The AQI is a calculated index that provides Ontarians with air quality information in near real-time. This index has its own concentration breakpoints for the descriptive categories (i.e. very good, good, moderate, poor, and very poor). It may be difficult to compare AQI calculations from jurisdiction to jurisdiction across North America because they are not standardized and the formulas involved are complex.
The relationship between AQI and the individual pollutant concentrations are shown below. For each category, a linear relationship is assumed between index values and the concentrations of the sub-index pollutant. At the end of each hour, the concentration of each pollutant measured at individual AQI sites is converted into a number ranging from zero upwards using a common scale or index. The calculated number for each pollutant is referred to as a sub-index. At a given site, the highest sub-index becomes the AQI reading for that hour at that location.
|
Ozone (O3) |
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|
AQI Category |
|
AQI Equation |
| Very Good | 0 to 23 | 0.6520 × [O3] + 0 |
| Good | 24 to 50 | 0.5800 × [O3] + 2.154 |
| Moderate | 51 to 80 | 0.5900 × [O3] + 2.1 |
| Poor | 81 to 149 | 0.7200 × [O3] – 8.37 |
| Very Poor | >149 | 0.7200 × [O3] – 8.37 |
|
Fine Particulate Matter (PM2.5) |
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|
AQI Category |
[PM2.5] 3-hour |
AQI Equation |
| Very Good | <12 | 1.364 × [PM2.5] + 0 |
| Good | 12 to 22 | 1.500 × [PM2.5] – 2.000 |
| Moderate | 23 to 45 | 0.7727 × [PM2.5] + 14.228 |
| Poor | 46 to 90 | 1.113 × [PM2.5] – 1.298 |
| Very Poor | >90 | 1.100 × [PM2.5] + 0 |
|
Sulphur Dioxide (SO2) |
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|
AQI Category |
[SO2] (ppb) |
AQI Equation |
| Very Good | 0 to 79 | 0.1899 × [SO2] + 0 |
| Good | 80 to 169 | 0.1685 × [SO2] + 2.520 |
| Moderate | 170 to 250 | 0.2125 × [SO2] – 4.125 |
| Poor | 251 to 1999 | 0.0280 × [SO2] + 42.97 |
| Very Poor | >1999 | 0.0500 × [SO2] + 0 |
|
Nitrogen Dioxide (NO2) |
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|
AQI Category |
[NO2] (ppb) |
AQI Equation |
| Very Good | 0 to 50 | 0.300 × [NO2] + 0 |
| Good | 51 to 110 | 0.2543 × [NO2] + 3.00 |
| Moderate | 111 to 200 | 0.191 × [NO2] + 10.79 |
| Poor | 201 to 524 | 0.1517 × [NO2] + 19.5 |
| Very Poor | >524 | 0.1903 × [NO2] + 0 |
|
Carbon Monoxide (CO) |
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|
AQI Category |
[CO] (ppm) |
AQI Equation |
| Very Good | 0 to 12.49 | 1.2500 × [CO] + 0 |
| Good | 12.50 to 22.49 | 1.6700 × [CO] – 5.67 |
| Moderate | 22.50 to 30.49 | 2.4300 × [CO] – 23.86 |
| Poor | 30.50 to 49.49 | 2.7200 × [CO] – 34.39 |
| Very Poor | >49.49 | 2.0000 × [CO] + 0 |
|
Total Reduced Sulphur (TRS) |
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|
AQI Category |
[TRS] (ppb) |
AQI Equation |
| Very Good | 0 to 5.49 | 3.0000 × [TRS] + 0 |
| Good | 5.50 to 10.49 | 3.7500 × [TRS] – 6.50 |
| Moderate | 10.50 to 27.49 | 1.0625 × [TRS] + 20.31 |
| Poor | 27.50 to 999.49 | 0.05046 × [TRS] + 48.59 |
| Very Poor | >999.49 | 0.1000 × [TRS] + 0 |
Notes:
- parts per billion (ppb)
- micrograms per cubic metre (µg/m3)
- parts per million (ppm)