Between the Vosges Mountains and the Black Forest, winds coming from either direction are often deflected by these
natural barriers (wind NNE-SSO). In Alsace, weather conditions are calm with warm Mediterranean air coming from "trouée
de Belfort", the average annual precipitation is low and the perceived summer temperatures can be inordinately high.
Finally, behind the Vosges Mountains, if Alsace wind's average is not strong, pollution does disperse well in
Mulhouse area with the "trouée de Belfort".
Nevertheless, ozone pollution peaks in summer or particulate peaks in winter occur on Mulhouse like on upper Rhine
valley with some meteorological conditions (bad pollutants dispersion, temperature, sunny…).
Because of the density of environmental polluting activities cities and their agglomerations are especially affected
by air pollution. In Mulhouse, particulate matter (PM10), nitrogen dioxide (NO2 – near road) and ozone (O3) may cause
problems. Main reason for the excessive load of these pollutants is the motorised traffic: Congested city streets lead
to breaches of pollution limits on arterial routes and in the city centre.
In the centre of town and along the main traffic axes the annual mean limit value for nitrogen dioxide (40
µg/m³) and particulate matter PM10 (40 µg/m³) is not exceeded. On contrary, the daily limit value
for particulate matter PM10 (50 µg/m³ in 24 hours max 35 days a year) and long term objective value for O3
(1 hour > 120 µg/m³ in 8 hours max 25 days a year) may exceed the limit.