What are one-hour and 24-hour averages?

Environment Protection Authority Victoria (EPA) continually monitors the air for PM2.5 at a number of locations around the state. EPA shows two types of PM2.5 data on EPA AirWatch: one-hour averages and 24-hour rolling averages.

PM2.5 data is displayed on EPA AirWatch using two different systems of categories: air quality categories and health categories.

Air quality categories use five colours to indicate how high or low pollutant levels are. The categories range from ‘very good’ (green) through to ‘very poor’ (black). The levels of each pollutant are compared to the corresponding ambient air standards. Air quality categories are not linked to cautionary health advice.

The health categories are a system of seven categories for smoky air (PDF 120KB; Emergency Management Victoria) that were developed by EPA and the Department of Health and Human Services (DHHS). Founded on international research, the categories are based on the concentrations of PM2.5 in the air. Cautionary health advice is automatically shown when PM2.5 levels reach a set threshold.

EPA displays one-hour averages using air quality categories.

EPA displays 24-hour rolling averages using either air quality categories or health categories. The categories shown on EPA AirWatch depend on the type of air monitor used to collect the data:

Both one-hour and 24-hour averages are important and provide different information about air quality.

One-hour average 24-hour rolling average
What is it? The average PM2.5 levels over a one-hour period. An average of the hourly readings of PM2.5 levels over the previous 24-hour period.
How often is the average calculated and updated on EPA AirWatch? Every hour. The previous 24 one-hour readings are averaged each hour.
What does it say about air quality? It tells us about air quality over the previous one-hour period. It tells us about air quality over the previous 24 hours.
Does this type of data automatically trigger cautionary health advice on EPA AirWatch? No.

Yes, but only for data collected by air monitors that meet Australian Standards.

Low and moderate levels of PM2.5 do not trigger cautionary health advice.

Why isn’t cautionary health advice shown for all types of PM2.5 data?

The evidence for the health impacts of PM2.5 is based on exposure periods of 24 hours or longer. For this reason, specific cautionary health advice is shown on EPA AirWatch only for 24-hour rolling average data and not for one-hour average data.

Why isn’t PM2.5 data from portable monitors and nephelometers shown using health categories? Some of the PM2.5 monitors in EPA’s air monitoring network produce indicative PM2.5 data. While portable monitors and nephelometers provide a good indication of local air quality, the data they collect is often not as accurate as the data produced by monitors that meet Australian Standards. This is why data from portable monitors and nephelometers is not used to generate cautionary health advice.
How can one PM2.5 average be high when the other is low?

High one-hour reading, low 24-hour reading:

PM2.5 levels were high over the previous one-hour period, but have not been high overall during the last 24 hours.

Example scenario:
The wind direction has just changed and the air has recently become smoky because of a fire.

High 24-hour reading, low one-hour reading:

PM2.5 levels were not high over the previous one-hour period, but PM2.5 levels have been high at some stage during the previous 24-hour period.

Example scenario:
There was a fire earlier in the day and the air was smoky for several hours. The fire is now extinguished and the air is no longer smoky.

Where is cautionary health advice found on EPA’s website?

One-hour PM2.5 data does not trigger specific cautionary health advice.

General advice about smoke exposure is available on Effects of smoke.

Cautionary health advice is automatically displayed on EPA AirWatch when the 24-hour rolling average reaches 26 μg/m3 or more. This only applies to data collected from air monitors that meet Australian Standards.

The health advice provides practical steps to reduce the impacts of smoke exposure. The advice varies according to the levels of PM2.5 in the air.

What is this data used for?

One-hour average data can quickly show when a smoke event is happening. It can also indicate how severe it is.

One-hour data shown on EPA AirWatch is the quickest way EPA can advise the community about high levels of PM2.5 in the air.

The categories ‘poor’ and ‘very poor’ indicate that the average PM2.5 levels in the previous hour were high to very high.

24-hour data is used for two main purposes:

NEPM reporting –  24-hour rolling average data is compared to the national standard of 25 μg/m3.

Smoke events – During smoke events, 24-hour rolling average data is used to issued specific cautionary health advice.

Page last updated on 12 Sep 2017