Air Quality Online
- Table of Contents
- Air Quality Introduction
- Air Quality Sub-Indexes and Odour Index
An Air Quality Index (AQI) is a way of transforming complex air quality
measurements into a single number or descriptive term. It describes both the
measured air quality and the publicly perceived air quality at any given time.
The British Columbia AQI are directly comparable to AQI's issued
in all major Canadian cities as we follow the same Federal guidelines.
How is an AQI interpreted?
The AQI numbers are interpreted thus: 0 to 25 is GOOD, 26 to 50 is FAIR, 51 to 100 is POOR, and 100+ is VERY POOR. An AQI in excess of 50 represents the point at which BC Environment normally becomes concerned about the level of human health impact.
Why issue an Air Quality Index?
The main purpose is to inform the public about the present state of air
quality. Having a daily Air Quality Index available via the media builds an
awareness in the public, who will for the first time be able to draw
comparisons between the measured air quality and what they directly sense. For
asthmatics and other concerned residents it may even influence their behavior.
For example, in a situation where the AQI is high (above 50) those with
respiratory problems may choose to refrain from strenuous exercise or
temporarily avoid the polluted region.
How is an AQI calculated?
Common air pollutants are measured continuously at state-of-the-art monitoring stations throughout British Columbia. This information is gathered by a central computer in Victoria and an hourly AQI value is calculated. The AQI is not an amalgamation of all measured pollutants, but rather the value of the pollutant with the highest AQI number. Every hour, each pollutants concentration is converted into an AQI number according to a methodology developed by Environment Canada and the Provincial Environment Ministries. The highest AQI number becomes that hours overall AQI value. When issuing the AQI it is common practice to note which pollutant is "driving" the AQI.
The AQI for a given concentration of each pollutant is determined from a set of straight line interpolations which start at zero and go through break-points at AQI values of 25, 50 and 100. These break-points represent the limits of good, fair and poor air quality. The ambient values of the break-points for the different pollutants measured over a set of averaging periods are as follows:
| Parameter | Sulphur Dioxide | Carbon Monoxide | Nitrogen Dioxide | Ozone | Particulates
<10 micrometers | Particulates
<2.5 micrometers |
|---|
| Averaging Time | 1 Hour | 24 Hour | 1 Hour | 8 Hour | 1 Hour | 1 Hour | 24 Hour | 24 Hour |
|---|
| Unit of Measure | ppm | ppm | ppm | ppm | ppm | ppm | ug/m3 | ug/m3 |
|---|
| Break-point: |
|---|
| AQI = 25 | 0.17 | 0.06 | 13 | 5.0 | 0.105 | 0.05 | 25 | 15 |
|---|
| AQI = 50 | 0.34 | 0.11 | 30 | 11.0 | 0.210 | 0.08 | 50 | 25 |
|---|
| AQI = 100 | 2.00 | 0.30 | 64 | 17.4 | 0.530 | 0.15 | 100 | 50 |
|---|
Particulate Matter (PM) refers to airborne solid and liquid particles. Our measurements focus on the microscopic particles that are small enough to be inhaled into the lungs. This includes PM10, which consists of particles 10 micrometres or smaller (the "inhalable" fraction), and PM2.5, consisting of particles 2.5 micrometres or smaller (the "respirable" or "fine" fraction).
PM10 is emitted from various natural and human-caused activities. The fine fraction -- PM2.5 - is largely attributed to combustion sources, including diesel vehicles and biomass burning. PM2.5 is also produced in the atmosphere from reactions involving gases such as sulphur dioxide, nitrogen oxides, hydrocarbons and ammonia. In contrast, the coarse fraction, consisting of particles between 2.5 and 10 micrometres diameter, are generally associated with crustal materials, such as fugitive dust.
Both PM10 and PM2.5 are associated with a broad range of health effects, predominantly to the lung and heart. However, current research indicates that PM2.5 is more closely linked to health impacts, including reduced lung function, increased respiratory symptoms, increased hospitalizations and increased risk of premature death from health and lung diseases. Those people most sensitive include children, the elderly, and those with pre-existing lung and heart disease.
Although PM2.5 is considered a better indicator of potential health impacts than PM10, the latter is retained in the index formulation as it provides a useful indicator of PM2.5 where these measurements are not available, and also a useful indicator of dust events (such as occur in the spring time in many interior communities) that may affect the health of some individuals.
The Odour Index is calculated separately
from the Air Quality Index, and is used to indicate the presence of
pollutants that aside from their odour, do not at low concentrations pose
a concern to health or the environment. Pollutants which create
an odour problem are generally comprised of compounds which contain sulphur
in its reduced chemical state. An Odour Index of 26 or greater indicates that
reduced sulphur compounds are present in a total concentration
greater than the "Desirable" Provincial Ambient Level. An Index
of 51 or greater indicates concentrations in excess of the
"Interim" Provincial Ambient Level.
The Odour Index for a given Reduced Sulphur concentration is determined from a set of linear interpolations starting at zero and having break-points at values of 25, 50 and 100. The ambient values of the break-points concentrations measured over 1 hour and 24 hour averaging periods are as follows:
| Parameter | Total Reduced Sulphur |
|---|
| Averaging Time | 1 Hour | 24 Hour |
|---|
| Unit of Measure | ppm | ppm |
|---|
| Break-point: |
|---|
| Odour Index = 25 | 0.005 | 0.002 |
|---|
| Odour Index = 50 | 0.020 | 0.004 |
|---|
| Odour Index = 100 | 0.050 | 0.006 |
|---|
