PM 2.5 is widely regarded as the pollutant with most health impact of all commonly measured air pollutants. Due to its small size PM2.5 is able to penetrate deep into the human respiratory system and from there to the entire body, causing a wide range of short- and long-term health effects.
In large parts of Europe and the U.S., the main air pollutants of concern are nitrogen dioxide (NO2) and ozone, so looking at PM2.5 only does not give a representative picture of air quality and health risks in some regions.

To learn more about PM2.5, see the link below:
PM2.5 (iqair.com)