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 issued an orange air quality alert for 12 states in the Midwest and Northeast today, advising that outdoor exercise may lead to coughing, eye irritation, or breathing issues.

In northern Wisconsin, a red alert is also in effect, with officials urging everyone to remain indoors where and when possible.

This alert follows the influx of wildfire smoke from Canada, which is currently facing its second most severe fire season to date. Over 4,000 fires have been documented, with approximately 700 still active.

The orange alert specifically targets sensitive groups, including young children, elderly individuals, and those with health conditions like asthma or other respiratory illnesses.

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. 

The above map shows the areas under a yellow, orange or red alert for air quality. Officials say orange and red alerts indicate outdoor exercise should be limited

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.

Shown above is New York City Monday. A haze is visible on the skyline and in the background, which is the wildfire smoke blanketing the US

A woman is pictured above photographing the haze covering the skyline of Toronto, Canada, over the weekend, where wildfires are also polluting the air and prompting warnings

The uptick is driven by surging levels of particulate matter 2.5 (PM2.5), or particles suspended in the air that measure less than 2.5 micrometers, or about a 10,000th of an inch.

These particles can be released by wildfires and then breathed into the lungs, where they penetrate deep into tissue and cause inflammation leading to coughing and shortness of breath. They can also penetrate the lining of the eyes, causing irritation.

The NWS also published yellow alerts for states neighboring those under orange advisories, warning of elevated levels of haze in the air.

These states include Connecticut, Rhode Island, New Jersey and Iowa.

Pollution in these areas is too low to cause complications for most people, but those with serious asthma or heart or lung complications could still be at risk.

Many states had their alerts first issued over the weekend. 

In Minnesota, officials say that their air quality alert, which started on Saturday, could be the state’s longest-running since 2008 when the alert lasted for seven days.

The state’s Pollution Control Agency warned that even healthy people may experience symptoms as a result of breathing in the polluted air.

The blazes in Canada are mostly in Manitoba, a province that lies just north of Minnesota, with winds then blowing the smoke south.

Most of Canada is also under air quality alerts because of the pollution, including in Montreal, which was shrouded in smoke over the weekend.