Key points from article :
Air pollution is the largest environmental risk to public health.
Outdoor air pollution causes around four million premature deaths globally each year.
Most harmful air pollutants in urban areas are NO2 gas and fine particulate matter.
PM2.5 sources include combustion, wood fires, industry, cooking, agriculture and vehicle exhausts.
Most countries have laws to maintain air pollution at safe levels in order to protect human health.
Researchers measure associations between health conditions and air pollution exposure.
They study the ways air pollutants enter the body and affect organs such as the lungs, heart and brain.
There is no evidence for a threshold in the health effects of fine particles.
Air quality standards need to be proportionate to society, as well as protect the people.
Governments set legally binding air quality targets.
There is an opportunity here to develop laws that make the air safer to breathe.
Carefully designed, air quality standards can balance both approaches.