When it comes to determining the concentration of air pollutants in the atmosphere, the units of measurement one would typically see are either mg/m3, µg/m3, ppm, or ppb. These are used in air quality thresholds and air quality indexes to help provide a baseline for what constitutes as clean air and for people to understand the acceptable level of pollutants in the air they breathe. But what do these measurement units stand for, exactly? And how can one be converted to the others? Let’s find out.

What does mg/m3, µg/m3, ppm, and ppb stand for?

Chemical concentrations in the air are typically measured in the mass units of the substance (milligrams, micrograms, nanograms, picograms) per volume of air (cubic metre or cubic feet). Thus, mg/m3 represents milligrams (one-thousandth of a gram) per cubic metre of air, while µg/m3 stands for micrograms (one-millionth of a gram) per cubic metre of air. However, these concentrations can also be expressed as parts per million (ppm) or parts per billion (ppb) by volume through a conversion factor. This is based on the molecular weight of the chemical, which is different between pollutants, and the atmospheric temperature and pressure. A temperature of 25 degrees Celsius and pressure of 1 atmosphere are what is normally assumed for the conversion factor. 

How to convert between µg/m3 – ppb and mg/m3 – ppm

The formula for conversion between µg/m3 (micrograms per cubic metre) and ppb (parts per billion) are as follows:

Converting from ppb to µg/m3 is:

Concentration (µg/m3) = molecular weight x concentration (ppb) ÷ 24.45

Converting from µg/m3 to ppb is:

Concentration (ppb) = 24.45 x concentration (µg/m3) ÷ molecular weight 

The number 24.45 in the equation is the volume (litres) of a mole (gram molecular weight) of a gas when the temperature is at 25°C and the pressure is at 1 atmosphere (1 atm = 1.01325 bar). 

The same equations above can be used for conversion between mg/m3  (milligrams per cubic metre) and ppm (parts per million) as well.

What are the conversion factors for the common major air pollutants?

Below is a table with the conversion factors and the molecular weights of the major air pollutants found to be the most damaging to human health and to the environment based on the formulas above at 25°C and 1 atm (standard atmospheric pressure):

Air Pollutant Conversion Factor Molecular Weight
Ammonia (NH3) 1 ppb = 0.7 µg/m 17.03 g/mol
Carbon monoxide (CO) 1 ppb = 1.15 µg/m3 28.01 g/mol
Nitric oxide (NO) 1 ppb = 1.23 µg/m3 30.01 g/mol
Nitrogen dioxide (NO2) 1 ppb = 1.88 µg/m3 46.01 g/mol
Ozone (O3) 1 ppb = 1.96 µg/m3 48 g/mol
Sulphur dioxide (SO2) 1 ppb = 2.62 µg/m3 64.07 g/mol

An exception to the conversion process is the air pollutant particulate matter (PM10 and PM2.5), as it consists of various tiny solid and liquid particles. It is virtually impossible to determine the molecular weights of its complex physical and chemical composition, which is why PM is read in µg/m3 or pcs/l (pieces per litre).