More than 40 million people affected by harmful day of air pollution in 2020 in US West

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Nearly 70% of western America was affected by harmful levels of air pollution on one particular day in 2020, affecting around 43 million people, according to a new study led by Washington State University.

It is the highest level of air pollution in 20 years and had been caused by large wildfires and severe heat events across the USA.

Conducted in partnership with the Cooperative Institute for Research in Environmental Sciences at the University of Colorado Boulder (CIRES) and NOAA’s Global Systems Laboratory, the study, published in Science Advances, found that such widespread air pollution events are not only increasing in frequency, but also persisting longer and affecting a larger geographic area. These events have become so bad that they have reversed many gains of the Clean Air Act – and the conditions that create these episodes are expected to increase, along with their threats to human health.

When wildfires and extreme heat occur at the same time, they magnify air pollution – wildfire smoke increases fine particulate matter in the air and the heat combines with the smoke and other pollutants to create more ground-level ozone. While ozone in the stratosphere is protective, ozone that forms at ground level is harmful to human health. Simultaneous exposure of millions of people to both high levels of ground-level ozone and particulate matter poses a substantial public health burden.

In this study, the researchers examined certain weather patterns that typically lead to higher levels of ground-level ozone during the summer months. High-pressure ridging, more commonly known as heat domes, occurs when an area of high-pressure air lingers over a region. This was observed trapping warm stagnant air and its pollutants close to the ground. Particulate matter used to be more common in the western USA during the winter, but wildfires have been found to have flipped that dynamic, bringing together the dangers of both particulate matter and ground-level ozone in the summer.

Co-occurring events were defined as days that registered in both the top 10% in particulate matter levels and the top 10% in ozone. The researchers found that annual population exposure to these extreme combined episodes is increasing by approximately 25 million person-days a year – a figure that counts both the number of people affected and the number of days they were impacted by air pollution.

The researchers reported that these extreme air pollution events potentially could be mitigated by taking measures to slow the temperature rise caused by climate change, as well as better managing wildfires with practices such as prescribed burns. Aside from those efforts, they suggested treating these air pollution events like a severe snowstorm or heatwave and making sure people have shelters equipped with air filters where they can go to get out of polluted air. Adopting policies that minimize exposure for people who typically work outside was also recommended.

Dmitri Kalashnikov, the paper’s lead author and a WSU graduate student, and colleagues from CIRES and GSL, the University of California Merced, UCLA, the National Center for Atmospheric Research (NCAR), and the Nature Conservancy of California, tracked air quality across the western states and parts of Canada from 2001-2020 with available monitoring station data. They combined this data with wildfire information derived from NASA satellites and weather data produced by the European Center for Medium-Range Weather Forecasts.

The size of the simultaneous air pollution events will make it difficult for many people to avoid their impacts, according to study co-author Deepti Singh, a WSU assistant professor. She said, “Preparing for these events is really important. We need to think about who is exposed, what capacity there is to minimize that exposure, and how we can protect the most vulnerable people.”

Kalashnikov said, “We have seen an increasing trend in the past 20 years of days when high levels of both particulate matter and ozone are occurring simultaneously. This is tied to two things – more wildfires and increases in the types of weather patterns that cause both wildfires and hot weather.”

Jordan Schnell, a CIRES scientist working in NOAA’s Global Systems Laboratory and one of the paper’s co-authors, said, “If the trend of more frequent sunny, hot, and dry conditions in the US West continues as projected in a warmer world, we can likely expect increases in summertime wildfires that cause these extreme ozone and particulate matter pollution episodes.”

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