In many offices or energy-efficient houses it is not possible to ventilate. 

And even then: ventilation reduces CO2 pollution in the air - but that is only a small part of the problem. A complete (!) exchange of air would have to be brought about at least three times a day, five times a day in the presence of people, in order to achieve proper air quality.

Source: http://www.energie-fachberater.de/heizung-lueftung/lueftung-klima/lueftung/stosslueften-so-lange-sollten-hausbesitzer-wirklich-lueften.php