The World Health Organization (WHO) recently released its latest global urban air pollution database, including information for nearly 3,000 cities—a doubling from the 2014 database, which itself had data from 500 more cities than the previous (2011) iteration. These increases in coverage in air pollution measurement and reporting is encouraging, but the WHO numbers reveal that we still have a ways to go to construct a comprehensive and accurate picture of global air quality.
WHO singles out Onitsha, Nigeria and Zabol, Iran as the cities with the world’s worst air pollution, the first for elevated coarse particulate, or PM10, levels and the second for extreme fine particulate, or PM2.5, concentrations. Yet these dubious rankings come with many uncertainties and stir more questions than they answer. “It is difficult to get accurate measurements in Africa,” a WHO spokeswoman said. “[I]deally the measurements should be done over a year to include different seasons and times of day. The reading in Onitsha may be representative but not altogether reliable.”
http://blogs.scientificamerican.com/guest-blog/a-new-air-pollution-database-is-good-but-imperfect/
