PM2.5; fine particulate matter
Refers to airborne particles with a diameter of 2.5 micrometers or less, often discussed in air quality reports and health advisories.
Since the PM2.5 concentration is high today, you'd better wear a mask.
In large Chinese cities, PM2.5 has become a serious problem.
Abbreviation of English 'Particulate Matter 2.5', referring to the particle size in micrometers.