July 4^th, 2021 marked the first Independence Day in the United States since the beginning of the COVID-19 pandemic when public health guidelines, such as social distancing and mask-wearing, were largely lifted across the country. This meant that many localities were permitted to hold large public events during Independence Day celebrations for the first time in over a year.

During the 2020 Independence Day, large-scale fireworks events had been mostly banned under guidelines from the Centers for Disease Control and Prevention (CDC).¹ Air pollution from fireworks that year was mostly limited to homes and neighborhoods in major cities, such as Los Angeles, and resulted in significant but fleeting air pollution across the entire region for hours after celebrations ended.

In 2021, the impact of fireworks on air pollution was drastic and noticeable in several major cities – Los Angeles, New York City, and Washington, DC – with air quality index (AQI) measurements reaching far into the “Hazardous” range in the hours following fireworks events.

Pictured: Air quality in the United States following Independence Day celebrations (July 4, 2021, 11:55 p.m. PST). At nearly midnight, over half of the country experienced air quality ranging from “Moderate” to “Unhealthy”, with New York City ranking as the 3rd most polluted city in the world for several hours. Source: IQAir

Los Angeles air quality

At the height of the Independence Day holiday, much of Los Angeles, California experienced air quality well beyond emergency conditions across hundreds of square miles.

Just before midnight on July 4th, 2021, most public air quality stations in the Los Angeles area recorded fine particulate matter (PM2.5) measurements ranging from the low 100s (“Unhealthy for Sensitive Groups” such as children, the elderly, and people with respiratory conditions) into the high 300s, emergency-level “Hazardous” air quality that can cause heart or lung damage even after short periods of exposure.

Pictured: Air quality in Los Angeles, CA in the late hours of Independence Day following widespread public fireworks events (July 4, 2021, 11:55 p.m. PST). Some areas in the Los Angeles reached well into the 300+ range – deemed as emergency conditions. Source: IQAir

The immediate impact of fireworks in Los Angeles is evident in these measurements, taken in the hours following fireworks shows that began as the sun set around 8:00 p.m. Pacific Standard Time (PST). Some stations in South and East L.A., including Inglewood and Huntington Park, recorded AQI measurements above 400 late into the evening.

However, this impact lingered into the morning of July 5th, 2021, with hazardous levels of air pollution still visible across much of the Los Angeles region into the neighboring foothills to the north and San Bernardino County to the east.

Even as PM2.5 concentrations within much of western Los Angeles had dispersed somewhat by the next morning, a combination of weather and geography trapped air pollution firmly in foothill suburbs like Pasadena, Arcadia, Monrovia, Claremont, and as far east as Fontana and Riverside.

Northeast-blowing wind patterns blew much of the Independence Day PM2.5 pollution into the San Gabriel Valley. Here, the surrounding hills and mountains as well as cool air in the lower atmosphere trapped air pollution in a temperature inversion until the early afternoon of July 5^th, when wind and warmer air allowed air pollution to disperse and rise.

Pictured: Air quality in Los Angeles, CA (July 5, 2021, 10:02 a.m. PST). Source: IQAir

San Bernardino County was similarly impacted by windblown air pollution and the geography of the Inland Empire following Independence Day celebrations. With a total population of over 4.2 million, the health impact of fireworks air pollution in the Inland Empire region ranged from “Very Unhealthy” to “Hazardous” for much of July 5th, with AQI measurements in suburban hubs like Ontario, Pomona, and Corona reaching beyond 300.

Washington, D.C. air quality

The hype surrounding fireworks in Washington, D.C. reached a fever pitch in the days leading up to July 4th. The presidential administration had cautiously heralded the return of public events in the nation’s capital due to success in controlling the spread of COVID-19 through vaccination, so many looked forward to resuming the public celebrations of years past.

But like Los Angeles, the U.S. capital city experienced dangerous levels of air pollution in the hours following July 4^th fireworks shows. Even in the early morning of July 5^th, air quality in the metropolitan area around the D.C. Beltway ranged from “Unhealthy” to “Very Unhealthy.”

Pictured: Air quality in Washington, D.C. and the surrounding metropolitan area (July 4, 2021, 2:57 a.m. EST). Source: IQAir

Many suburbs of Maryland around D.C., such as Silver Spring and Chillum, recorded PM2.5 concentrations in the low to mid-200s, posing a significant health risk to all residents. Across the Potomac, cities in Virginia also experienced air pollution from fireworks shows around the DMV region, with cities like Tysons Corner, Vienna, and Falls Church measuring PM2.5 around 200 or above.

In the U.S. capital itself, PM2.5 concentrations remained “Very Unhealthy” across broad swathes of the city, particularly in the far eastern regions along the Anacostia River and near RFK stadium.

By the late morning of July 5th, PM2.5 concentrations had lowered considerably, with most of the DMV area only experiencing “Moderate” levels of PM2.5 between AQI measurements 50-100. However, no level of air pollution is safe – even as air pollution was dispersed on July 5^th, over 6 million people in the region were still subject to heart, lung, and organ damage from long periods of exposure to air pollution.

Pictured: Air quality in Washington, D.C. and the surrounding metropolitan area (July 5, 2021, 1:03 p.m. EST). Source: IQAir

New York City air pollution

Despite being one of the biggest cities in the U.S. with a population of over 22 million in the greater metropolitan area, New York City experiences much less severe air pollution than other major urban ports like Los Angeles. AQI measurements in New York City typically remain below 50 (considered “Good” and posing little health risk) throughout the year.

But for several hours on July 4th and 5th, New York City was the 3rd most polluted city in the world – at its peak, New York City recorded AQI measurements of 168 – trailing Krasnoyarsk, Russia as the most polluted city and Johannesburg, South Africa as the second most polluted.

Pictured: The most polluted major cities in the world late in the evening on July 4, 2021 – New York City ranked third. Source: IQAir

For most of the evening on July 4^th, New York City did not appear in the World AQI Ranking.

But PM2.5 concentrations rose to dangerously high levels as fireworks continued across New York State and southeastward winds blew in additional air pollution from the city’s northern suburbs, including White Plains and as far north as Poughkeepsie and Albany.

Cool marine air kept air pollution trapped in the lower atmosphere throughout the city for many hours. New York City AQI remaining well into the “Unhealthy” category until falling into the “Moderate” category around noon on July 5^th.

Even as air pollution from fireworks has dispersed, poor air quality lingered even into July 6^th, with the hourly average AQI hovering between 70-90 beginning at midnight on July 5^th and showing little sign of dispersing back to the city’s normal “Good” AQI below 50.

Pictured: New York City AQI from Sunday morning, July 4, 2021, until Tuesday afternoon, July 6, 2021. The red and orange bars represent increased hourly average air pollution following July 4th evening celebrations, from about midnight on July 4 until 9:00 a.m. on July 5. Source: IQAir

The takeaway

July 4th, 2021 saw the return of large-scale Independence Day events for the first time since the COVID-19 pandemic and, consequently, widespread fireworks air pollution that lasted for hours and spread many miles beyond their source.

Fireworks shows may be brief, but the impact of residual air pollution can be felt long afterwards. Airborne pollutants associated with fireworks such as PM2.5, ultrafine particles (UFPs), and volatile organic compounds (VOCs) can pose a health threat to residents. The best means to protect against these threats can be to close up doors and windows, run an air purifier, and ask local officials to plan alternatives to fireworks for a safer celebration.