Millions of Americans have been urged to stay indoors and close their windows as a thick smoky haze blankets large swaths of the US.
The National Weather Service has placed 12 states in the Midwest and Northeast under an orange air quality alert today, warning outdoor exercise could cause coughing, eye irritation or breathing problems.
In northern Wisconsin, a red alert is also in effect, with officials urging everyone to remain indoors where and when possible.
The alerts were issued after wildfire smoke blew across the border from Canada, which is experiencing its second worst fire season on record. More than 4,000 blazes have been recorded so far, with around 700 still burning.
The orange alert is aimed at sensitive groups, such as very young children, older adults or those with underlying conditions such as asthma or other respiratory diseases.
But officials at the NWS told DailyMail.com that even healthy adults in affected states should consider limiting outdoor activities.
Breathing in smoke from wildfires can irritate the airways and cause trouble breathing. In serious cases, it can also raise the levels of inflammation in the blood and cause clots to form that may block a blood vessel and cause a stroke or heart attack.
Warnings over wildfire smoke are expected to remain in place through Monday and into Tuesday, with officials saying they could be extended to the end of the week.
Mac Bhenard, a lead forecaster at the NWS, told this website: ‘The smoke is probably going to still be around today or tomorrow… and it does look like some smoke may be present through to the end of the week.’
He added: ‘The orange alerts pretty much just recommend that people should consider limiting strenuous outdoor physical activity.
‘That typically includes those in the most sensitive groups, such as very young people and those with pre-existing respiratory problems. But, in general, the recommendation for everyone is to limit strenuous outdoor activities if you can.
‘Closing windows may also be a good idea. Trying to preserve indoor air quality, that certainly can’t hurt.’
The states under orange air quality alerts are: Delaware, Maine, Massachusetts, Michigan, Minnesota, New Hampshire, New York, Vermont and Wisconsin. There are also orange alerts covering parts of Indiana, Ohio, Pennsylvania and Vermont.
The red alert covered northern Wisconsin.
Over the weekend, parts of Illinois were also under warnings because of the smoke, although that alert is now no longer in place.
In the affected states, the Air Quality Index, a measure of pollution in the air, has surged to between 101 and 150, the weather service says. In northern Wisconsin, it is between 151 and 200.
A healthy level is between 0 to 50, while an acceptable level is between 51 and 100. Most of the US is typically in the healthy, or green, level.
