Particulate matter, or PM, refers to fine particles in the air that are detrimental to your health.  The Air District bans wood burning during Winter Spare the Air Alerts when PM pollution is expected to be unhealthy in the Bay Area.

PM: Particulate Matter


Particulate Matter, also known as soot or PM, consists of microscopically small solid particles or liquid droplets that can either be emitted directly into the air, or formed from secondary reactions involving gaseous pollutants that combine in the atmosphere.


Wintertime PM

On cold, stagnant winter evenings, surface-based radiation inversions form quickly in the Bay Area and PM levels rise rapidly. 

When PM concentrations are forecast to be unhealthy, the Air District issues a Winter Spare the Air Alert.  When a Winter Spare the Air Alert is in effect, it is illegal  for Bay Area residents to burn wood, pellets, or other solid fuels in woodstoves, fireplaces, or other wood-burning devices.

In the wintertime, wood smoke from the 1.4 million woodstoves and fireplaces in the Bay Area contributes about one-third of the overall PM pollution.  Cars and other motor vehicles also contribute significant amounts.


Types of PM

PM is usually measured in two size ranges:  PM10 and PM2.5. 

PM10 refers to particles with diameters that are less than or equal to 10 microns in size (a micron, or micrometer, is one-millionth of a meter), or about 1/7 the diameter of a human hair. 

PM2.5, also called "fine particulates," consists of particles with diameters that are less than or equal to 2.5 microns in size.  PM2.5 is a more serious health concern than PM10, since smaller particles can travel more deeply into our lungs and cause more harmful effects.


Diesel PM

Diesel particulate matter is listed as a toxic air contaminant by the state of California.

Diesel PM is released to the environment in exhaust from diesel engines, such as trucks, trains, buses, boats, generators, agricultural pumps, and other sources.

The Air District is concerned about the impacts of diesel exhaust in local Bay Area communities, and has implemented a number of programs to address this problem, including the Community Air Risk Evaluation (CARE) program.



During northern California’s wildfire season, the Air District monitors general air quality in the Bay Area and will issue a health advisory if wildfire smoke appears to be causing elevated levels of particulate pollution in the region.

Residents should take precautions if they are experiencing localized effects of wildfire smoke. For information, see our wildfire safety tips and CDC’s wildfire resources page.

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