WORCESTER — Out-of-control wildfires rage in Canada, and Worcester County’s air quality continues to feel the negative effects. A statewide air-quality alert is in effect until midnight Wednesday, the second straight day that has happened.

Satellite imagery showed heavy, dense smoke lingering over the city Wednesday morning. It’s expected to hover across Southern New England into Thursday, according to a tweet from the National Weather Service in Boston.

Fine particle levels will likely average in the "unhealthy for sensitive groups" range in much of Massachusetts, according to the state Department of Environmental Protection, with the highest readings in western sections of the State where Unhealthy readings are also likely at times.

Sensitive groups, including those with heart and lung disease, should avoid prolonged outdoor activity on Wednesday, warned the National Weather Service.

Why is the smoke lingering?

The reason is an upper-low air pattern that has trapped smoke over much of Massachusetts, said meteorologist Bill Simpson at the National Weather Service. That means air flow that moves west to east is backed up, locking us in a smoky haze. It will run into the weekend.

"It looks like that upper pattern will continue into Saturday,” Simpson said. "In this scenario it's a seven-day pattern. Air usually flows west to east, but it's backed up little before it moves east. That's why it's hung around for so long."

Unhealthy air readings

Meanwhile, Worcester’s air quality is moving in and out of the unhealthy range for sensitive groups, according to IQAir, the world’s largest real-time air quality platform.

• At 7 a.m. Wednesday, the air quality in Worcester was unhealthy for sensitive groups at 119 AQI.

• For comparison, Tuesday’s readings hit a maximum unhealthy level of 177 at 5 p.m.

Last week, Worcester experienced good air quality, except on Wednesday and Thursday, when the reading was at a moderate level.

Tuesday's air-quality alert included advice that sensitive groups cut prolonged or heavy outdoor activities, and watch out for symptoms such as coughing and shortness of breath.

Worcester not alone in air quality problems from wildfires

Heavy smoke from the Canadian fires stretches from the upper Midwest down to the Carolinas.

In total, more than 4 million acres have burned in 2,305 fires in Canada so far this year. As of Wednesday, Canada has 245 wildfires burning out of control, according to the Canadian Interagency Forest Fire Centre, including dozens in Ontario, Quebec and Nova Scotia. Quebec alone has 154 active wildfires.

On its website, the Environmental Protection Agency offers a number of recommendations, including:

• Keep your windows and doors closed.

• If you have an HVAC system with a fresh air intake, set the system to recirculate mode or close the outdoor intake damper.

• If you have an evaporative cooler, avoid using it because it can bring more smoke inside.

• If you have a window air conditioner, close the outdoor air damper. If you can’t close it, do not use the window air conditioner. And make sure the seal between the air conditioner and the window is as tight as possible.

• If you have a portable air conditioner with a single hose (typically vented out of a window), do not use it because it can bring more smoke inside.

• If you have a portable air conditioner with two hoses, make sure that the seal between the window vent kit and the window is as tight as possible.

• Use a portable air cleaner or high-efficiency filter to remove fine particles from the air, and run it on the highest fan speed.

• Avoid activities that create more fine particles indoors: using gas, propane or wood-burning stoves and furnaces; spraying aerosol products; frying or broiling food; burning candles; vacuuming, unless you use a vacuum with a HEPA filter.

The Telegram & Gazette is investigating the effects of a rapidly heating planet on people who live in our city. Follow along with "City on Fire" as we report the struggle with summer temperatures, even in New England. This is part of the USA TODAY project Perilous Course. Contact reporter Henry Schwan to be included in a story if you have been affected by heat: expense of air conditioning or lack of it, health risks, less access to green space, concern about pets and animals in the summer conditions, worry about an older loved one, etc.

Contact Henry Schwan at henry.schwan@telegram.com. Follow him on Twitter: @henrytelegram.

This article originally appeared on Telegram & Gazette: Worcester air quality remains poor with wildfire smoke from Canada