Classification of air quality in European cities

The Romanian city of  Cluj-Napoca ranks first among 100 larger cities of the European Union in terms of air quality, according to a research published by French magazine We Demain together with Respire association.
Cluj-Napoca, which is located in the center of Romania’s historical province of Transylvania, is the country’s third largest city, with 304,500 people.
According to the research, Cluj-Napoca is one of the only two large cities in Europe where the average number of days of air pollution exceeding normal levels is zero. The other city is Edinburgh in Scotland, but Cluj wins due to lower average levels in the analyzed air pollution indicators.
The research takes into consideration three air polluting factors, which are small particles, nitrous dioxide and ozone and calculates the average number of days per year in which the level for these pollutants are exceeding the normal levels, which makes the air unhealthy to breathe.
However, the authors of the research show that the data measurements are imperfect which makes the data not that relevant for comparison. For example, Cluj has only two stations for measuring air pollution, while Berlin has 48 such stations. Berlin is ranked 65th with 83 days a year of exceeding levels for air pollution.
Read more at;
http://www.romania-insider.com/romanias-cluj-napoca-best-city-in-the-eu-for-air-quality/124062/
http://abonnes.lemonde.fr/planete/article/2014/06/05/dans-quelle-ville-en-europe-respire-t-on-le-mieux_4433073_3244.html
http://www.respire-asso.org/classement-de-la-qualite-de-lair-en-europe-100-plus-grandes-villes/