Air pollutant units and conversion factors

Air pollutants are measured in various units. Find out about the units and how to convert them so you can compare our data with data from other sources.

We report air pollutant measurements in 2 different ways: as Air Quality Index (AQI) values and as concentration values.

Air Quality Index (AQI) values calculated using measurements of ozone, carbon monoxide, sulfur dioxide, nitrogen dioxide, airborne particles and visibility. The AQI provides a comparison of air pollutants, standardising these measurements into one easy-to-understand index. See details on the AQI calculations.

Concentration values are the primary measurements of parameters obtained from air quality monitoring stations. Concentrations are reported as:

  • particles (as PM10, PM2.5) – micrograms per cubic metre (µg/m3)
  • NEPH or visibility – reported in units of 10-4 m-1. This means that a NEPH value of 1.5 should be read as 1.5x10-4 m-1. On multiplying by 100, you can convert this number to the widely used unit for visibility, known as inverse megametres (Mm-1).
  • gases – parts per million (ppm) for CO and parts per hundred million (pphm) for others. Gaseous concentration values are reported referenced to a temperature of 0°C and an absolute pressure of 101.325 kilopascals. This conversion table will allow you to compare data in the DPIE air quality monitoring reports with those from other sources.

Pollutant

Concentration units we report in…

to convert from our reported units to…

multiply by

Ozone

pphm

µg/m3 (0°C) 
µg/m3 (25°C)

21.4
19.6

Nitric oxide (NO)

pphm

µg/m3 (0°C) 
µg/m3 (25°C)

13.4
12.3

Nitrogen dioxide (NO2)

pphm

µg/m3 (0°C)

20.5

Nitrogen oxides (NOx)

pphm

µg/m3 (25°C)

18.8

Sulfur dioxide (SO2)

pphm

µg/m3 (0°C) 
µg/m3 (25°C)

28.6
26.2

Lead

µg/m3 (0°C)

µg/m3 (25°C)

0.92

Carbon monoxide

ppm

mg/m3 (0°C) 
mg/m3 (25°C)

1.25
1.15

NEPH or visibility 10-4 m-1 Mm-1 or inverse megametres 100