The best way to improve air quality, certainly in well-insulated homes and buildings, is to ventilate frequently. If not, pollutants, odours and CO2 levels will rise as the day goes on, causing drowsiness and, in the worst case, making you feel ill. But:

- In harsher climates or in fall or winter one does not open up windows, certainly not with the current energy prices;

- In big city locations or industrial areas with a high degree of pollution outside, it would be wiser NOT to open a window; 

- Your age and pre-existing medical conditions, such as allergies (due to pollen i.e.), are also of an important influence;

- If the moisture level in your home or building is too high, bacteria and moulds will grow abundantly, often in places you don’t regularly look, such as basements, closets or crawl spaces underneath floor boards. These mould spores are ‘silent kilers’ and can make you very sick (i.e. asthma flare-ups, fungal sinusitis, or pneumonitis).