Despite the fact that India lies below USA and China on the list of carbon emitters in world, why is Delhi the most air-polluted city of the world?
Also why are so many Indian cities (and no US city), mentioned on the list by USA TODAY, with high levels of air pollution?
Source: USA TODAY
Several reasons come to mind.
1. Population density
Air pollution is mostly caused by people and the more people live together in a particular area the higher the air pollution generally is. If people are spread more evenly over an area, so is the pollution which means that overall pollution levels are lower.
Both China and India have several cities in the top 20 of largest cities in the world sorted by population-density, for example Mumbai, Kolkata, Shenzen, Delhi and Beijing. You can see that Delhi and Beijing have a similar density, but Delhi has a much bigger population so more air pollution is to be expected.
The first US city on the list is Los Angeles (position 90) with a population density only 1/4 of that of Delhi.
2. Number of cars
Traffic is a notorious contributor to air pollution, especially if you are measuring particulates, which the WHO study did. Common sources are exhaust fumes from fossil-fuel engines and dust from unpaved roads.
Based solely on population density you'd expect Mumbai or Kolkata to be the most polluting cities, but Delhi has by far the most vehicles of all cities in India: 7.35 million in 2012 which is about 2.5 times more than Mumbai (source). Beijing has an estimated 5 million cars in 2011 (source), Shenzhen about 3 million (source)
3. Income
Areas with relatively low-income usually have more air pollution. This has to do with not being able to afford cleaner alternatives or filtering systems for manufacturing, travel and energy production.
Per capita GDP in Delhi is Rs. 210,000 (source), which is about USD 3,150
Per capita GDP in Mumbai is Rs. 486,000 (USD 7,300) (source)
Per capita GDP in Beijing is CNY 99,214 (USD 16,150) (source)
Per capita GDP in Shenzhen is CNY 149,500 (USD 24,084) (source)
4. Local conditions
Local conditions can play an important role, for example coastal cities usually have lower pollution levels compared to more in-land cities due to more windy weather (pollution is blown away) and the fact that there are less nearby polluters (no industry on the water).
5. Air pollution laws
I'm no expert on this, but it's been my impression that air pollution laws and regulations are stricter and better enforced in the US and China than they are in India.
Remarks about the WHO study
The WHO study did not investigate all cities in the world, but about 1600 cities for which data on PM10 and PM2.5 particulates is available. Many cities were not included simply because there is no local monitoring system. There is no data from most large cities in Africa, and very little from South America, Russia and the Middle East. So, it's possible that there are cities that are even more polluted than Delhi.
Although this may not be a direct answer to the question "Why is Delhi the most air-polluted city of the world?" I want to go against the statement that Delhi is the most polluted city since there are a lot of ways to measure this.
The USA today article gives a list of "the world cities with highest pollution levels" ranked on "pollution level". Actually the original data (over 2014) of the WHO air pollution site shows that the values are in fact the Annual mean PM2.5 μg/m3. Where PM2.5 are fine particles of 2.5 μm (micrometre) or less.
High PM2.5 levels are obviously unhealthy:
Exposure to fine particles can cause short-term health effects such as eye, nose, throat and lung irritation, coughing, sneezing, runny nose and shortness of breath. Exposure to fine particles can also affect lung function and worsen medical conditions such as asthma and heart disease. Scientific studies have linked increases in daily PM2.5 exposure with increased respiratory and cardiovascular hospital admissions, emergency department visits and deaths.
The PM2.5 particles are not the only causes of low air quality, as indicated by this WHO article:
The 2005 "WHO Air quality guidelines" offer global guidance on thresholds and limits for key air pollutants that pose health risks. The Guidelines indicate that by reducing particulate matter (PM10) pollution from 70 to 20 micrograms per cubic metre (μg/m), we can cut air pollution-related deaths by around 15%.
The Guidelines apply worldwide and are based on expert evaluation of current scientific evidence for:
particulate matter (PM) ozone (O3) nitrogen dioxide (NO2) and sulfur dioxide (SO2), in all WHO regions.
Also the slightly bigger PM10 particles (until 10 μm) are dangerous for health:
Major concerns for human health from exposure to PM-10 include: effects on breathing and respiratory systems, damage to lung tissue, cancer, and premature death.
And in fact, taking the original data, we find that Peshwar, Pakistan had the highest mean PM10 concentration with 540 μg /m3 average, while Delhi is on the 8th place with 286 μg/m3.
An intend to combine the different pollutant concentration in one number is the Air Quality Index (AQI):
EPA calculates the AQI for five major air pollutants regulated by the Clean Air Act: ground-level ozone, particle pollution (also known as particulate matter), carbon monoxide, sulfur dioxide, and nitrogen dioxide
An actual and continuously updated map of the AQI of the world can be seen on the aqicn.org world map. I have to make two notes on this data: 1) I don't know the update frequency, and 2) although some value may look better (lower), this may be because the lack of a sensor for a certain pollutant (for example PM2.5).
Another site, numbeo, gives ranking of pollution based on surveys of visitors in which they score different thinks: air quality, water quality, garbage, etc.
