- Onitsha has the worst PM10 levels in the world, according to a new report
- Four of the worst cities in the world for this were located in Nigeria -- Africa's largest economy
- The WHO urges all cities to start monitoring their pollution levels
(CNN)The word "Africa" often evokes romantic images of elephants crossing the Kalahari, thundering water at Victoria Falls, or panoramic views from Table Mountain.
But an increasingly common sight for Africans -- especially those in Nigeria -- is that of smog, rubbish and polluted water, according to a new report.
Four of the worst cities in the world for air pollution are in Nigeria, according to data released by the World Health Organization (WHO).
Onitsha -- a city few outside Nigeria will have heard of -- has the undignified honor of being labeled the world's most polluted city for air quality, when measuring small particulate matter concentration (PM10).
A booming port city in southern Nigeria, Onitsha recorded 30 times more than the WHO's recommended levels of PM10.
Named and shamed
The other three cities named and shamed in the WHO report for high PM10 levels are the transport hub of Kaduna, in the north, which came fifth, followed by the cities of Aba -- in sixth place -- and Umuahia, in 16th position, which are both trade centers in southern Nigeria.
Last year, the World Bank reported that 94% of the population in Nigeria is exposed to air pollution levels that exceed WHO guidelines (compared to 72% on average in Sub-Saharan Africa in general) and air pollution damage costs about 1% post of Gross National Income.
The WHO study tracked the growth in the two different sizes of particulate matter, PM10 and PM2.5, per cubic meter of air.
PM2.5 particles are fine, with a diameter of 2.5 micrometers (µm) to more than 40 micrometers, according to the Environmental Protection Agency.
PM10 particles are less than or equal to 10 micrometers in diameter.
Nigeria did not feature in the top 10 for PM2.5 levels.
Why is Nigeria so polluted?
The cause of Nigeria's pollution problem is a complex story.