The Air Quality Index (AQI) is a scale of air pollution that gives Canberrans an indication of how clean the air is so we can change our outdoor activities if pollution levels are high. The AQI levels are as follows:

0-33

Very Good ● 34-66

Good ● 67-99

Fair ● 100-149

Poor ● 150-200

Very Poor ● 200+

Hazardous ●

A general term for any atmospheric particle smaller than 2.5 micrometers (μm) in diameter (3% diameter of a human hair). Bushfire smoke contains large amounts of PM2.5. When reports say that "AQI is really high" or "Air quality is terrible", they're referring primarily to the high PM2.5 concentration. An excellent further reading: What is PM2.5 and Why You Should Care

They contain all sorts of toxins, and are so small that they can penetrate deep into the blood stream and all your body's tissues. Unlike large particles, your body has a much harder time filtering these particles out. Consistent high levels of PM2.5 exposure have been linked to a large number of health conditions, including respiratory trouble, heart disease, depression, and cancer.

Anyone with a respiratory condition, children, the elderly, pregnant women.

Absolutely not. Everyone is harmed by smoke pollution. Even if you don't have any immediate symptoms, prolonged expsoure can cause serious long-term health problems.

Not necessarily. PM2.5 particles are so small that they are difficult to smell, or see in the air. I have measured hazardous levels of PM2.5 in buildings which look and smell clear.

Again, not necessarily. Some aircon systems filter out larger (e.g. PM10) particles, and cooler air can often feel nicer. Unless their aircon has the right kind of filter, the building may still have toxic levels of PM2.5. (My local gym had this exact problem. People should not work out in such conditions.)

Air particles are measured in terms of "micrograms per cubic meter of air" (µg/m3). AQI is a normalized score designed to show the health impact of the current air, and is calculated by taking the maximum of each individual AQI produced for each possible pollutant (there are many other pollutants, such as PM10, VOCs, CO, etc.). Each pollutant uses a different calculation to produce the final individual AQI value (based on concentration and estimated negative health effects of that pollutant). In the ACT, AQI for PM2.5 is calculated by multiplying the actual particle readings by 4. So a 50 ug/m3 particle concentration would yield an AQI of 200 (Hazardous). Since PM2.5 is the primary pollutant produced by bushfire smoke, during the bushfire crisis the reported AQI value will almost always be based on the PM2.5 levels. During dust storms, for example, the pollutant with the highest AQI is usually PM10, so the final reported AQI will be based off the individual PM10 AQI score (which is calculated based off of the µg/m3 concentration, but using a different formula from the PM2.5 calculation).

There are several reasons for this: Even though the actual particle concentration is the same, different groups multiply this number by different amounts to derive their AQI score. Because of this, you can not compare AQI scores between apps or organizations. Instead you need to get that original PM2.5 concentration number. Many apps pull from the publicly available data on the ACT Health Air Quality page. This data is a 24-hour rolling average of PM2.5 readings, and does not reflect the very latest air quality readings. It can take 12+ hours for the ACT Health numbers to reflect the actual outdoor air quality. Our numbers at the top of this page do not have this problem, and are accurate to the latest hour.

I've had many folks write in to point this out. While they say their PM2.5 readings are hourly, they are actually the 24 hour rolling average. You can verify this yourself by multiplying their PM2.5 numbers by 4, and getting the exact "24 hour rolling average AQI" value (ie. PM2.5 * 4 = AQI_PM2.5). It took me quite a while to get the real hourly data. (I asked them directly, and they declined to publish it themselves.) Update: The ACT Health team has started publishing actual hourly numbers on this page. The data are accurate, and timely.

We take data from PurpleAir monitoring stations, which update in real time. Because these stations use many different kinds of sensors, the data from them are not directly comparable with the sensors used by ACT Health. They are, however, reasonably close, and can give you a rapid view on what the air quality is doing right now.