Officials in Nevada warned that July 4 fireworks—combined with smoke from California wildfires—may cause unhealthy levels of air pollution. So did the weekend fireworks cause Las Vegas to exceed clean-air standards?
Yes, at least for a few hours. This isn't at all unusual: State environmental agencies often report Independence Day spikes in their PM2.5 readings—a measure of the concentration of very tiny airborne particles, no bigger than 2.5 micrometers in diameter. Under federal law, a location's air quality is judged, in part, by the number of days on which the PM2.5 average exceeds 35 micrograms per cubic meter. Fireworks shows on the Fourth of July can put out enough pollution to break that threshold.
In Las Vegas, preliminary data from at least one site shows PM2.5 readings greater than 200 micrograms per cubic meter at 10 p.m. on July 4. The city's air quality was clean enough earlier in the day to prevent the 24-hour average from cracking 35, so the show won't affect the city's clean-air record. In other places, the average reading might have been higher. Last year, for example, an air-quality monitor located in South Bend, Ind., registered an average reading of 39 micrograms per cubic meter on July 4, up from 16.6 the day before.
Fireworks-related pollution may not affect a city's air-quality rankings, even if the average readings do surpass the federal threshold. That's because the Environmental Protection Agency has a special provision allowing states to discount a high reading in the case of "exceptional events." This is supposed to ensure that unforeseen events like wildfires or terrorist attacks don't artificially boost a location's air-pollution readings. Because the agency classifies July 4 fireworks as a special cultural tradition, state and local agencies can apply to have their abnormally high readings struck from the record. (Chinese New Year fireworks are exempt, too.) Last year, cities including Fresno, Calif., (PDF) and Salt Lake City (PDF) filed reports explaining why July 4 readings shouldn't count toward their stats. These reports also make a clear case that the higher pollution levels are due to the fireworks themselves and not a spike in automobile traffic for the holiday.
Fireworks-related air pollution is not a uniquely American problem. One team of scientists in India reported that sparklers set off during the Diwali festival raise ozone levels; another recorded sulfur dioxide concentrations as much as 10 times higher during the holiday. A Chinese study found similar results during a lantern festival in Beijing. Research suggests (PDF) that weather conditions make a big difference, too: If the wind is weak, then the particles released during the fireworks display will take longer to disperse.
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Explainer thanks Alan Gertler of the Desert Research Institute, Cathy Milbourn of the U.S. Environmental Protection Agency, and Phil Silva of Utah State University.