Ground-Level Ozone and Smog

Smog is a mixture of pollutants made up mostly of ground-level ozone that can damage the respiratory system.  They form through chemical reactions among nitrogen oxides (NOx) and volatile organic compounds (VOC).  Most of the air pollutants come from vehicles, power plants, refineries, chemical plants, and other industrial sources that create combustion from fossil fuels,  which chemically react with sunlight to form ozone.  

Aaron Baker

Sep 20, 2022 • Updated Jan 25, 2023 • 3 min read


Smog is a mixture of pollutants made up mostly of ground-level ozone that can damage the respiratory system.  They form through chemical reactions among nitrogen oxides (NOx) and volatile organic compounds (VOC).  Most air pollutants come from vehicles, power plants, refineries, chemical plants, and other industrial sources that create combustion from fossil fuels,  which chemically react with sunlight to form ozone.  

Ozone can be harmful or helpful.  The ozone layer high up in the atmosphere protects us from harmful ultraviolet radiation.  But, when ozone is near the ground, it mingles with other irritants and, with repeated exposure,  can damage the respiratory system. It’s especially dangerous for people with respiratory illnesses like asthma.

Smog also includes airborne particle pollution that reduces visibility.  Most of the smog we see today is photochemical smog from cars and industry, but particulate matter released by wildfire smoke, coal-fired power plants, dust, and pollen can also contribute to hazy air.  Fine particulate matter (PM2.5), less than 30 times the thickness of a human hair,  is released from all of these sources and is especially dangerous because it can be inhaled deep into the lungs and even enter the bloodstream.

Smog and ground-level ozone are more likely to form in urban areas because of their higher concentrations of vehicles and industry.  This can be made worse by thermal inversions that trap air pollution under a layer of warm air near the ground in a layer of cool air, effectively concentrating smog and ground-level ozone.  Thermal inversions typically happen in cities near mountain ranges along the coast or in deep valleys.  Coastal cities, such as Los Angeles, CA, and cities in valleys like Denver, Colorado, experience thermal inversions that can lead to some of the worst air quality in the US.

Ozone, particulate matter (PM2.5 and PM10), carbon monoxide, and sulfur dioxide are measured continuously at over 500 monitoring stations around the US.  These air quality measurements are available for any location in real-time through AreaHub. See our article on air quality for additional information on air pollution and AQI.

According to the 2022 “State of the Air” report from the American Lung Association, over 40% (137 million) of Americans lived in areas with unhealthy ozone levels or particulate matter pollution between 2018-2020.  This is an increase of 2.1 million from the previous three-year reporting period.  They also found that during this time, the US experienced more days of “very unhealthy” and “hazardous” air quality than ever before in the 20-year history of their reporting.  These three years are among the seven hottest ever recorded, and climate change-fueled increases in ozone and particulate matter from extreme heat and wildfires are putting millions more at risk.  

The geographic distribution of the cities in the US with the worst ozone has followed a trend over the last decade–fewer eastern cities and more western cities.  Population growth and oil and gas extraction in the southwest, coupled with the cleanup of power plant emissions in the east, have shifted the cities that experience the greatest number of unhealthy ozone days.  

Smoke from wildfires in the western US is increasing the number of days, locations, and severity of particle pollution.  The “State of the Air” 2022 report includes 47 counties in eight western states, home to about 34 million people, where the AQI spiked to the “very unhealthy” level on 116 days.  This is twice the number recorded in the two previous reports and nearly ten times that reported in “State of the Air” 2018. 

Your Home and Ground Level Ozone and Smog#

Health and Safety#

Breathing air with ozone or smog can harm your health, especially on hot, sunny summer days when air pollution is more likely to reach unhealthy levels.  The EPA describes the following health risks of both short-term and long-term exposure to smog, including ground-level ozone.  Short-term exposure (8 hours or less) can lead to:

  • Being unable to inhale a full normal breath.

  • Coughing and a sore or scratchy throat.

  • Pain, burning, or discomfort when taking a deep breath.

  • Chest tightness, wheezing, or shortness of breath.

  • Aggravating lung diseases such as asthma, emphysema, and chronic bronchitis.

  • Increasing the frequency of asthma attacks.

Recurrent or long-term exposure can also  lead to:

  • Reduced small airway function as a result of repeated inflammation.

  • Possible development or progression of chronic lung disease or asthma.  

Those most at risk include people with asthma, children, older adults, and those that are active or work outdoors. Children are at a higher risk because their lungs are still developing; they are more likely to be active outdoors when ozone and smog levels are high, and they are more likely to have asthma than adults.  According to the EPA, people with certain genetic characteristics and individuals with reduced intake of vitamins C and E are also at a greater risk.

Property Values#

Air pollution, including ground-level ozone and smog, typically has a negative effect on property values. Researchers have consistently found that property values are higher in areas with lower air pollution and have even increased in counties in the US when air quality improved.  During the 20 year period from 1970 through the 1980s, researchers from Brown University found that home prices in US counties with reduced air pollution increased between 3.9 and 4.8% more in value than homes in counties that did not reduce air pollution significantly.  There are exceptions; a UC Davis study found that home prices in the San Francisco Bay area increased as air pollution increased; the authors suggest this is most likely due to high demand and low-stock housing in this area. Researchers at Harvard also found that attendance at outdoor attractions like zoos and sporting events drops significantly in response to smog alerts, suggesting that people are willing to undertake costly avoidance behavior regarding air pollution.

What You Can Do#

Because of the role of sunlight and heat in the photochemical reaction that produces ozone and smog, these pollutants are usually highest on hot, sunny summer days – especially between noon and early evening.  Keep track of the AQI in your area and limit your exposure and exertion levels when air pollution levels are high.  Use AreaHub to learn more about the AQI in an area or specific address, including annual averages and the number of days the AQI exceeded the healthy range.  

If the AQI levels in your area are unhealthy:

  • Stay indoors with filtered air.

  • Keep your activity levels low.

  • If you cannot buy filters for your entire home, create a “clean room” for sleeping supplied with filtered air through an air conditioner or air filter.

  • Use an N-95 or P-100 respirator when outdoors.

In addition, here are some of the actions everyone can take to reduce air pollution that contributes to ground-level ozone and smog:

  • Use public transportation, bike, or walk whenever possible.

  • Combine errands and carpool to reduce driving time and mileage.

  • Set air conditioners to a higher temperature and turn off lights, TVs, and computers when not in use.

  • Avoid using small gasoline-powered engines, such as lawnmowers, string trimmers, chain saws, power washers, air compressors, and leaf blowers.

  • Avoid outdoor burning, including leaf burning and use of firepits and campfires.

For product options to reduce indoor air pollution, check out AreaHub’s product guide.

Highly Rated Products#

These highly-rated air purifiers and monitors may help measure and improve your indoor air quality:

Check Pricing
Levoit Core Mini
Larger Spaces

These products are not endorsed by AreaHub and AreaHub will not be liable for your use of the product if you purchase it. If you buy something through these links, AreaHub will be compensated and/or receive a commission.

Find information for climate, natural, and environmental hazards in the United States with AreaHub in seconds. With a free AreaHub report, access data about 18 potential dangers in your zipcode, like wildfires, extreme weather, hurricanes, and radon. With a Premium AreaHub report, access 27 potential dangers near your home, like heatwaves, solid waste landfills, and facilities in the toxics release inventory and more, and monitor your area for severe weather alerts and pollution incidents.