This classification system was established by Ministry of the Environment, Conservation and Parks (MECP) scientists to help you gain a general understanding of how hourly air contaminant concentrations compare to MECP criteria.
The classification system is primarily based on the MECP’s Ambient Air Quality Criteria (AAQC). For pollutants that do not have 1-hour AAQC, comparable reference values have been derived.
A detailed breakdown of the concentrations associated with the Good, Moderate, Poor, and Very Poor categories for each pollutant is provided below.
Common Air Pollutants
The classification for the common air pollutants (NO2, O3, PM2.5, SO2, and TRS) is based on Ontario’s Air Quality Health Index category divisions and the former Air Quality Index category names.
CASA Classification System for 1-Hour Concentrations1
Common Air Pollutants
|
Pollutant
|
Units
|
1-hour AAQC
|
Good
|
Moderate
|
Poor
|
Very Poor
|
NO2
|
ppb
|
200
|
0 – 110
|
110 – 200
|
200 – 524
|
>524
|
O3
|
ppb
|
80
|
0 – 50
|
50 – 80
|
80 – 149
|
>149
|
PM2.51
|
µg/m3
|
-
|
0 – 22
|
22 – 45
|
45 – 90
|
>90
|
SO2
|
ppb
|
40
|
0 – 39
|
40 – 119
|
119 – 249
|
>250
|
TRS
|
ppb
|
-
|
0 – 10
|
10 – 28
|
28 – 999
|
>999
|
1The PM2.5 classification is based on the 3-hour running average (the average concentration over the previous 3 hours).
Notes
Total Reduced Sulphur (TRS) compounds at very low concentrations have an unpleasant, pungent odour and are not typically considered a health hazard. The categories for TRS do not consider the potential for impacts due to odour. It is possible for TRS concentrations to be above the odour threshold, but fall within the Good classification.
Volatile Organic Compounds
Some CASA stations monitor volatile organic compounds (VOC) on an hourly basis, including benzene, 1,3-butadiene, and ethylene. 1-hour AAQC do not exist for these compounds, so comparable reference values have been derived.
The classification system is based on a conversion of the 24-hour AAQC for these compounds to a 1-hour equivalent value, using a formula described in Ontario Regulation 419/05. The divisions between the categories Good, Moderate, Poor, and Very Poor are set at 1, 10, and 100 times the 1-hour equivalent AAQC.
CASA Classification System for 1-Hour Concentrations Volatile Organic Compounds
|
Pollutant
|
Units
|
24-hour AAQC
|
1-hour AAQC equivalent
|
Good
|
Moderate
|
Poor
|
Very Poor
|
1,3-butadiene
|
µg/m3
|
10
|
24
|
0 – 24
|
24 – 240
|
240 – 2400
|
>2400
|
Benzene
|
µg/m3
|
2.3
|
5.6
|
0 – 5.6
|
5.6 – 56
|
56 – 560
|
>560
|
Ethylene
|
ppb
|
33
|
80
|
0 – 80
|
80 – 800
|
800 – 8000
|
>8000
|
Notes
The AAQC for 1,3-butadiene and benzene were derived from cancer risk levels based on continuous lifetime exposure. The AAQCs can be converted to 1-hour equivalent values for informational purposes, and as an indicator of whether the longer term AAQC may be exceeded. An exceedance of the AAQC 1-hour equivalent does not mean that cancer risks will increase unless the frequency of the exceedance and the magnitude above the AAQC is experienced over a long term.
The AAQC for ethylene is based on vegetation effects. An exceedance of the AAQC 1-hour equivalent does not mean that effects will occur unless the frequency of exceedance and the magnitude above the AAQC is experienced over several (2-3) days.