How we gauge the health risk from smoke

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When smoke is in the air it is monitored with three different instruments.

  • ‘Coarse’ particles are measured as PM10, as this is a standard and widespread measure.
  • ‘Fine’ particles are measured as PM2.5, as this is now the main indicator of human health effects.
  • Also measured is ‘visibility reduction’, which is an indicator of how much distant objects are obscured by smoke.

PM10 and PM2.5 are measured as concentrations in micrograms per cubic meter, whereas visibility reduction is measured as inverse distance, which is directly converted to kilometres, indicating how far a person can see through the smoke, using a distant landmark.

Visibility and PM2.5 are very closely correlated, since it is the fine particles in the smoke causing the visibility reduction.

EPA uses visibility here as the primary indicator because there are more stations measuring this at present (until a planned upgrade to PM2.5 monitors is implemented for all stations). Visibility is also preferred in some circumstances as it directly relates to what people see (or cannot see) due to the smoke, and can judge for themselves once suitable landmarks are identified.

Monitoring fine particles

The main pollutant EPA monitors in smoke-affected areas is fine particles (PM2.5). EPA and DHHS have developed a system of categories for smoky air based on international research. The categories are set by PM2.5 levels. Each category has cautionary health advice that suggests practical ways to reduce possible health impacts of smoke.

Table 1 shows the health categories based on PM2.5 levels.

Increased smoke levels decrease visibility, which makes it harder to see objects far away. Table 2 provides landmark visibility distances that can be used to estimate smoke levels.

Table 3 outlines suggested actions to minimise the impacts of smoke exposure. The advice depends on the concentration of PM2.5 in the air.

Visit EPA AirWatch to access the latest PM2.5 data measured at EPA’s air monitoring stations.

Health categories for fine particles

Table 1 shows the concentration of PM2.5 for each of the seven health categories.

Each health category has cautionary health advice that suggests practical ways you can reduce your exposure to PM2.5 in smoke. See EPA AirWatch health categories.

Table 1: Health categories based on PM2.5 levels

Health category 24hr PM2.5
Low 0–8.9
Moderate 9.0–25.9
Unhealthy – sensitive 26.0–39.9
Unhealthy – all 40.0–106.9
Very unhealthy – all 107.0–177.9
Hazardous (high) – all Greater than 177
Hazardous (extreme) – all Greater than 250

How to self-assess air quality during smoky conditions 

Increased particle levels reduce visibility. Observing landmarks is a good way to estimate this reduction. This can be used to self-assess the air quality. This may be especially useful when air monitoring data is not available.

Assess the air quality using Tables 2 and 3 below to decide what to do when smoke is in your area:

  1. When there is no fire in the area, identify landmarks visible from your home at the distances shown in Table 2.
  2. Use these landmarks as a guide to estimate the air quality in your area when smoke is present.
  3. When you can no longer clearly see a landmark this means air quality has deteriorated. Estimate the visible distance (by observation) of the nearest landmark that is just obscured by the smoke.
  4. Use the visibility distance determined in step 3 to identify the applicable health category. Look this up in Table 3, read the advice and take any precautions you think are necessary.

Table 2: Health categories linked to landmark visibility distances

Health category Visibility
Low More than 20 km
Moderate 10–20 km
Unhealthy – sensitive 5–10 km
Unhealthy – all 2–5 km
Very unhealthy – all 1.5–2 km
Hazardous (high) – all 1–1.5 km
Hazardous (extreme) – all 0.5–1 km

Table 3: Smoke advisory levels linked to visibility

Smoke advisory level Visible landmark Sensitive to smoke Less sensitive to smoke
Low/moderate 20 km  normal activity normal activity
Unhealthy – sensitive 10 km reduce prolonged or heavy physical activity normal activity
Unhealthy 5 km avoid prolonged or heavy physical activity reduce prolonged or heavy physical activity
Very unhealthy 2 km avoid all physical activity outdoors avoid prolonged or heavy physical activity
Hazardous 1.5 km if there are no fire threats, remain indoors and keep physical activity levels as low as possible  avoid all physical activity outdoors

Page last updated on 6 Mar 2019