Overall, the number of days when the air was considered "poor" and dangerous to public health jumped 21 per cent from last year.
The number of days when the air quality was deemed "acceptable" remained stable, while the number where the air was deemed "good" dropped. The number of days when it was poor rose from 53 in 2013 to 64 in 2014.
Until 2014, air quality in the city had actually been improving over the past decade.
André Bélisle, head of the Quebec Association against Air Pollution, said the 2013-2014 winter — the coldest in 20 years — is likely a major reason for the reversal.
"The colder it is, the more people tend to use wood to heat their homes,” Bélisle said.
Other factors may include an increase in traffic as well as the mixture of chemicals used to keep roads free from ice and snow. According to Bélisle, those particles can be swept into the air in dry and windy conditions.
Karine Price, a toxicologist at the Public Health Department of Montreal, said the findings are troubling.
"When we talk about people more vulnerable to the effects of pollution, we talk about older people, people with chronic diseases, such as heart and lung diseases, as well as children with asthma," she said.