In the US, exposure to very fine particulate matter known as PM2.5 is considered safe by the US Environmental Protection Agency’s national ambient air quality standards so long as a person breathes in an average of 12 micrograms per cubic meter of air (μg/m3) or less per day over the duration of a year. In the short term, the US considers it safe if PM2.5 levels don’t go over 35 μg/m3 within a day, so long as the yearly average comes out to 12 μg/m3 per day (in other words, a few days of higher PM2.5 here and there is acceptable, so long as there are only a few).
The “safe” levels laid out by national health agencies everywhere are inherently far from safe. “Any level of air pollution, no matter how low, is harmful to human health”
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