mg/Nm3 (mg/Nm3)

mg/Nm³ stands for milligrams of pollutant per normal cubic metre of gas. It is the standard unit in which India expresses stack emission concentration limits — for particulate matter, SO₂, NOₓ, HCl, HF, dioxins and the rest. The "N" (normal) is the crucial part: the gas volume is corrected to standard reference conditions of 0°C and 1 atmosphere on a dry basis, so that measurements taken under different real-world temperatures, pressures and moisture levels can be compared on a consistent footing.

The correction matters because hot, humid flue gas occupies more volume than the same gas cooled and dried. Without normalisation, an operator could appear compliant or non-compliant purely because of the gas temperature at the time of measurement. Many standards add a further correction to a reference oxygen or CO₂ percentage (for example 11% O₂ for incineration), which stops dilution with excess air from artificially lowering the mg/Nm³ figure.

For recyclers, mg/Nm³ is the unit that appears against every air pollutant in the Consent to Operate. Typical figures to recognise: particulate matter around 50-150 mg/Nm³ depending on source, HCl 35 mg/Nm³, chlorine 15 mg/Nm³, fluoride 25 mg/Nm³, sulphuric acid mist 50 mg/Nm³, and dioxins/furans expressed in the much smaller ng TEQ/Nm³ (0.1 ng TEQ/Nm³ for incineration). Knowing the unit lets an operator read consent conditions and stack-test reports correctly.

The practical point is to ensure stack-test reports state results in mg/Nm³ at the correct reference conditions (dry, 0°C, 1 atm, corrected oxygen where applicable). A common dispute during inspection is whether a measured value was properly normalised — an apparently passing raw reading can fail once corrected, so the reference basis must always be checked.