The comfortable indoor can be achieved via mechanical ventilation which takes the outdoor air to supply the air indoors at the comfortable level of temperature, relative humidity and acceptable levels of pollutants such as CO2.
The concentration level of outdoor air are specifies in EN 13779 with one of the main characteristic pollutant CO2. In this standard, the concentration levels for CO2 are defined for the rural areas as 350 ppm, in towns 400 ppm and city centres at 450 ppm.
But what happens if the outdoor air is so polluted that even the filters in the mechanical ventilation system will not be able to handle it? In extremely heavily polluted cities, for example in China and India, the concentration levels are today above 400 ppm.
The increase of CO2 levels puts a strain on mechanical ventilation system and cleanness of air delivered indoors. More demands are put on the effectiveness of filters in mechanical ventilation and full recirculation of indoor air will have to be considered. More demand will be put on how to either clean the outdoor air so it can be supplied in buildings or how to clean the indoor air so it can be recirculated.