Two cities – one in Iran and
another in Nigeria – can
claim title because WHO
measures pollution in two
different ways.

The new WHO database of
worldwide air pollution
measures it in two different
ways, and as a result two
cities – one in Iran and
another in Nigeria – can lay
claim to the unenviable title
of world’s most polluted city.

It all comes down to which
minute particles, or
particulate matter (PM), in
the air are being measured.
These particles are between
2.5 and 10 microns in
diameter, roughly 30 times
smaller than the width of a
human hair.

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 PM2.5s can only
be seen with microscopes
and are produced from all
kinds of combustion. 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 PM2.5 pollution
is seen as the best measure
of how bad air pollution is
for health. Richer countries
usually have higher levels of
PM 2.5s, while low income
countries have higher levels
of PM10s. Both, says the
WHO, are deadly.

Onitsha: highest for PM10s
In 2013, two people died of
heat exhaustion after a six-
hour gridlock on the city’s
bridge over the river Niger.

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 594 micrograms
per cubic metre of
microscopic PM10 particles,
and 66 of the more deadly
PM2.5s. Onitsha’s figures
are nearly twice as bad as
notoriously polluted cities
such as Kabul, Beijing and
Tehran and 30 times worse
than London.

“We know pollution is very
bad here. But this city must
be much better than Lagos,”
said Solomon Okechukwa, a
sceptical Anambra state
official, on Wednesday.
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.

A recent study of Onitsha’s
water pollution found more
than 100 petrol stations in
the city, often selling low-
quality fuel, dozens of
unregulated rubbish dumps,
major fuel spills and high
levels of arsenic, mercury,
lead, copper and iron in its
water. The city’s many metal
industries, private hospitals
and workshops were all said
to be heavy polluters emitting
chemical, hospital and
household waste and
sewage.

“The level of pollution in
Onitsha is getting
increasingly serious,” said
the authors.

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.

“It is difficult to get accurate
measurements in Africa. You
can get super-high readings,
but ideally the measurements
should be done over a year
to include different seasons
and times of day.
The reading in Onitsha may be
representative but not
altogether reliable,” said a
WHO spokeswoman.