Air Quality Index
What is the Air Quality Index?
The Nova Scotia Air Quality Index (AQI) is a numeric scale of air quality based on hourly measurements of some or all of the following air pollutants: ground-level ozone (O3), sulphur dioxide (SO2), nitrogen dioxide (NO2) and carbon monoxide (CO).
The AQI increases as air quality worsens. The AQI decreases as air quality improves. The AQI value is linked to one of four air quality categories: "good", "fair", "poor", "very poor". Environment Canada provides health information associated with each category as outlined below:
| Category | Colour | AQI | Health Information |
|---|---|---|---|
| Good | green | 0-25 | The air quality is considered relatively good. |
| Fair | yellow | 26-50 | There may be some adverse effects on very sensitive people. |
| Poor | orange | 51-100 | There may be some short-term adverse effects on the human or animal populations, or may cause significant damage to vegetation and property. |
| Very Poor | red | 100+ | There may be some adverse effects on a large proportion of those exposed. |
How the AQI is determined - an example
Each hour, pollutant readings are collected from each of our monitoring stations (PDF: 523k). A sub-index of air quality is calculated for each pollutant and the pollutant with the highest sub-index is used to describe the air quality at that station for that hour.
| Pollutant (hourly average) |
SO2 (1hr) |
SO2 (24hr) |
NO2 (1hr) |
O3 (1hr) |
CO (1hr) |
CO (8hr) |
|---|---|---|---|---|---|---|
| Pollutant Readings | 0.8 | 0.78 | n/a | 57 | 0.9 | 0.86 |
| (units of measure) | pphm | pphm | ppb | ppb | ppm | ppm |
| Calculated Air Quality Sub-indicies | 1 | 3 | n/a | 30 | 1 | 4 |
| Greatest Air Quality Sub-index |  |
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Legend
The greatest of the six calculated sub-indices becomes the AQI. In this example, the greatest sub-index is for O3 at 30. Therefore, the AQI is 30 as determined by O3.
How can you use the AQI?
You can use the AQI to help make decisions about your daily activities by exercising and enjoying time outdoors when the AQI is good, or by taking protective action for yourself and those under your care when the AQI level increases.
You can access the Air Quality Index by calling 902-424-2775 or by visiting Environment Canada's Air Quality website. In the future we will be reporting the current AQI from this page of our website.
Limitations of the AQI
Given the large number of pollution sources and the limited number of monitoring stations in the province, the AQI may not always reflect local conditions and personal exposure.
Some monitoring stations are not measuring all of the pollutants currently included in the AQI (O3, SO2, NO2 and CO). We have plans to improve monitoring by adding more equipment to our stations. Our stations are currently monitoring the following AQI pollutants:
| Monitoring station | Pollutants monitored |
|---|---|
| Aylesford | O3 |
| Dayton | O3 |
| Halifax | O3,SO2,NO2,CO |
| Kejimkujik National Park | O3 |
| Kentville | O3 |
| Pictou | O3 |
| Port Hawkesbury | SO2 |
| Sable Island | O3,NO2 |
| Sydney | O3,SO2 |
The AQI does not consider the additive effects of air pollutants. Rather, it is based on only the highest pollutant. This is not just an issue in Nova Scotia, but across the country. We are participating in work being done in Canada to develop a new index that will reflect the additive effects of pollutants including O3, SO2, NO2, and fine particulate matter (PM2.5) on human health.
The AQI does not currently include PM2.5. Since the impact of PM2.5 on health and air quality has recently been recognized, we are working to include this pollutant in our AQI.
It is difficult to compare AQI values in Nova Scotia to AQI values from areas outside the province, even within Canada. Even though the name is the same, indices of air quality may be calculated differently in other jurisdictions. There may be differences in the types of pollutants that are included, the averaging periods used, and the thresholds of acceptable pollutant concentrations. Work is being done to have a common index that can be compared across Canada, to address the limitations of the current AQI, and to improve air quality reporting in Nova Scotia.
The AQI is based on raw data that are automatically collected from monitoring stations every hour. These raw data have not undergone quality control and quality assurance procedures and may contain errors due to instrument malfunctions, connection problems, QA/QC requirements, instrument calibration times and power failures. The raw data should not be used for analysis or in published documents.
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