That's why you need ventilation! The house itself needs to be airtight, for energy preservation. The heat-recuperating system and natural ventilation takes care

That's why you need ventilation! The house itself needs to be airtight, for energy preservation. The heat-recuperating system and natural ventilation takes care of taking out the dirty air. Of course when renovating a house (a renovation will never get it airtight) with antiquated dangerous heating systems that can exhaust fumes when malfunctioning, those need to be removed. Modern building standards (I don't mean building codes, I mean state of the art in building engineering) says to make houses 100% air tight, but with proper ventilation systems, of course. Opening the windows and doors is not enough! CO levels will get back to 80% of their old levels in 30 minutes after closing them! Modern houses need to be continuously ventilated to achieve the highest levels of living comfort. (PS I'm a real estate developer)