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Annual regional air quality index statistics for 2004

In 2004, the AQI was calculated for 18 weather regions and 4 subregions of the Montréal Island. The percentage of days during the year where the air quality index was deemed “good” or “acceptable” was, for most regions, above 91% (approximately 333 days). This was not the case for the regions of Vallée-du-Richelieu and Montréal Island, however, where this percentage was approximately 87% (317 days out of the 363 for which values could have been calculated during the year) and 82% (301 days out of 366) respectively. Air quality was “poor*” in these two sectors most often, with a respective percentage of 13.2% (48 days) and 17.8% (65 days). In terms of hours, these percentages totalled 3.8% (320 hourly indexes out of the 8,382 calculated) and 4% (349 hourly indexes out of 8,781).

The further away a region is from Montréal, the better the air quality, regardless of direction.

* Occurs where the concentration of fine particles is above at least 35 µg/m3 (over a three-hour period) or the hourly ozone concentration is above at least 82 ppb at at least one station in the


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