The AQI levels are as follows:
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.
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
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).
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.
Air purifiers are rated by CADR (Clean Air Delivery Rate) or m3/h (cubic meters per hours). These values are used together with the area/volume of your room to figure out how many times per hour the purifier filters all your air, a number often called the ACH (Air Changes per Hour).
You want your purifier to filter all the air in your room at least 3 times per hour, though 5+ times is ideal. I have created a simple Air Purifier Power Calculator which can help you figure out what CADR (or m3/h) rating works best for your room/house.
Consumer air quality monitors are nowhere near as accurate as lab equipment, but they can still be useful to get a sense of how bad it is, and to measure changes in conditions, such as when more smoke rolls in, or you've set up a new purifier in your home.
I personally use the Temtop M10 Air Quality Monitor, and have found it to be convenient, portable, and accurate enough for personal use. When I turn on my purifier in my bedroom, for example, the numbers dropped from 600 (outside) to about 5 over the course of 25 minutes. (See this tweet for an example, tracking incoming smoke.)
Yes! The PurpleAir service makes air quality sensors you can install around your home. Data from here shows up on their map in real-time, available to all. If folks all around the ACT did this, we could have accurate AQI numbers for the entire state!
The Aqicn network also has some good monitoring stations you can set up, though there aren't any running at present in the ACT, maybe you can be the first!