How Air Quality Index Numbers Are Calculated
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An air quality index takes readings for several pollutants, such as fine particles, ozone and nitrogen dioxide, and converts each into a common scale. Whichever pollutant scores worst on a given day becomes the number the public sees, which is why the reading can shift sharply between neighbouring suburbs or from one hour to the next.
Behind the single figure sit health based thresholds. Agencies build these using long term studies linking pollutant concentrations to respiratory and cardiovascular symptoms, then group results into plain bands such as good, moderate and poor, so the outcome is something anyone can read at a glance rather than a chemistry lesson.
The index is a snapshot, not a forecast. It reflects conditions at a monitoring station over the recent hour or day, so it can lag behind fast changing events like a wind shift or nearby smoke. Checking it more than once a day gives a truer picture of what a household is actually breathing.