Visibility reduction

Visibility reduction is a measure of how visual range is affected by very small particles in the air. The higher the visibility reduction measurement, the lower the visual range is. For example, very smoky conditions at an air monitoring station would show a high value for visibility reduction.

Visibility reduction is shown on EPA’s website as an airborne particle index (also called a visibility reduction index). This is different to how EPA reports on concentrations of PM2.5 and PM10 particles.

Self-assessment of visual air quality

Observing landmarks is a good way to make a quick, visual assessment of air quality. You can use it yourself to assess the air quality. This can be especially useful when air monitoring data is not available.

See Effects of bushfire smoke for information on conducting your own visual assessment of air quality

Health effects of reduced visibility

Visibility reduction measurements are based on the levels of fine particles in the air. These particles are often small enough to be breathed deep into the lungs. This can cause health effects. Children, the elderly, pregnant women and people with existing heart or lung conditions (including asthma) are more sensitive to the effects of breathing in fine particles. Symptoms may include wheezing, chest tightness and difficulty breathing.

For more information see Effects of bushfire smoke.

Sources of fine particles

The fine particles that reduce visual range are produced from the burning of fossil fuels (such as coal) and organic matter (including wood and grass). Motor vehicles, powerplant emissions and bushfires are all major sources of fine particles.

How EPA measures visibility reduction

EPA measures visibility reduction using a monitor called a nephelometer. It measures the number of particles in the air and then calculates an airborne particle index (API).

Visibility reduction air quality categories

The visibility reduction data on our website will be shown in different colours, depending on the amount of particles in the air. Green means particle levels are low and the visible range is very good. Black means particle levels are high and the visibility range is greatly reduced.

Air quality category 1-hour visibility reduction index (API) Distance in km
Very good 0–0.77 More than 45
Good 0.78–1.56 30–45
Fair 1.57–2.34 20–30
Poor 2.35–3.52 10–20
Very poor 3.53 or greater Less than 10

Current standard for visibility

EPA uses the State Environment Protection Policy (SEPP – Ambient Air Quality) standard for visibility reducing particles. Unlike the other key air pollutants EPA measures, this objective is not included in the national air quality standards (Ambient Air Quality NEPM).

State environment standard (SEPP) Averaging time
Minimum visible distance – 20 km
(equivalent to a visibility reduction index [API] of 2.35)
1 hour

Page last updated on 22 Dec 2015