A thick haze taking over the skies of much of the Northeast U.S. has prompted numerous cities to urge people to stay indoors – and for good reason. The smoke trailing in from wildfires in Canada has spiked air pollution to levels that could cause health issues for those exposed, particularly people in vulnerable groups. 

According to the Environmental Protection Agency, wildfire smoke is a “complex mixture” of pollutants can cause anywhere from minor health effects to those that are more serious. That’s because the particulate matter within the smoke irritates the respiratory system, impacting the body’s ability to function even among those who are healthy, and even short-term exposure of just a few days can have serious impacts. 

“Sensitive groups,” including children, the elderly, pregnant people and those with pre-existing respiratory and cardiovascular issues, are most vulnerable to these impacts. 

“The particulate matter that’s in this haze is significant because it does irritate the bronchials, the small tubes that go down into your lungs and connect to the alveoli, which are the sacs that allow you to breathe,” Dr. Bob Lahita, a rheumatologist, told CBS News, saying that anyone who belongs to the sensitive groups should avoid going outside.

According to the National Weather Service, “poor air quality can be hazardous.” Here’s what to look out for. 

Headaches, irritation and fatigue

Among the most minor of symptoms when it comes to health effects of poor air quality are headaches, sinus and eye irritation, and fatigue. Even though they are not as severe as some other potential effects, they could cause significant discomfort or worsen other impacts. 

“If you look at your car this morning and it’s been parked outside and there’s a fine layer of soot over your car, well, it’s often gonna be inside your lung, inside your chest,” Lahita said. “And that is a big problem. A lot of people can’t tolerate it and will be coughing and sneezing all day.” 

Respiratory issues

Those who have pre-existing respiratory issues, including asthma, are more susceptible to the impacts of wildfire-fueled poor air quality. Difficulty breathing, asthma attacks, irritated throat, bronchitis, reduced lung function, coughing and chest pains are all health effects of wildfire smoke and bad air quality. And according to the EPA, it often leads to an “increased risk” of emergency room visits. 

You don’t have to be in direct line with the wildfires to have those impacts.

“Pollution from wildfire smoke can rise up to 14 miles into the air and then is carried with wind currents which is why it affects everyone,” Cleveland Clinic pulmonologist Neha Solanki said in 2021. “So even if you don’t live directly near wildfires, you’re still exposed to all of that toxic pollution.”

More than 9.3 million acres have been “charred” by dozens of ongoing fires in Canada, The Weather Channel’s Stephanie Abrams said on “CBS Mornings” Wednesday. And the smoke that has since drifted to the U.S. “could last for a while.” 

“There’s going to be thick smoke pollution at least through Saturday, especially in the Northeast,” she said. 

Cardiovascular issues

Similar to respiratory issues, pre-existing cardiovascular problems also pose a problem when it comes to air quality. Heart failure, heart attacks and strokes are all possible when exposed to poor air quality, even for short amounts of time. Chronic heart issues, such as congestive heart failure and high blood pressure, have been linked to premature death.

Weakened immune system

According to the Cleveland Clinic, there is evidence that smoke inhalation weakens the immune system. 

“We breathe in smoke and it gets into our bloodstream,” Dr. Solanki said. “Then the particles stick to a location in our body and the immune system activates and can create an inflammatory response.”

In 2021, a Harvard study found that thousands of COVID cases and deaths across California, Oregon and Washington could be linked to the increases in air pollution caused by wildfire smoke.

How bad is the air quality? 

On Wednesday, New York City, which typically has a “good” air score on the Air Quality Index, ended up with one of the highest amounts of air pollution across tracked global cities at a level considered “unhealthy” by national standards. 

Much of the Northeast was under that same “unhealthy” level as of Wednesday morning, according to federal tracking, with some areas – including portions of New York, Pennsylvania, New Jersey, Delaware and Maryland hitting “very unhealthy” levels, meaning the general population, not just sensitive groups, is susceptible to health impacts.

Li Cohen

Li Cohen is a social media producer and trending content writer for CBS News.