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Stagnant Air on the Rise, Upping Ozone Risk

Research Report by Climate Central

Summers in the U.S. bring more than just searing, dangerously hot days. When the air is stagnant and there is little air circulation, hot weather can trigger high levels of air pollution that can have health consequences for millions of Americans. With stagnant air now occurring more frequently in much of the country, and projected to continue increasing, the combination of heat and stagnant air are primed to counteract efforts to reduce ground-level ozone pollution and continue to put thousands of lives at risk every year.

In a new analysis, Climate Central’s States at Risk project has investigated how stagnant summer air conditions have been increasing in much of the nation and explored the relationship between this stagnant air and ozone levels in major U.S. cities in the Lower 48 states. We found that:

  • 66 percent of the country has seen an overall increase in days with stagnant summer air conditions since the early 1970s. This increase is consistent with what we expect in a warming world and climate models project that if greenhouse gas emissions continue at the current rate, stagnant air events will continue to increase.

  • While average summer ozone levels have decreased in most major U.S. cities since the 1980s, there are still several days every year where ozone levels reach unhealthy levels.

  • In several cities, like Baltimore, St. Louis, Pittsburgh, and Cincinnati, summers with more stagnant air events also tend to be the summers with more ozone air quality violations, based on ratings from the Environmental Protection Agency (EPA). In the future, more air stagnation could trigger smog events, offsetting some of the progress already made to clean up the air.

Health Impacts of Ground-Level Ozone

Heat, sunlight, and pollution from cars and power plants combine to form ground-level ozone, the main component of smog. Ozone, along with particulate matter, is responsible for most air pollution-related health impacts, and is estimated to be responsible for thousands of premature deaths every year in the U.S.

Ground-level ozone can cause a wide range of respiratory issues, including exacerbating asthma suffered by millions of Americans. Children, the elderly, and those with pre-existing respiratory conditions disproportionately feel the health impacts of ozone. Dangerous ground level ozone should not be confused with ozone in the upper atmosphere, which helps shield the planet from harmful levels of ultraviolet light. The EPA monitors ozone levels on an hourly basis from stations across the country, and categorizes levels according to their degree of health hazard. In the U.S., ozone is estimated to be responsible for thousands of premature deaths each year. This is in addition to leading to tens of thousands of emergency room and hospital visits, and millions of cases of other respiratory conditions resulting in missed school and work.

Credit: EPA

The U.S. has made significant gains in improving air quality due to the Clean Air Act, helping to reduce the hazardous effects of smog and other air pollution. Concentrations of the major air pollutants have been decreasing overall, since 1990. Yet, while average ozone concentrations have decreased, many major U.S. cities still face unhealthy levels of ozone several days each year. In 2015, despite decreases in pollutants, 127 million Americans lived in counties that exceeded the EPA standard for safe levels of ozone. Air pollution also remains as a major health issue in major cities around the world.

High temperatures on sunny days make ground-level ozone (a major component of smog) form much more readily. An EPA study looking at more than 20 years of measurements across most of the rural areas in the eastern U.S. found that harmful ozone concentrations increased nearly linearly as temperatures increased and named the effect the “climate penalty on ozone." 

Also of concern are days with stagnant air that allow air pollutants to build up and not be flushed out of an area by wind. Stagnant air has been shown to drive up concentrations of both ozone and particulate matter, unlike temperature, which only directly affects ozone.

More Stagnant Air

We have found that 66 percent of the U.S. has experienced an overall increase in the number of days with stagnant air since the 1970s. This increase is consistent with what you expect in a warming world, where rising temperatures are expected to increase stationary air masses as overall air circulation slows down.

The regions that have seen the largest increases in stagnant air are the Southeast and parts of the West Coast, but the Northeast and Midwest have also seen measurable increases in stagnant air events. This is over the same period of time that summer temperatures and extreme heat have been warming across most of the country.

Projections of stagnant air days under continued heat-trapping gas emissions and warming show future increases in the number of these days in both the growing western U.S. and the highly populated eastern areas. As global temperatures warm, the difference between temperatures at the poles and the equator are decreasing, and this is projected to influence the jet stream, bringing an overall decrease in storm tracks through the midlatitudes (including areas over the continental U.S.) and decreasing windiness. The result could be longer spans of stagnant air hanging over U.S. regions.

Stagnation and Ozone in U.S. Cities

Given that stagnant air events in many U.S. cities have been rising, and are projected to continue increasing if greenhouse gas emissions keep growing, we have also explored the relationship between stagnant air and high ozone levels in several major U.S. cities.

Drawing on air quality data since 1980, we found that in many cities, and particularly those in the Northeast and the Midwest, summers with more stagnant air events also had more days with unhealthy levels of ozone. A high ozone day was one when concentrations exceeded those levels classified by the EPA as unsafe into one of the four unsafe categories.

Midwestern cities like Chicago, Detroit, Columbus, Cincinnati, Cleveland, and St. Louis show clear correlations between the years with more stagnant air events with the number of ozone violations. Similarly, several large Northeastern cities also exhibit this relationship with stagnant air and ozone levels. These cities have all seen average ozone levels decrease in recent decades, and yet all have also experienced single years where ozone violations have spiked when there was more stagnant air.


Cities with the most stagnation days

Not all cities that have seen an increase in stagnant air exhibit this same relationship with ozone, particularly those that have little-to-no local ozone issues. However, in Midwestern and Northeastern cities that continue to struggle with air pollution, the problems is exacerbated by stagnant air conditions that keep atmospheric pollutants stationary over the city.

In the coming decades, continued emissions and the warming climate are projected to increase stagnant air conditions. With more stagnant air events comes the risk that the air quality gains made in recent decades will be offset because conditions will be more favorable for forming pollutants, such as ozone, and having them build up in populated areas.

Methodology: Climate Central analyzed records of stagnant air days (data from NCEI) and days with unhealthy levels of ozone (data from EPA AirData) in the 35 largest U.S. cities in the Lower 48 states to investigate associations between the two. Changes in stagnation days are based on trends between 1973-2015. The focus is on large cities as this is where both pollution sources and people at risk of health impacts are more highly concentrated. We analyzed 36 years of data beginning in 1980 and focused on the summer months each year as that is the time when stagnation days and ozone occur most frequently. Ozone violations were counted as any day (24hr average) where ozone levels were classified by the EPA as being "unhealthy for sensitive groups," "unhealthy," "very unhealthy," or "hazardous."

Research and analysis by Alyson Kenward, PhD, Jennifer Brady, Todd Sanford, PhD, and Greta Shum.