New data on urban air quality from the World Health Organisation recently led Onitsha, Nigeria to be given the title of “most polluted city” in the world. International media also singled out the “most polluted city” in their region, and highlighted countries such as India which had several cities in the “top five”.

Of course, it’s important to highlight cities where pollution is a big concern. Air pollution is a “silent killer”, which can increase the risk of stroke, heart disease, lung cancer and respiratory diseases such as asthma. Often, poorer and more vulnerable groups, such as children and the elderly suffer these effects the most.

But this “name and shame” approach risks being inaccurate and misleading. For one thing, the pollutants that cause poor air quality can vary significantly between cities. Saying one city is more polluted than another is a bit like comparing apples and oranges—particularly when it comes to developing countries. To understand why, we need to dig down further into the data.

Attention to detail

For its analysis, the WHO looked at levels of two types of particulate matter—PM10 (which has a mean aerodynamic diameter of ten micrometres) and PM2.5(which has a mean aerodynamic diameter of 2.5 micrometres)—for 796 cities from 2008 to 2013. PM10 and PM2.5 include pollutants such as sulphates, nitrates and black carbon (soot), which penetrate deep into the lungs and into the cardiovascular system, posing the greatest risks to human health.

Unsurprisingly, the WHO analysis shows higher levels of urban air pollution in low and middle-income regions such as Africa, Latin America and South-East Asia. In these areas, a lack of funding and inadequate capacity to monitor air quality make it difficult to effectively reduce air pollution.

Based on PM10 data, Onitsha, Nigeria topped the list in Africa, while Bamedna, Tanzania, had the highest PM2.5 levels. In the Americas, Santa Gertrudes, Brazil, had the worst PM10 pollution, and Lima, Peru, topped the list for PM2.5. In the Eastern Mediterranean and South-East Asia (which are clustered together in the database), Peshawar, Pakistan, is number one for PM10, and Zabol, Iran, for PM2.5.

And these examples focus only on particulate matter. The rankings would be more diverse if other common pollutants—such as nitrogen dioxide, sulphur dioxide and ozone—were included in the database.

Apples and oranges

There are several other reasons why the air quality of different cities cannot be directly compared. For example, different seasons have unique meteorological conditions and activities, which may cause dips or spikes in pollution. For instance, a city will have greater pollutant concentrations in a dry year than in a wet year, and higher levels of particulate matter at times when agricultural refuse is burned. So using data for one season to extrapolate an annual mean pollution level could skew the results.

What’s more, some cities will not make the “most polluted” list simply because they do not monitor air quality as is the case in some African cities. And even cities that do monitor pollution levels will have different numbers of monitoring stations in different sites. For example, the stations may be concentrated in less-polluted, residential areas in one city, and on busy roads with high pollution in another. A minimum number of monitoring stations is needed to obtain spatially representative data.


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LOL

Long overdue !!!!

&

Well deserved

onithsa is not more populated than kano and lagos... Kano is 8lgas,onistha metropolis is 4,while PH is 2

We are happy for our brown roof


At least we are not polluted


Thanks

joeprince23:
onithsa is not more populated than kano and lagos... Kano is 8lgas,onistha metropolis is 4,while PH is 2

Not about population, but being popular. Onitsha is one of the most visited cities in Nigeria I think after Lagos.

Dreal11247:


Not about population, but being popular. Onitsha is one of the most visited cities in Nigeria I think after Lagos.

,,,, see olodooooooo. So polluted means most visit state to u. Chaiiiii, it means most dirty state.

Thesame people that are depleting the Ozone layers...

What of ibadan

What about Lagos that has the highest number of automobiles and industries.?

sarrki:
Long overdue !!!!

&

Well deserved

Child of hate. Just dnt knw y u hate d eastern guys. Bin monitoring ur comments here, grow up pls

U can imagine that

saintkel:
Child of hate. Just dnt knw y u hate d eastern guys. Bin monitoring ur comments here, grow up pls

you are wrong not me.

Check most of my comments i am for everybody and for nobody

Dreal11247:


Not about population, but being popular. Onitsha is one of the most visited cities in Nigeria I think after Lagos.

is onithsa more industrialised than lagos,abi na cars? What is causing the pollution? Oga your report is wrong. Although there are a lot of industries within the nkpor-obosi axis,but you are still wrong

Onicha is still a Town not a city, if it is about the dirtiest city, it is Lagos.