Air quality alerts remain in effect in 19 states Friday afternoon, but that number will likely drop throughout the weekend.

Air quality will gradually improve during the holiday weekend as the wildfire smoke clears out of the eastern U.S. But with no end in sight to the wildfires that have burned a record of 20 million acres across Canada so far this year, the smokey skies are likely to return.

The Centers for Disease Control and Prevention on Friday issued a health alert on wildfire smoke exposure.

People are urged to watch out for symptoms including: headache, eye irritation, trouble breathing, coughing, wheezing, chest pain, palpitations and fatigue.

The smoke on Friday is stretching from the eastern Great Lakes to the Interstate 95 travel corridor and down to parts of the Southeast.

The Air Quality Index in New York City hit 169 Thursday afternoon, which falls under the "unhealthy" category. Any number over 100 is considered unhealthy for sensitive groups.

At least 20 U.S. states are under air quality alerts on Friday morning as Canadian wildfire smoke lingers in the East for another day before it's forecast to dissipate over the weekend.

The thickest smoke on Friday will stretch from the eastern Great Lakes to the Interstate 95 travel corridor and down to parts of the Southeast.

Skies will be clearer by Saturday, but some light haze could still hang over the East Coast.

Air quality alerts are in effect in 21 states Thursday afternoon as the Canadian wildfire smoke infiltrates the Midwest, Northeast and parts of the South.

Heavy blankets of smoke billowing over the United States from wildfires burning in neighboring Canada are threatening the health of millions of people -- even non-vulnerable populations with no preexisting conditions. But what about the smoke makes it so hazardous for humans to be around?

Wildfire smoke contains fine particulate matter, known as PM2.5, which are microscopic solid or liquid droplets -- often 30 times smaller than the diameter of a human hair -- that can be inhaled and cause serious health problems, according to the U.S. Environmental Protection Agency.

Most particles form in the atmosphere as a result of complex reactions of chemicals, such as sulfur dioxide and nitrogen oxides -- pollutants emitted from power plants, industries and automobiles. But wildfires likely contain PM2.5 that is up to 10 times more harmful than the same type of air pollution coming from combustion activity, according to a 2021 study conducted in California.

PM2.5 is considered unhealthy for "Code Orange" and sensitive groups once the Air Quality Index surpasses 100, according to AirNow, a website that publishes air quality data. Once the AQI surpasses 150, it is considered "Code Red," unhealthy for some members of the general public who may experience health effects, with sensitive groups experiencing more severe effects.

The AQI is at "Code Purple" once it surpasses 200, considered "very unhealthy" with increased health risk for all populations. "Code Maroon" is labeled as "hazardous" and a health warning for emergency conditions once the AQI reaches 300 and higher.

At Code Maroon, "everyone is more likely to be affected," according to AirNow.

A "good" AQI is measured at 50 and below, and a "moderate" air quality index ranks between 51 and 100.

Read more here.

-ABC News’ Julia Jacobo