Air Quality

Last Updated: 04/28/2019

The Air District maintains one of the most comprehensive air quality monitoring networks in the country, consisting of over 30 stations distributed among the nine Bay Area counties. This network measures concentrations of pollutants for which health-based ambient air quality standards have been set by the U.S. Environmental Protection Agency and the California Air Resources Board. The network also measures concentrations of various pollutants designated as Toxic Air Contaminants by the state of California.

How clean or polluted the air is and the level of health concern is in the Air Quality Index (AQI). The AQI categorizes air quality based on air measures collected from Department of Environmental Quality (DEQ) air monitors. The AQI provides real-time monitoring and alerts in response to changing air quality levels. The AQI accounts for five different pollutants, including: 1) ground-level ozone; 2) particle pollution (also known as particulate matter); 3) carbon monoxide; 4) sulfur dioxide; and 5) nitrogen dioxide. Of these, ground-level ozone and particulate matter are the most common and most concerning pollutants for outdoor physical activity. The AQI value for the day, is based on the 24-hour average concentration as established by the U.S. EPA, so hourly readings are only estimates. The one-hour average is meant to protect against acute exposure and the 24-hour average is meant to protect against more long-term exposure at lower levels. Both will impact your health.

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 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


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.

In addition to the AQI, you can use your own observations to determine the air conditions in your area. To do a visual inspection:

  • Go outside.
  • Face away from the sun.
  • Determine the limit of your visible range by looking at objects at known distances (miles).
  • Visible range is the point at which even high contrast objects totally disappear.

Low-cost air quality sensors are a relatively new technology that measure specific air pollutants, typically particulate matter and some gaseous pollutants, and cost much less than traditional air quality monitors. Since this emerging technology is still under development, little information exists on the quality of data that these sensors produce. In the United States, air quality has traditionally been measured according to standards established by the United States Environmental Protection Agency (EPA) using regulatory monitors that are designated as federal reference method (FRM) or federal equivalent method (FEM). These monitors cost tens of thousands of dollars and require significant infrastructure and trained personnel to operate, whereas, low-cost sensors may only cost a few hundred dollars and can be simple to operate. Low-cost air quality sensors cannot replace traditional regulatory monitors, but they do create new opportunities to increase and expand access to air quality monitoring and can play a part in tracking air quality.

PurpleAir Sensor FAQ

The PA-II Dual Laser Air Sensor uses laser beams to detect the particles going past by their reflectivity, like dust shimmering in a sunbeam. The PM 2.5 and PM 10 micro-gram weights are calculated from the counts. The displays points correlates with the Federal Environmental Protection Agency (EPA) Air Quality Index (AQI) scale. The AQI allows comparison for different pollutants with an easy to visualize color scheme. The values are averaged every 20 seconds versus the 24-hour average concentration as established by the U.S. EPA.


Most studies comparing Purple Air sensors to regulatory monitors have found Purple Air sensors report higher levels than regulatory monitors. Recognizing this discrepancy, Purple Air has added the ability to apply "Conversion" factors to their website map. AQandU applies to University of Utah and the Salt Lake City region. LRAPA applies to Lane Regional Air Protection Agency located in Springfield Oregon. There currently isn't a conversation formula established for this region.

  • Wildfire smoke can make asthma symptoms worse. It can trigger asthma attacks. Symptoms of asthma include coughing, shortness of breath, wheezing and chest tightness. Even students without known asthma can have symptoms when exposure to unhealthy levels of wildfire smoke pollutions.
  • Students with asthma should follow their Asthma Action Plan. This will help them decide if they need to take special precautions while engaging in outdoor activities. Athletes with asthma should have rescue inhalers readily available. Use should be as directed by their health care provider.
  • Anyone experiencing symptoms should contact a health care provider.
  • Call 911 in an emergency.

For more information visit the American Lung Associations webpage:

  • The American Lung Association encourages everyone to get involved in the fight for cleaner, healthier air. Here are some simple, effective tips for protecting you and you family from the dangers of air pollutions:

  • Check daily air pollution forecasts in your area. The color-coded forecasts can let you know when the air is unhealthy in your community. Sources include local radio and TV weather reports, newspapers and online at

  • Avoid exercising outdoors when pollution levels are high. When the air is bad, walk indoors in a shopping mall or gym or use an exercise machine. Limit the amount of time your child spends playing outdoors if the air quality is unhealthy.

  • Always avoid exercising near high-traffic areas. Even when air quality forecasts are green, the vehicles on busy highways can create high pollution levels up to one-third mile away.

  • Use less energy in your home. Generating electricity and other sources of energy creates air pollution. By reducing energy use, you can help improve air quality, curb greenhouse gas emissions, encourage energy independence and save money! Check out the U.S. Environmental Protection Agency's easy tips for conserving energy at home.

  • Encourage your child's school to reduce exposure to school bus emissions. To keep exhaust levels down, schools should not allow school buses to idle outside of their buildings. Many school systems are using the U.S. EPA's Clean School Bus Campaign to clean up these dirty emissions.

  • Walk, bike or carpool. Combine trips. Use buses, subways, light rail systems, commuter trains or other alternatives to driving your car.

  • Don't burn wood or trash. Burning firewood and trash are among the major sources of particle pollution (soot) in many parts of the country.

  • Use hand-powered or electric lawn care equipment rather than gasoline-powered. Old two-stroke engines like lawnmowers and leaf or snow blowers often have no pollution control devices. They can pollute the air even more than cars, though engines sold since 2011 are cleaner.

  • Don't allow anyone to smoke indoors and support measures to make all public places tobacco-free.

  • Get involved. Start by checking out Fighting for Air which has more information about what you can do.

  • For 20 years, the American Lung Association has analyzed data from official air quality monitors to compile the "State of the Air" report. The more you learn about the air you breathe, the more you can protect your health and take steps to make our air cleaner and healthier.  Click on the link for more information



Last Updated: 02/28/2018