Look up the chemical formula of the substance in a list of common air pollutants; for example, the formula for carbon dioxide is CO2.
Use a periodic table to find the molecular weight of each atom in the substance. The molecular weight is the number beneath the abbreviation for the element's name. In the case of carbon dioxide, the molecular weight is 12 for carbon and 16 for oxygen.
Add together the molecular weights of every atom in the molecule to get the molecular weight of the molecule. For carbon dioxide, add the molecular weights of one carbon atom and two oxygen atoms: 12 + 16 + 16 = 44.
Multiply the concentration of the pollutant in mg per cubic meter by 24.45. If the concentration is 3 mg/m^3, for example, 24.45 * 3 = 73.35.
Divide the result by the molecular weight to find the concentration in ppm. For the example of carbon dioxide, divide 73.35 by 44 to get 1.67 ppm.