ug/m3 (ug/m3)

µg/m³ stands for micrograms of pollutant per cubic metre of air (one microgram is a millionth of a gram). It is the standard unit for ambient air quality — the pollution level in the open air people actually breathe — and is the unit in which India's National Ambient Air Quality Standards (NAAQS) are written for PM10, PM2.5, SO₂, NO₂, ozone, lead, ammonia and benzene. For the most toxic trace pollutants — benzo(a)pyrene, arsenic, nickel — the even smaller unit ng/m³ (nanograms, billionths of a gram) is used.

The key distinction is between µg/m³ and mg/Nm³. µg/m³ measures ambient air (what is in the general environment, at a monitoring station), while mg/Nm³ measures stack emissions (concentrated gas leaving a chimney). They differ by a factor of a thousand in the prefix and by their reference: ambient air is measured at actual conditions in the field, not normalised to 0°C/1 atm. Confusing the two — reading an ambient limit as a stack limit or vice versa — is a common and consequential error.

Typical NAAQS values an operator should recognise in µg/m³: PM10 100 (24-hr), PM2.5 60 (24-hr), SO₂ and NO₂ 80 (24-hr), ozone 100 (8-hr), benzene 5 (annual); and in ng/m³: benzo(a)pyrene 1, arsenic 6, nickel 20 (all annual).

For recyclers, µg/m³ is the unit that appears in baseline ambient studies, EIA reports, and any ambient monitoring the SPCB requires. The practical relevance is in siting and impact: a plant's predicted or measured contribution to ambient PM, SO₂ or NO₂ in µg/m³ is checked against NAAQS to judge whether the surrounding airshed can absorb it — especially important in non-attainment cities where headroom is limited.