An average of 230 Americans end up in the emergency room every day in the month around July 4 because of firework-related injuries, but pyrotechnic mishaps are not the only potential setback of this Fourth of July tradition.
A new study published this week in the journal Atmospheric Environment found that fireworks release high levels of pollution into the sky on July 4 and 5.
“When people think of air pollution, they think of other kinds of things—smoke stacks, automobile exhaust pipes, construction sites,” study author Dian J. Seidel told TIME. “I don’t think most people think of fireworks.”
As a national average, culled from 315 different testing sites, Independence Day fireworks introduce 42 percent more pollutants into the air than are found on a normal day.
Part of that increase is a spike in emissions of perchlorate, a chemical that the Environmental Protection Agency says may “disrupt the thyroid’s ability to produce hormones needed for normal growth and development.”