Below is information on the current background air quality in some of Europe's major cities based on data taken from http://www.airqualitynow.eu and recorded at 8:50 am GMT May 28.
City Quality Amsterdam Good  Berlin Very good  Brussels N/A Krakow N/A Oslo Very good  Prague Very good  London Very good  Madrid Good  Paris Good  Stockholm Good  Zurich Good 
Air Quality Very low 'Very good' 0/25 Low 'Good' 25/50 Medium 'Moderate' 50/75 High 'Poor' 75/100 Very high 'Very poor' >100 N/A N/A
The above data was collected from http://www.airqualitynow.eu, an interactive webservice designed to display and compare air quality and developed as part of the European Union's ‘Common Information To European Air' (CITEAIR) project.
The data is taken from the ‘general background air quality index' and represents the minimum level of air quality experienced by a citizen in each of the above cities, the data represents an hourly index and is subject to change throughout the day.
The indices have five levels from ‘Very low' to ‘Very high' and are based upon the quantity of three major pollutants PM10, NO2, O3 and where data is available, will also take into account pollutants CO, PM2.5 and SO2. These pollutants are of major concern throughout Europe due to their various detrimental effects on health.
Particulate Matter (PM10) particles are small enough to penetrate deep into a person's lungs and as such pose a significant health risk, these particles are most commonly attributed to road traffic emissions and in particular diesel vehicles. NO2 refers to nitrogen dioxide and is detrimental to the bronchial system. Ground level Ozone (03) is, unlike other pollutants listed above, not emitted directly into the atmosphere but rather is a secondary pollutant formed by reaction between Nitrogen Dioxide (NO2), hydrocarbons and sunlight.
For more information on the index calculation see: http://goo.gl/jlbfs