The air quality index Wednesday morning reached the “unhealthy” benchmark in cities across the United States including Seattle, Los Angeles and Pittsburgh. Millions of Americans have felt the effects of thick smoke from Canadian wildfires in recent weeks, but experts said the smog this time was likely to be a combination of Fourth of July fireworks, stagnant air and, in some areas, West Coast wildfires.
Major cities most often see these air quality index spikes after Fourth of July celebrations, said Dan Westervelt, an associate research professor at Columbia University and air pollution adviser to the State Department.
“Basically, there was a huge spike in particulate matter pollution last night. It’s still lingering around today, but I expect it won’t last tomorrow,” Dr. Westervelt said. “I think the risk to healthy individuals is fairly low, since it is so short-lived.”
According to a 2015 study from Atmospheric Environment, the national average of air pollutants is about 42 percent higher in the 24-hour period after 8 p.m. on July 4, compared with an average day.
Fireworks displays were hosted across the country. Many were official Independence Day shows held by cities, while others were informal firecrackers and bottle rockets set off in neighborhoods. Even in places like New York and California, where residents in many areas technically aren’t allowed to set off aerial fireworks, the sounds of Roman candles and bottle rockets filled the air and lit up the sky late Tuesday and into Wednesday.
In Seattle, firework activity and local wildfires, as well as smoke drifting south from fires in British Columbia, contributed to a haze over the city, the National Weather Service said, but it dispersed a few hours after sunrise.
The Weather Service put a dense fog advisory in place for southern Connecticut and Long Island until 7 a.m. Wednesday morning, citing trapped smoke from fireworks as one of the reasons.
The New York State Department of Environmental Conservation also issued an air quality advisory for the New York City metropolitan area and Long Island region for Wednesday until 11 p.m. The higher levels of particulate matter pollution that triggered the advisory “were recorded primarily due to fireworks displays” and “not the Canadian wildfires,” the state’s environmental conservation department said in a statement.
In Los Angeles, meteorologists warned that the Air Quality Index could reach hazardous levels from the evening of July 4 into the morning of July 5, but then decrease throughout the day on Wednesday.
An air quality alert that covers much of Los Angeles County is in effect until 11:59 p.m.
Residents in cities that do not often face wildfire smoke have been paying closer attention to air quality metrics since smoke from the Canadian fires blanketed Chicago in a thick smog last week and filled New York City with an acrid campfire smell, sending residents to the hospital for respiratory issues, in June.
In lieu of fireworks, some cities, like Salt Lake City, opted to provide celebratory drone displays to avoid worsening air quality.
Typically, temperatures cool higher in the atmosphere, but at night, air near the ground can cool faster and cannot rise because it is so cold. So, the warmer air above acts as a cap or a lid — trapping the cool air and pollutants like firework and wildfire smoke near the surface. This is called an inversion, and even a mild one, like in New York City Tuesday night, can worsen air quality conditions through the overnight hours and into the early morning.
As the sun rises and the surface heats, the residual smoke will rise and disperse, as it did in the city right after sunrise Wednesday morning.