Environment Canada has put in place a smog warning for the greater Montreal area, saying that air quality is especially poor in urban areas where pollutant emissions are higher.

The warning affects Châteauguay, Laval, Longueuil and the island of Montreal.

Simon Legault, a meteorologist with the weather agency, said the main emissions causing the smog come from transportation and wood-burning.

Light wind and cold temperatures allow particles to remain trapped near the earth's surface, resulting in smog, he said.

"It's the time of year where you have the most accumulation of particles."

The weather agency is warning asthmatic children and people with respiratory ailments or heart disease to avoid intense physical activity outdoors until the smog warning is lifted.

"If you have any health considerations regarding your breathing ... just take your time if you need to do something," Legault said.

He said the smog will likely remain over the weekend, though perhaps not consistently.

Residents in the affected areas are asked not to use their wood-burning stoves or fireplaces until the smog warning has been lifted. There is also a city ban on the use of wood-burning appliances during smog warnings.

The smog warning affects the Châteauguay, Laval, Longueuil and Montreal Island regions. (Charles Contant/CBC)

In 2018, the city of Montreal banned the use of fireplace and stove units that emit more than 2.5 grams of fine particles per hour.

"In Quebec, wood heating is the main source of fine particles that contribute to smog during winter," Environment Canada said.