Gopolang Makou ANALYSIS: Can we tell which African cities have the most polluted air?

Dakar and Joburg were near the top of a Twitter post listing world cities with the worst air quality. But how is the air pollution of different cities measured, or even ranked in a list?

Do you know what you’re breathing in big cities like Dakar and Johannesburg?

Spectator Index tweeted a list of “selected cities” with the “worst air quality” in July, crediting the World Health Organization. Senegal’s capital Dakar and South Africa’s financial hub Johannesburg were among the five most polluted cities.

(Note: @spectatorindex and the Spectator magazine in the UK are “completely unrelated”, Lara Pendergrast, online editor of the Spectator, told Africa Check.)

But how do you even rank air quality? Two readers asked us to clear the air.

Air pollution generally means ‘particulate matter’

The WHO’s Global Ambient Air Quality database collects information on the air quality of 4,387 towns and cities in 108 countries, 10 of them in sub-Saharan Africa.

Many pollutants can be found in the atmosphere, a technical officer specialising in air pollution at the WHO’s department of public health and environment told Africa Check. But, Sophie Gumy said, the WHO database only collects data on particulate matter.

“What is generally meant by air pollution in the media is particulate matter, as this is a very good indicator for pollution in general,” she said. Particulate matter can come “from many sources, and is the one most studied in epidemiological studies, and which has large detrimental effects on health”.

According to Samantha Keen, an environmental scientist at the University of Cape Town’s Energy Research Centre, particulate matter (PM) is “a mixture of solids and liquids in the atmosphere of a specific size” and includes dust.

She said PM10 (measuring fewer than 10 microns, which is about one-fifth of the width of a human hair) is more commonly monitored while PM2.5 (2.5 microns or less) is considered more harmful to human health.

So how is air quality measured?

Special meters measure the amount of pollutants in the air in a period of time, usually a day. These measurements are used to calculate an annual total.

Air quality is judged as good or bad according to guidelines on the effects the pollutants have on human health, Gumy told Africa Check.

The WHO has a set of pollutant concentration guidelines for outdoor (ambient) air quality, but countries also develop their own standards. From a public health perspective, “the WHO guideline values are the ones defining good or bad for health”, Gumy said.

Air pollution in Dakar and Joburg exceeds guidelines

The latest WHO guidelines were published in 2005. They recommend that particulate matter of 10 microns and smaller shouldn’t exceed an average concentration of 50 micrograms per cubic metre of air over any 24 hour period or 20 micrograms per cubic metre of air over a 12 month period. 

Johannesburg far exceeded the annual guideline in 2011, the year of its last entry in the WHO database. The city of gold recorded an annual average air quality of 85 micrograms per cubic metre.

Senegal’s capital city has more recent figures, showing an even higher figure. In 2017 Dakar had, on average, 149 micrograms of particulate matter in each cubic metre of air.

‘Many polluted cities not included in database’

But it’s not simply a matter of ranking the levels of pollution as entries in the database are not comparable. This is due to a number of factors, such as:

  • Differences in the location of measurement stations
  • Differences in the methods and quality of measurements
  • Variations in seasons

And “many polluted cities are not included in the database”, Gumy said – even those expected to be among the most polluted. This is because the WHO database only includes cities that report data to the organisation.

Not much monitoring of air quality happens in developing countries “especially on a continuous basis”, which can skew the results, Raeesa Moolla told Africa Check. She is a lecturer in physical geography at the University of the Witwatersrand who specialises in air quality and pollution.

Ranked or not, air pollution is dangerous

Bearing all of this in mind, the Spectator Index ranking “does not mean much,” Gumy told Africa Check.

But Moolla stressed that “these figures, whether comparable or not, are very important”.

“If the air quality is ‘poor’ this means that you are inhaling a higher level of pollution than is recommended. Pollutants could be hazardous to human health in many different ways, and for this reason, keeping our air clean is vital for good health.”

Edited by Anim van Wyk


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