In Switzerland the Environmental Law and its decrees were enforced in 1986. The cantons have the duty to set implementation plans to meet air pollution standards. To show the efficiency of measures, diffusive samplers are in important tool.

Figure 1 shows the decrease of air pollution level in the city of Aarau in Switzerland over a period of 9 years.

Figure 2 show an similar example, but the city is situated in valley. So stable weather situations are dominating the NO2 level and the decrease is not only dependent of traffic measure but also from the meteorological situation during the year.

A change of 5% should be reached to be said to be significant.