Based on World Health Organization Recommendations
There is a direct relationship between exposure to high concentrations of particulate matters (coarse particles PM10 and fine particles PM2.5) and an increased morbidity or even mortality, both immediate and in the long run.
Conversely, when exposure to breathable particles is limited, impact on health eases rapidly.
This direct and positive relationship allows policymakers to project the population rapid health improvements that can be easily achieved when air quality is successfully improved, as pioneered for instance in Tokyo after voluntarily reducing all diesel fleets.
More specifically, fine particles PM2.5 (less than 2.5 micrometers in size, meaning a diameter twenty to forty times smaller than hair) are tiny enough to deeply penetrate the respiratory tract, down to the bloodstream interface that can even be crossed by the smallest ones, subsequently affecting most organs.
These have health impacts even at low concentrations – no threshold has been identified below which no damage to health is observed. Consequently, WHO Guidelines were issued to encourage the lowest possible exposure to these peculiarly dangerous fine particles.
WHO recommends that the 24-hour mean of 15 µg/m3 should not be exceeded more than 3 days per year.
Many of the 300+ world-cities (2 million+ inhabitants) are way beyond this yearly ceiling, especially in Europe (predominance of diesel vehicles), Middle-East/Africa (oil & drilling, cooking, sand storms), South-East Asia (all kinds of burning) and China (coal & industry).