The micrograms per cubic metre (μgm⁻³) air quality measure is actually a bastardised SI unit & should be outlawed.
If the emission is in micrograms (μg; millionths of a gram) the accompanying volume should be measured in cubic centimetres, not cubic metres.
So 40μg/m³ is 40 millionths of a gram (40 micrograms) per million cubic cm (cm³).
40 x 10-6 g per million cm³ = 40 x 10-6 x 10-6 = 40 x 10-12 gcm⁻³ (grams per cubic centimetre).
That is 40 trillionths of a gram (or 40 picograms) per cubic centimetre (cm⁻³).
We can only detect such levels because we have extremely sophisticated measurement equipment nowadays.
Modern urban background levels are typically of the order of 7 μg/m³ (= 7 picograms cm⁻³) — as is the ambient level in the Amazon rainforest.
Don’t forget also that humans evolved for millennia living in caves (& later rudimentary dwellings) burning hydrocarbons (typically wood, then coal) indoors for cooking, heating and lighting – and consequently habitually breathing in their combustion products at levels many orders of magnitude above current maximum permissible levels.
In that primitive epoch, average life expectancy was about 40 years at best – doubtless partly due to habitual long-term indoor exposure to very high levels of hydrocarbon combustion products – and lack of any kind of healthcare.
Nevertheless, this also ensured that our species evolved a heightened genetic tolerance for particulate (PM2.5 & PM10s), NOx & SOx levels significantly above background ones, before the emergence of any significant health impacts occurs.
There is no statistically validated scientific evidence that emissions at these (parts per trillion) levels have any measurable health impacts1; only attributional epidemiological studies by health ‘professionals’ with “skin in the game”; who: (a) want justifications for harvesting subsidised funding for ever more tenuous (junk-?)science; and (b) view Marxo-Fascist BANANA (Ban Anything Near Anyone Near Anywhere) science as “A Good Thing” – since it means no-one can do anything anywhere & our society therefore collapses under the weight of its own “safety” regulations.
There is a clear parallel here with the nuclear power industry; the perverse safety rules for which nowadays require lower radiation levels inside nuclear reactors than prevailing ambient levels in the areas surrounding it: a prime example of spurious, false, overbearing and extremely costly “safety” regulation for its own sake.
References:
1. Fact sheet – particulate matter in outdoor air does not cause death
