Wildfire smoke leads to hazy skies, air quality questions in Chicago area. What to know

Hazy skies are expected in the Chicago area starting Friday and continuing into the weekend, but should you be concerned?

As of 1 p.m. Friday air quality levels remained in the "moderate" category in Chicago, with the city's PM2.5 -- an air pollutant also known as fine particulate matter -- sitting at around 67, according to AirNow. An "unhealthy for sensitive groups" level is reached when the PM2.5 rises above 100.

"If you are unusually sensitive to particle pollution, consider reducing your activity level or shorten the amount of time you are active outdoors," the government website states at the city's current level.

Hazy conditions were reported in and around the city, leading to concerns as residents may recall a series of previous alerts for unhealthy conditions numerous times this summer due to Canadian wildfires. And the worst fire season on record in Canada shows no signs of easing.

According to NBC 5 Storm Team Meteorologist Kevin Jeanes, the hazy conditions are still "due to the return of wildfire smoke and haze."

"The smoke isn’t too dense and most of it is expected to stay up in the atmosphere rather than getting mixed down to the ground," Jeanes said, noting, however, that unhealthy levels could still be reported during the afternoon hours.

Nearby, all of Wisconsin is under an air quality advisory.

"Canadian wildfire smoke, although less dense and delayed in arrival, is moving into the state from the northwest and will travel south southeast [Thursday night] into Friday morning," the advisory stated.

The National Weather Service predicts levels could become "unhealthy for sensitive groups" over the weekend "due to favorable weather conditions alongside the presence of wildfire smoke."

The haze could linger in the Chicago area into Saturday as well, but is expected to clear up Saturday evening and into the overnight hours, Jeanes said.

Meanwhile, the city will usher in a potentially dangerous heat wave, which could lead to different air quality concerns.

"While the smoke may no longer be an issue, higher levels of ozone could," Jeanes said. "I wouldn’t be surprised if we get some air quality alerts for ozone Sunday and again around Wednesday and Thursday next week."

Canada has seen a record number of wildfires this year — contributing to choking smoke in parts of the U.S. — with more than 5,700 fires burning more than 53,000 square miles from one end of Canada to the other, according to the Canadian Interagency Forest Fire Centre.

As of Friday morning, more than 1,000 wildfires were burning across the country, over half of them out of control.

What is an unhealthy air quality level?

AirNow said its air quality index determines the level of air pollution and the correlating health concerns.

"When AQI values are above 100, air quality is unhealthy: at first for certain sensitive groups of people, then for everyone as AQI values get higher," the website states.

Once levels reach above 300, the area enters the highest level of concern known as "hazardous."

In total, there are six categories: green, or good; yellow, or moderate; orange, or unhealthy for sensitive groups; red, or unhealthy; purple, or very unhealthy; and maroon, or hazardous.

Daily AQI Color	Levels of Concern	Values of Index	Description of Air Quality
Green	Good	0 to 50	Air quality is satisfactory, and air pollution poses little or no risk.
Yellow	Moderate	51 to 100	Air quality is acceptable. However, there may be a risk for some people, particularly those who are unusually sensitive to air pollution.
Orange	Unhealthy for Sensitive Groups	101 to 150	Members of sensitive groups may experience health effects. The general public is less likely to be affected.
Red	Unhealthy	151 to 200	Some members of the general public may experience health effects; members of sensitive groups may experience more serious health effects.
Purple	Very Unhealthy	201 to 300	Health alert: The risk of health effects is increased for everyone.
Maroon	Hazardous	301 and higher	Health warning of emergency conditions: everyone is more likely to be affected.

What is PM2.5?

According to the Environmental Protection Agency, "PM stands for particulate matter (also called particle pollution): the term for a mixture of solid particles and liquid droplets found in the air."

"Some particles, such as dust, dirt, soot, or smoke, are large or dark enough to be seen with the naked eye. Others are so small they can only be detected using an electron microscope," the EPA states.

PM2.5 in particular involves "fine inhalable particles, with diameters that are generally 2.5 micrometers and smaller." By comparison, the average human hair strand is about 70 micrometers in diameter, or 30 times larger than these particles.

PM2.5 is one of five major air pollutants regulated by the Clean Air Act, which also includes ground-level ozone, carbon monoxide, sulfur dioxide and nitrogen dioxide.