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ABC News

Illustration of a bar chart made from cigarettes of varying length The World Health Organisation keeps a close eye on air quality data, providing a global insight into just what we are all breathing every day. The relevant indicator the WHO measures is PM2.5, the "concentration of particulate matter of less than 2.5 micrometres of diameter" — that's about 3 per cent of the width of a human hair. These particles are similar to the 'tar' that smokers voluntarily inhale through cigarette filters.