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.