A new wave of wildfire smoke from Canada reached the U.S. on Tuesday, triggering air quality alerts for millions of people.
Why it matters: Breathing in the unhealthy levels of smoke and other air pollution can increase a person's risk of developing lung and heart conditions, according to the Environmental Protection Agency (EPA).
- It can also exacerbate pre-existing lung and heart conditions and can trigger asthma and heart attacks, which can be fatal.
- Air pollution contributes to almost 11,000 deaths in the U.S. annually, according to a 2021 estimate, while a 2022 study estimated that it contributed to at least 1.8 million excess deaths in 2019.
How it works: Air pollution is solid and liquid particles, called aerosols, from many different sources — like vehicle exhaust or emissions from industrial facilities — that become suspended in air.
- The main component of wildfire smoke is particulate matter less than 2.5 microns (PM2.5) in diameter, which is far smaller than the diameter of human hair and fine beach sand.
- Particles less than 10 micrometers (PM10) in diameter are the most concerning air pollutant — they can be inhaled into the deepest part of the lungs and can potentially enter a person's bloodstream, according to the EPA and the California Air Resources Board.
By the numbers: PM2.5 and PM10 are one of the five main major pollutants that are measured to determine an area's air quality, which is reported using the EPA's Air Quality Index (AQI).
- An AQI of 0 to 50 is considered "good" air quality, while 51 to 100 is considered "moderate" quality.
- Any value above 100 is considered to be unhealthy, especially for sensitive groups of people, such as those with lung diseases, older adults, children and teenagers and people who are active outdoors.
- At levels between 151 and 300, everyone is recommended to either reduce or avoid intense activities outdoors, while values between 301 and 500 are considered "hazardous" and everyone should avoid all physical activity outdoors.
Driving the news: Canadian wildfires on Tuesday brought a thick haze to the U.S. Midwest that will likely last for a couple of days, according to the National Weather Service.
Threat level: Among the U.S. cities with the poorest air quality Tuesday were Grand Rapids, Michigan; Milwaukee; Chicago; and Minneapolis, per AirNow.
The big picture: The U.S. National Weather Service had issued air quality alerts for a swath of the northeast and parts of the East Coast earlier this month, including over all of Massachusetts, Rhode Island, Connecticut, Pennsylvania, Delaware, Maryland and North Carolina.
- The service's Chicago office recommended people with chronic respiratory issues to limit time outdoors.
- The American Veterinary Medical Association recommends that pet owners keep animals indoors as much as possible and keep windows shut during periods of poor air quality, as they are also affected by air pollution — especially birds.
Go deeper: Smoke from Canadian wildfires leads to record poor air quality in East U.S.
Editor’s note: This story has been updated with additional developments.