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. By combining PM2.5 data with mortality rates, Berkeley Earth developed a method of calculating the equivalent number of cigarettes you'd smoke in a day.
If you're curious about how many you're 'smoking', the free Sh*t! I Smoke app will give you the latest data from where you are right now.
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