These maps show how quickly Fourth of July fireworks diminish air quality

There’s one night of the year where celebrations across the United States light up the map like no other. It’s the Fourth of July — when fine particulates fill the air.

The impact fireworks can have on air quality can be quite striking and seemingly outsized given their relatively small explosive force. And there have been stark examples in recent years of poor air quality spiking after the Fourth of July. In 2019 in Washington, D.C., for one, the smoke got so bad that many couldn’t actually see the fireworks, and crews filming the spectacle flipped back and forth to file footage.

In that case, something called an inversion – where a cap of warm air sits on top of cooler air at the ground – trapped fireworks smoke in the low levels of the atmosphere. Add in a lack of wind, and there wasn’t much to see after the first few shells. Wandering the streets of the city afterward, there was a distinctive smell. Many reported experiencing burning eyes and coughing.

At times it can seem the nation’s smoky celebration is poorly timed. Heading into the heart of summer, there can be stagnant air in much of the country.

This was the case last year, when high pressure dominated both coasts, and there was a bit of a jet stream dip in the north-central U.S. That weather pattern configuration allowed for widespread poor air and probably helped amplify smoky conditions in places such as the Midwest.

Looking at a snapshot of the Northeast over the past several years, the plumes of bad air can generally be seen pointing from northwest to southeast with usual evening wind. Years with lower wind levels tend to be the ones with more blobs of bad air quality.

In recent years, some of the areas that have seen notable bouts with very unhealthy or hazardous air after the Fourth include the New York City area, Minneapolis, Chicago, Detroit and Los Angeles. Some more unexpected areas include Boise, Idaho; Omaha and Akron, Ohio.

And an analysis of the median over the last six years shows several cities and states where air quality diminished to a lower but still unhealthy range. These are some of the affected locations:

Parts of Southern California, in and around Los Angeles
Portions of Idaho, such as Boise
The D.C. area, particularly in southeast suburbs of Maryland
Eastern Nebraska, in and around the Omaha area
Northeastern Ohio, such as areas downwind of Cleveland
The area around Billings, in south-central Montana

Conditions that can fuel air quality issues

In addition to the inversion scenario noted above in D.C., other conditions can complicate smoke dispersal as well.

High humidity can work hand in hand with a temperature inversion, and it may help cause a smoke/fog mix to develop in the worst cases. In others, it keeps the smoke hanging low and dense, and lowers visibility somewhat.

Topography is another. It’s a reason a place such as Boise ranks among the worst offenders despite its smaller size compared with other cities. Boise sits in the Snake River Valley, which helps trap particulates by blocking their natural escape routes.

Los Angeles has tricky winds and terrain. While the breeze will often be offshore, helping disperse some of the smoke over the Pacific Ocean, at other times it will blow onshore keeping smoke bottled up. Smoke also tends to get trapped in the metro area, as it sits in a bowl surrounded by mountains, a cause for the region’s iconic smog.

Wind is typically key to keeping smoke issues at bay.

When there’s breeze, the mixing of the air does much of the work to disperse the smoke. And in summer, if there is some wind, there is also a decent chance that a fresher air mass is involved, such as behind a cold front.

Fireworks smoke is unhealthy to breathe, as it contains the tiny particulates associated with burning material that can travel deep into the lungs. It also contains various chemicals and metals that create the sparkles and colors.

“Short-term exposures can aggravate lung disease, causing asthma and acute bronchitis, and increasing the susceptibility to respiratory infections,” according to the Wisconsin Department of Natural Resources.

One saving grace: Though the poor air quality can often linger into the morning after the holiday, the high concentrations of smoke are generally short lived.