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.
