Answer:
According to the WHO, PM 2.5 levels should not exceed 25 micrograms per cubic meter over a 24-hour period and 10 micrograms per cubic meter on average over a year. But in cities like Delhi and Beijing, there are days when PM 2.5 levels surge to almost 1,000, which is so high that it’s literally off the scales of many pollution monitoring devices.
PM 2.5 refers to fine particulate matter – microscopic particles that are less than 2.5 micrometers in diameter, minuscule enough to be absorbed right into the lungs and blood. Sustained exposure to high levels of PM 2.5 can cause respiratory diseases like bronchitis, asthma and inflammation of the lungs, and even heart attacks and strokes.