There was haze over parts of the city Wednesday afternoon as Environment Canada placed Hamilton, Burlington, Ont., and much of the Greater Toronto Area under an air quality warning.

“Stagnant winter weather conditions are creating elevated pollution levels,” the federal agency said in a notice around 4:30 p.m.

Over the last few days, a mass of air has been stagnating over southern Ontario, operational meteorologist Lucas Alexopoulos told CBC Hamilton.

“That’s allowed the pollutant concentration to build up,” he said. “Today it’s reached quite noticeable levels.”

Air quality warnings ‘uncommon’ in winter when air more active

Alexopoulos, who’s in Toronto, said he hadn’t seen any haze but had heard reports of it in some parts of that city.

Mainly, he said, Environment Canada thinks the pollution is made up of fine particulates and nitrogen oxides, which can come from traffic, fires and power plants.

In the winter, these warnings are “uncommon,” Alexopoulos said. That’s because the air is usually more active than other times of year, with things like snow storms.

In Hamilton as of publication, the air quality risk was listed as moderate.

Avoid strenuous outdoor exercise: Environment Canada

When air pollution is high, Environment Canada said, pregnant people, seniors, young children, people with health conditions and those who work outdoors should reduce strenuous activities.

When indoors, close your windows as much as possible, the agency said. It also recommended using good quality air filters in ventilation systems and portable air cleaners.

It’s likely air quality will improve Friday, Alexopoulos said.

He said there’s rain in the forecast “that for sure will help clear it all out.”