The air quality index on Wednesday morning reached the “sick” 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 haze this time is likely to be a combination of Fourth of July fireworks, stagnant air and, in some places, West Coast wildfires.

Major cities most often see these air quality indexes after the Fourth of July festivities, said Dan Westervelt, an associate research professor at Columbia University and an air pollution adviser to the State Department.

“Basically, there was a huge spike in particulate pollution last night. It’s still going on today, but I expect it won’t continue tomorrow,” Dr. Westervelt said. “I think the risk to healthy individuals is pretty low because it’s so short-lived.”

According to a 2015 study of 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 to an average day.

Fireworks were set off 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 are technically not 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, fireworks and local fires, 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 dissipated 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 metropolitan area and Long Island region for Wednesday until 11 p.m. The higher levels of particulate pollution that triggered the advisory “were recorded primarily due to fireworks.” ” and “not the Canadian wildfires,” the state environmental conservation department said in a statement.

In Los Angeles, meteorologists warned that the Air Quality Index could reach dangerous levels from the evening of July 4 to the morning of July 5, but then decrease throughout the day on Wednesday.

An air quality alert that covers much of Los Angeles is in effect until 11:59 pm

Residents in cities that don’t often face wildfire smoke have been paying closer attention to air quality measures since smoke from Canadian wildfires blanketed Chicago in a thick haze last week and filled New York with a pungent campfire smell, sending residents to the hospital for. respiratory problems, in June.

Instead of fireworks, some cities, such as Salt Lake City, have chosen 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 can’t rise because it’s so cold. So, the warmer air above acts like a cap or blanket – trapping the cool air and pollutants like fireworks and wildfire smoke near the surface. This is called an inversion, and even a mild one, like in New York on Tuesday night, can worsen air quality conditions through the overnight hours and into the early morning.

As the sun rises and the surface warms, the residual smoke will rise and disperse, as it did in the city just after sunrise Wednesday morning.

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