The term ‘concentration’ is used when we measure gasses like carbon dioxide, methane or oxygen. Concentration of a particular gas is defined as the amount of gas by volume in the air. The two most common units of measurement for concentration are parts-per-million (PPM), and percent concentration (%)

What is PPM

PPM (Parts-per-million)is the ratio of one gas to another. For example, 500 ppm of CO2 means that if you could count a million gas molecules, 500 of them would be carbon dioxide and 999,500 molecules would be some other gasses.

Parts per million calculations are often used when measuring concentrations in gasses. PPM is calculated by dividing the mass of the solute (the minor component in the solution) by the mass of the solution and then multiplying by 1,000,000.

PPM = mass of solute / mass of solution x 1,000,000

Converting PPM to Percentage (%)

When PPM levels of any gas are over 10,000 PPM, we usually move from PPM to ‘percent concentration’. For example, instead of describing 15,000 ppm CO2, we talk about 1.5% CO2.

15,000 / 1,000,000 = 0.015 (1.5%)

1 ppm = 0.0001% gas

Acceptable CO2 Concentrations

A table showing safe levels of indoor CO2 is shown below.

250-400 ppm Normal background concentration in outdoor ambient air
400-1,000 ppm Concentrations typical of occupied indoor spaces with good air exchange
1,000-2,000 ppm Complaints of drowsiness and poor air.
2,000-5,000 ppm Headaches, sleepiness and stagnant, stale, stuffy air. Poor concentration, loss of attention, increased heart rate and slight nausea may also be present.
5,000 ppm Workplace exposure limit (as 8-hour TWA) in most jurisdictions.
>40,000 ppm Exposure may lead to serious oxygen deprivation resulting in permanent brain damage, coma, even death.