Sadly, Swizerland's air quality is not reliable (and I know the country well) because it is surrounded by countries that have substantial installations of heavy industry and power stations not very far away (in the scheme of things). Therefore, wherever the wind (i.e. the air) has blown from it will include exhaust fumes that are not more than a day or so old. The air in Northern Scandinavia generally arrives from across the Atlantic (weeks on the way) but also from Europe (including air from Britain which caused acid rain and killed lots of lakes). Iceland is in a similar position but extremely rarely gets European air - when that happens people notice it and remark on the poorer quality. The important point is time for dispersal of impurities and their capture in precipitation which has the effect of filtration.