Airborne
fine particles are largely due to long range transport. The air in the Helsinki
Metropolitan Area includes also exhaust gases from traffic, emissions from
fireplaces and energy generation. The
abbreviation PM2.5 refers to fine particles.
Concentrations
are high when the 24-hour average is higher than 25 µg/m3. The annual average in the Helsinki Metropolitan
Area is 8–13 µg/m3.