New York City (NYC) residents are masking up again, but not to protect against a virus. 

East Coast residents awoke Tuesday morning (June 6) to their cities and towns shrouded in a thick, smokey haze from the current wildfires in Quebec, Canada. The smoke is so thick that NYC moved up to number one on the World Air Quality Index (AQI), according to IQAir, with a rating of over 200 — which is considered “very unhealthy.” The rating has since dropped to 161 – “unhealthy” — and the city still ranks near the top of the list.

“I used to wear my mask only inside because of being in a close office space,” says Nina, a Brooklyn resident. “But I would take my mask off outside because the air is usually fresh.” Now, Nina wears a mask outside to protect against the low air quality.

Fine Particles 

As of June 7, the air quality is still low and rapidly changing as winds push the smoke south from Canada.

Wind carried the smoke from over 160 wildfires currently burning in Quebec and could keep the smoke there for a few days. With the smoke comes not only the haze but also fine particulate matter. According to NYC health officials, fine particles — PM2.5 — are among the most harmful air pollutants.

Blood can easily absorb these particles through the lungs. And officials say that long-term exposure to PM2.5 can lead to “an estimated 2,300 excess deaths from lung and heart disease each year in NYC.”

What is Particulate Matter 

Particulate matter (PM), according to the Environmental Protection Agency (EPA), is “a mixture of solid particles and liquid droplets found in the air.” PM2.5 are particles with a diameter of 2.5 micrometers and smaller. According to the EPA, 2.5 micrometers is about 30 times smaller than the diameter of a human hair. 

PM2.5 are often found in vehicle emissions, industrial pollution and wildfire smoke. Thanks to the Canadian wildfires, the PM2.5 levels in NYC are currently 15 times higher than the World Health Organization guidelines, according to IQAir. 

Read More: New Study Links Traffic Pollution to Pediatric Asthma

An Unsettling Feeling 

With the world still recovering from the COVID-19 pandemic, another event that impacts lung health can unsettle some residents. 

“I went from not wearing a mask outside to wearing an N95 [outside],” says Nina. “I’ve noticed a lot more people not only wearing masks on the train again but wearing masks outside because you can see how bad the air is; it feels unnerving.” 

Another thing that is unnerving to Nina is that for the pandemic, one of the ways to combat the virus was to be outside, have windows open and have fresh air circulating. For the smoke, however, it’s the opposite. 

Across the city, officials have canceled outdoor events, and schools are keeping children inside for recess. City health officials urge residents to limit outdoor activity and keep windows closed.  

What Caused the Canadian Wildfires?

It’s uncertain what caused the wildfires in Canada. Most wildfires are caused by natural phenomena such as lightning strikes or by people — accidental or not. Throwing cigarette butts out car windows and dragging metal-like chains from behind vehicles can spark a wildfire. So can misuse of fireworks and campfires. 

Factors such as drought and hot weather can also increase the likelihood of wildfires. According to The Washington Post, parts of Canada are experiencing extreme heat and higher-than-normal temperatures. 

Wildfires take a lot of time and resources to extinguish. Usually, they can’t be completely put out without the help of Mother Nature. According to PBS, nearly 500 wilderness firefighters are working to put out the Quebec fires, with nearly 200 more wilderness firefighters coming from the U.S. and France to help. 

Read More: Canadian Wildfire Smoke Was So Widespread it Was Visible From Nearly a Million Miles Away