Record heat and large fireworks displays forecast to reduce air quality on July Fourth

Cities across the United States are preparing larger fireworks displays for the country's 250th anniversary while a heat dome is forecast to push temperatures above 100 degrees in multiple locations. Washington, DC, plans to launch 850,000 fireworks in 40 minutes from 10 sites, aiming for a Guinness World Record.

New York intends to fire 85,000 shells, Nashville expects thousands of launches, and San Francisco will stage a display from the Golden Gate Bridge for only the third time.

Heat and ozone conditions The National Weather Service projects at least 100 record-high temperatures through Saturday, mostly along the East Coast. Washington, DC, could reach 102 degrees, Philadelphia may hit 104, and New York is forecast to reach triple digits for the first time in a decade.

Ground-level ozone forms when vehicle emissions react with sunlight and heat. Forecasters expect moderate ozone levels during daylight hours in several cities before evening fireworks begin.

Particulate matter from fireworks Fireworks produce fine particulate matter known as PM 2.5, particles roughly 30 times smaller than a human hair. Researchers have recorded a roughly 42 percent increase in these particles in the hours after displays.

Russell Dickerson, a professor at the University of Maryland, College Park, said stagnant air under the heat dome will likely keep smoke near the ground longer than usual. Dan Jaffe, a professor at the University of Washington, stated that every city with a fireworks display experiences large PM 2.5 spikes once the show starts.

Air quality guidance The Environmental Protection Agency advises people who are pregnant, have asthma, or are caring for young children or older adults to limit outdoor exposure if the air quality index exceeds 100. An index above 300 is considered hazardous for everyone.