It all comes down to which minute particles, or particulate matter PM , in the air are being measured. These particles are between 2. The coarser PM10s include dust stirred up by cars on roads and the wind, soot from open fires and partially burned carbon from the burning of fossil fuels like coal, oil and wood.
The particles are small enough to be inhaled deep into the lungs. But the ultra-fine particles known as PM2. These are small enough to get from the lungs into the blood supply and are possibly more deadly because they affect the cardiovascular system. Many cities in developing countries traditionally monitor only PM10s. But increasingly PM2. Richer countries usually have higher levels of PM 2.
Both, says the WHO, are deadly. Cars and trucks on the main road to Lagos belch fumes from burning low-quality diesel, and the air often stinks of burning waste from rubbish dumps, the smoke from old ships on the river and discharges from the metal workshops. But people did not expect Onitsha in Anambra state on the eastern bank of the mile-wide river Niger, to be named the most polluted in the world.
According to the WHO, an air quality monitor there registered micrograms per cubic metre of microscopic PM10 particles, and 66 of the more deadly PM2. But Onitsha, say academics, is a textbook example of the perils of rapid urbanisation without planning or public services creating a sustained pollution assault on its water and air.
As a tropical port city which has doubled in size to over 1 million people in just a few years, it is frequently shrouded in plumes of black diesel smoke from old ships; it has no proper waste incineration plants; its construction sites and workshops emit clouds of dust and its heavy traffic is some of the worst in Nigeria. However, the WHO also said on Wednesday that the pollution data from Onitsha was not necessarily reliable because it came from a single monitoring station.
