Poor air quality days
A poor air quality day occurs when air quality is poor for at least one hour at a minimum of one measuring station. The hourly index is deemed “poor” when the average concentration of fine particulates over a three-hour period exceeds 35 micrograms per cubic metre (µg/m3) or when the average concentration of ozone over a one-hour period exceeds 82 parts per billion (ppb). This indicator is calculated by administrative region and takes account of both intense regional smog days and days when poor air quality is observed locally over a short period of time.
Average annual number of poor air quality days by administrative region* (2004–2019)
*Based on daily occurrence of at least one hourly concentration of ozone or fine particulates (averaged over three hours) that respectively exceed 82 ppb and 35 µg/m3 at at least one station in the network in each administrative region.
The average number of poor air quality days by administrative region increased in 2019, while only the Capitale-Nationale, Mauricie, Montréal and Abitibi-Témiscamingue exceeded the 23.5 average number of poor air quality days.
Weather conditions impact the dispersion of contaminants, which explains the variation in the number of poor air quality days between years. Nonetheless, the downward trend observed since 2004 suggests that anthropic factors such as legislation, more efficient antipollution systems and the closure of polluting industries (among others) also play a role. Fine particulates are the main cause of poor air quality days. Ozone caused none in 2019, which is consistent with observations made in recent years.
Number of poor quality air days in 2019 per station*, grouped by administrative region
*Based on at least one daily occurrence of an hourly concentration of ozone or fine particulates (averaged over three hours) that respectively exceed 82 ppb and 35 µg/m3.
The number of poor air quality days by station determines which stations have the greatest weight in the statistic. The highest numbers were observed at urban stations in the Capitale-Nationale, Mauricie, Montréal and Abitibi-Témiscamingue regions and is due to the greater density of polluting emission sources (transport, wood heating and industry) in urban settings. Some stations, such as the one in Témiscaming, stand apart for very specific reasons. In this case, the station is close to an industrial source whose emissions directly influence measurements, thereby explaining the elevated number of poor air quality days. In rural settings, poor air quality days are in general rarer, due to the fact that measuring stations are set up far from sources of pollution.