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Graph showing air pollution over 24 hours, from 10 o’clock on 31 December to 10 o’clock on 1 January. Pollution is measured as concentration of fine particles PM 2.5. Concentrations are 1-6 microgram per cubic metre, except for midnight, when they shoot up to 125 microgram per cubic metre before slowly falling again back to baseline by 4 o’clock in the morning. For context, in the US the Environmental Protection Agency set “the level of the primary (health-based) annual PM2.5 standard at 9.0 micrograms per cubic meter to provide increased public health protection, consistent with the available health science”, while it set “the 24-hour PM 2.5 standard at the level of 35 micrograms per cubic meter”: https://www.epa.gov/pm-pollution/national-ambient-air-quality-standards-naaqs-pm