Skip to main content

Air Pollutants Defined


Ozone is a secondary pollutant that is formed by the reaction of other primary pollutants (precursors), such as Volatile Organic Compounds (VOCs) and Nitrogen Oxides (NOx), to the presence of sunlight.
Sources: VOCs are emitted from motor vehicles, chemical plants, refineries, factories and other industrial sources. NOx is emitted from motor vehicles, power plants and other sources of combustion.
Potential health impacts: Exposure to ozone can make people more susceptible to respiratory infection, result in lung inflammation and aggravate pre-existing respiratory diseases such as asthma. Other effects include decrease in lung function, chest pain and coughing.

Unit of measurement: Parts per billion (ppb)
Averaging interval: Highest eight-hour period within a 24-hour period (midnight to midnight) 
Reduction tips: Curtail daytime driving, refuel cars and use gasoline-powered equipment as late in the day as possible.


The term “particulate matter” (PM) includes extremely small solid particles and liquid droplets that circulate in the air. Commonly called dust, particles 10 micrometers in diameter or less (PM10) can be inhaled and accumulate in the respiratory system. Commonly called soot, particles 2.5 micrometers in diameter or less (PM2.5) are responsible for visibility degradations such as the “Valley Brown Cloud." High levels of PM occur when the air is especially stagnant or windy.
Sources: All types of combustion (motor vehicles, industry, wood burning, etc.) and some industrial processes cause fine PM (less than 2.5 micrometers in diameter). Crushing or grinding operations and dust from paved or unpaved roads cause coarse PM (between 2.5 and 10 micrometers in diameter).
Potential health impacts: PM can increase susceptibility to respiratory infections and can aggravate existing respiratory diseases, such as asthma and chronic bronchitis.
Units of measurement: Micrograms per cubic meter (µg/m3)
Averaging interval: 24 hours (midnight to midnight)
Reduction tips: Stabilize loose soils, slow down on dirt roads, carpool and use public transit. 


Carbon Monoxide is a colorless, odorless, poisonous gas formed when the carbon in fuels is not burned completely.
Sources: In cities, as much as 95 percent of all CO emissions emanate from automobile exhaust. Other sources include industrial processes, non-transportation-related fuel combustion and natural sources such as wildfires. Peak concentrations occur in colder winter months.
Potential health impacts: Reduces oxygen delivery to the body’s organs and tissues. The health threat is most serious for those who suffer from cardiovascular disease.
Unit of measurement: Parts per million (ppm)
Averaging interval: Highest eight-hour period within a 24-hour period (midnight to midnight)

Reduction tips: Keep motor vehicles tuned properly and minimize nighttime driving.