Getting Acquainted With AQI

PUBLISHED MAY 15, 2021 12:00 A.M.
Share this:  
A smoky blanket of particulate matter hovers over San Francisco’s skyline.

A smoky blanket of particulate matter hovers over San Francisco’s skyline.   Photo by Steve Jurvetson   CC BY 2.0


AQI, or the Air Quality Index, is explained as a “thermometer for air quality.” It ranges from zero to 500; as the number increases, so does the level of pollution in the air.

In the not-too-distant past, most Californians felt no need to check their local AQI unless they lived in a traffic-clogged, smog-choked metropolitan area.

But ever-worsening fire seasons and the attendant concentrations of smoke have resulted in wider usage of the acronym. Californians and many other Western United States residents, accustomed to beautiful blue skies, have become familiar with this air quality evaluation tool; they now look for the AQI number at the bottom of their smart phone’s weather application.

The Clean Air Act of 1970 was the first comprehensive federal law that regulated emissions. The act set the precedent for current air quality standards, regulated by the Environmental Protection Agency. Today, the AQI reflects the following compounds in our air: ground-level ozone, particulate pollution, carbon monoxide, sulfur dioxide, and nitrogen dioxide.

  • Ground-level ozone comes from chemical reactions in sunlight between oxides of nitrogen and volatile organic compounds—highly reactive free-floating chemicals in the air. Our cars, power plants, industrial boilers, refineries, chemical plants, and other manufacturing processes emit these substances. When they collide, they form a haze known as ground-level ozone. This haze can be transported long distances by wind, so rural environments and other states may experience the residual ozone from polluted areas.
  • Particulate pollution is also called particulate matter, or PM. According to the EPA, “It is a general term for a mixture of solid and liquid droplets suspended in the air.” There is typically a number that follows the letters PM which explain the size of the particles in relation to the air around it, but that’s a bit too sciency for our purposes. The important thing to know is that “they can be made up of a number of different components, including acids (such as sulfuric acid), inorganic compounds (such as ammonium sulfate, ammonium nitrate, and sodium chloride), organic chemicals, soot, metals, soil or dust particles, and biological materials (such as pollen and mold spores).” The air we breathe inside and outside always has some combination of this matter, though in many cases the concentration is insignificant.
  • Carbon monoxide, sulfur dioxide, and nitrogen dioxide are byproducts of fossil fuels and natural gases.

Most of our weather apps show words such as good, moderate, unhealthy for special groups, unhealthy, very unhealthy, and hazardous. Nowadays it’s unnecessary to calculate the AQI and interpret the levels for ourselves. But individuals do need to understand what prolonged levels of exposure mean for their health.

During 2020—a record-setting year for wildfires—parts of California reached 300 on the AQI meter and higher from mid-August to mid-September, ranking the air quality very unhealthy and in some cases hazardous.

According to an article by UC San Francisco News and Media, most healthy adults are able to recover from exposure to extreme smoke.

Long-term exposure, however, will continue to bombard the lungs with particulate matter. Though some individuals will only have symptoms of coughing or wheezing for a few weeks, anyone with a predisposition is likely to feel longer effects of this exposure. When PMs are inhaled they get lodged in the lungs as hydrocarbons, causing some short-term damage and inflammation. Dr. John Balmes of UCSF shared that the risk is threefold with more vulnerable populations, children or seniors with chronic ailments and or asthma: “Inflammation in the lungs can become systemic, affecting the whole body’s vascular function; the risk for blood clots increases, which can lead to heart attack or stroke; and the autonomic nervous system is stimulated, which can cause heart arrhythmia.” In addition, Balmes said, “There’s good evidence that these effects can accelerate atherosclerosis, the plaques that build up inside blood vessels.”

In 2021, persistent drought could exacerbate wildfires. So if and when the fires start, it could be time to purchase high-quality air filters—or get out of Dodge until the particles disperse.


Support California Local

$10 • $25 • $50 • Our Impact

Long form articles which explain how something works, or provide context or background information about a current issue or topic.

Join Us Today!