activity and agriculture. Unlike ground-level ozone, particle pollution occurs year-round.