First, the obvious. Delhi is polluted. Most Indian cities are, but Delhi probably trumps them all. Its 25 million residents are by now habituated to all forms of pollution – land, water, food, and air. But with air pollution, because most times we cannot “see” it – given that its most dangerousform is PM2.5 (Particulate Matter of size less than or equal to 2.5 microns) – we tend to ignore it, or mumble that, like the first snake bite provides us protection from the second, we are only strengthening our immunity by waiting at a Delhi bus stop, allowing all those fluttering PM2.5 fairies to wave their magic wand on our lungs. In reality, what we are doing is hammering nails into our decrepit coffins, one by one, day by day. But we have our excuses ready. Economic growth has found its friend in the shortened life span of philosophers, as it did for the West a century or more ago. Yes, let us all join hands and die slowly for posterity’s sake.
But how slowly? This article attempts to answer just this question.
Not a day goes by without Delhi being talked of as the world’s most polluted city. Sample the headlines doing the rounds since 2014: “Breathing poison in the world’s most polluted city”; “Delhi most polluted city in the world: WHO”; “Delhi world’s most polluted city: Study”; “Delhi is world’s most polluted city: WHO study”.
Calling a city polluted – based on anecdotal or scientific evidence – is one thing; calling it the “World’s most polluted” is quite another. Especially when the World Health Organisation (WHO), the organisation that carried out the study had this to say as a disclaimer:
Primary source of data are official national/subnational reports and national/subnational web sites containing measurements of PM10 or PM2.5 and the relevant national agencies.
Data from different countries are of limited comparability because of:
(a) Different location of measurement stations;
(b) Different measurement methods;
(c) Different temporal coverage of certain measurements; if only part of the year was covered, the measurement may significantly deviate from the annual mean due to seasonal variability;
(d) Possible inclusion of data which were not eligible for this database due to insufficient information to ensure compliance;
(e) Heterogeneous quality of measurements;
(f) Omission of data which are known to exist, but which could not yet be accessed due to language issues or limited accessibility.
Setting aside these limitations for a minute, the WHO standard limit for PM2.5 is 10 μg/m3 (annual mean). India’s standard limit for PM2.5 is 40 μg/m3. Yes, our lungs are presumably well equipped to suck in four times more filth. To be sure, a decade ago 89% of the world’s population lived in areas that exceeded the WHO PM2.5 limit.
2005 annual average PM2.5 concentrations, from Brauer et al. (http://pubs.acs.org/doi/abs/10.1021/es2025752)
Returning to the 2014 WHO report, Delhi – having a mean PM2.5 reading of 153 μg/m3 in the year 2013, the figure a collate of the data supplied by six monitoring stations – was labelled by the Indian media as the world’s most polluted city. That’s right – six PM2.5 samplers, for a population size that is six times that of New Zealand, gifted Delhi this soubriquet.