A smog day is a day in which air emissions and weather conditions result in the formation of high concentrations of fine particulates and/or ozone over a large area, lasting many hours. The following three criteria are used to define and determine smog days:
Number of fog days caused by fine particulates and ozone, by administrative region (2004–2019)
ND = no data
Average annual number of smog days by administrative region (2004–2019)
In 2019, the number of smog days varied by region, with none recorded in many regions and up to a maximum of six days in the Montréal and Mauricie regions. Forest fires remained impactful, causing July smog episodes in the Mauricie, Estrie, Montréal, Abitibi-Témiscamingue and Centre-du-Québec regions.
As is the case each year, cold temperatures had a major impact on results, with more than 70% of smog episodes observed in the winter months (December, January, February and March). If we compare 2019 data to figures for earlier years, we see that the average number of smog days (2.1) was among the lowest measured since 2004. Ozone was not a contributing smog day factor.
The longest smog episode over the 2004–2019 period lasted nine consecutive days, running from January 31 to February 8, during which atmospheric stagnation affected a large part of southern Québec. That single event caused the average number of smog days by region to rise to 18.8 in 2005 (nine times greater than in 2019). The highest concentration of fine particulates measured during the episode was 117 µg/m3, in Montréal. Elsewhere in southern Québec, the highest concentrations of fine particulates varied between 52 µg/m3 (Estrie) and 102 µg/m3 (Capitale-Nationale).
Learn more at Poor air quality statistics.