A UN report released this month has shown that the vast majority of the world's population, a whopping 6.76 billion people, live in places where air pollution exceeds recommended levels. That's 92 percent of the entire human race. But getting to the nitty-gritty (pun intended) of air pollution is more complicated than it seems. Measurements by the World Health Organization can seem bewildering and labyrinthine, and the fact that small places rather than huge metropolitan areas often top the rankings of the most polluted cities in the world can be frankly confusing. But we need to understand this data for a very good reason: Air pollution is highly deadly. The UN estimates the annual death toll of air-pollution-related diseases at around 6.5 million, and a study in March indicated that the annual economic cost of pollution to the United States alone was around $4.3 billion. This is neither a cheap nor an irrelevant issue. So what do the numbers say?
The main source of air pollutant measurements is the World Health Organization, which issues annual reports on the state of the world's air health. They haven't just been collecting numbers, either. In May of this year, they created a "road map" to lower pollution levels worldwide, and their new data for 2016 includes information from over 3,000 locations around the world. (That's still not enough in some places, as we'll cover in a bit.) But measuring air pollution levels isn't as easy as going outside, smelling the air, having a coughing fit, and moving to another country. The WHO uses two different measures for air pollution: "fine" particles of around 2.5 micrometers or less (The Washington Post explains this is about 1/30th of the width of a human hair), and "coarse inhalable" particles of 2.5 to 10 micrometers. These aren't the only types of air pollution in the world, but they're the types that seem the most harmful to human health. Different processes produce PM2.5 and PM10, as they're called by scientists, and each have different effects on humans.
The PM2.5 particles are the worst for respiratory health. When we breathe them in, their tininess means that they can carry toxic pollutants into the lungs, and this "causes asthma, respiratory inflammation, jeopardizes lung functions and even promotes cancers," according to a review of the latest science about it in January 2016. An increase in daily levels of PM2.5 by just 10 micrograms per square meter raises incidents of respiratory disease by over 2 percent, hospitalization rates by 8 percent, and overall mortality by 0.2-0.6 percent. PM10s are regarded as marginally less dangerous, but inhaling them is still tied to respiratory problems, asthma aggravation, and lung and cardiovascular issues.
The WHO lines up the sources of PM2.5 and PM10 in the world, and they're pretty diverse: combustion engines, coal, oil and other heavy fuels, building, cement and brick manufacture, erosion of pavement, traffic, dust in the air, and a host of other things. Yes, dust and soil in the air are valid pollutants.
The Worst Cities For Fine Particles
The new model rolled out by WHO includes a view of the entire word map in terms of PM2.5 levels, and it's not a pretty sight. While much of North America lies within acceptable levels, huge parts of Africa, China, India, and Saudi Arabia show up as glaring purple, representing air pollution levels of PM2.5 which far exceed the WHO's recommended levels. (You can go and look up your own city if you want.)
But when it comes to PM2.5 levels worldwide, the winner (if you can call it that) of particulate levels is the city of Zabol in Iran. Never heard of it? That's unsurprising. But you may be surprised to know that it's not an industrial hub, or some kind of notorious coal-burning site. Zabol's level of air pollutants is because of its unprecedented dust storms, which were exacerbated, the Guardian reports, when a nearby wetland dried up in 2000. It can have up to 120 continual days of dust storms a year -- a crisis that is literally choking the city.
The Indian press has been surprised and pleased by the remainder of WHO's list, which features a majority of Indian cities in the top ten, for one reason: Delhi, usually known as India's most polluted place, is beaten out by four other Indian cities. (Delhi's PM2.5 levels are down to 122, after a rating of 153 by WHO in 2014 led it to try to increase air quality generally. To demonstrate just how bad this still is, WHO's ideal annual mean is a PM2.5 rating of 10.) The city of Gwailor in the state of Madhya Pradesh came in second after Zabol, with Indian metropolises Allahabad, Patna, and Raipur following after. Rounding out the top ten are the city of Bamenda in Cameroon and Xingtai, which has been the most polluted city in China for several years thanks to its steel factory and coal power station. (It beat out Baoding, which is also located in the province of Hebei, known as China's "ground zero" for air pollution.)
When it comes the United States, the worst PM2.5 levels aren't necessarily found in mega-cities either. The American Lung Association, which collects data on PM2.5 levels across the US, ranks Bakersfield, California as the most polluted, followed by Fresno, San Jose, Los Angeles, and San Francisco. California may seem like the healthy state, but it certainly isn't doing a lot for the lungs of its citizens.
The Worst Cities For Coarse Air Pollutants
When it comes to PM10 pollution, Zabol is third, beaten out by two others: Onitsha in Nigeria and Peshawar in Pakistan. Onitsha is the hub of a national problem -- CNN reported a World Bank statistic that 94 percent of Nigeria experiences levels of air pollution above WHO guidelines. Apparently, the cause of PM10 levels in the area tends to be reliance on fuels, dirty generators, and cars with enormous emission levels. It's a tricky combination to fix.
The World Economic Forum, looking at the PM10 pollution levels, highlighted something about all the cities in the top 20 (which range from Iran to India to Afghanistan, with only one Chinese entry): They're in economically similar regions. These "rapidly growing economies," as the WEF calls them, tend to be using a lot of resources quickly as more people go there to work, and are still reliant on old-school devices like wood-burning and diesel fuel. The result? Seriously awful air.
If you're less interested in smaller places and more into larger cities, the PM10 data indicate that Delhi and Cairo are your worst bets, followed by Dhaka in Bangladesh, Kolkata, Mumbai, and Beijing. (Some of the best, according to the WHO? Wellington in New Zealand, Toronto, Madrid, Sydney, and London.) In the United States, the two worst PM10 levels in mega-cities are in Los Angeles and New York, but their levels are still pretty low, at 20 and 16, respectively. Delhi, in contrast, is at 229.
But there are limits. WHO points out that there's a serious lack of monitoring stations across Africa, Eastern Europe, and Russia, and that more of those will give us a much more accurate look at air pollutants worldwide. Hopefully, more cities follow in Delhi's lead. It was shamed by previous poor WHO ratings into making systemic changes, and appears to have dented its PM levels, even if it's still rated poorly.