A “smog day” is a day during which the combination of emissions and weather conditions will cause the formation or the accumulation of high concentrations of ozone or PM2.5 over a wide area, for several hours. Three criteria are used to determine smog days:
Number of smog days due to PM2.5 and ozone, by administrative region, 2004–2016
N/A = not available
Average annual number of smog days by administrative region, 2004–2016
The number of smog days varied from one region to another in 2015; No smog days were recorded in several regions, while a maximum of 8 smog days were recorded in the Montréal region. As is the case each year, cold temperatures impacted the results significantly. Nearly 90% of all smog episodes were observed in winter (December, January, February and March). When comparing 2016 data to that of previous years, we see that the average number of smog days decreased with respect to 2015, which is the best result since 2004. Ozone contributed to a single smog day in Montréal.
The longest smog episode in the 2004–2016 timeframe ran for 9 consecutive days, from January 31 to February 8, 2005, when a stagnant air mass covered most of southern Québec. The highest concentration of PM2.5 during that episode was 117 µg/m3, measured in Montréal. Elsewhere in southern Québec, the highest concentrations of PM2.5 ranged from 52 µg/m3 in Estrie to 102 µg/m3 in the Capitale-Nationale region.
Between 2004 and 2016, the highest levels of PM2.5 that caused smog were of natural origin. There were forest fires in Haute-Mauricie in May and July 2010, which caused concentrations of PM2.5 to reach a maximum of 380 µg/m3.
For more information, see: Poor air quality statistics