Inadequate ventilation inevitably generates excess humidity caused by showers, baths and cooking. Excess humidity inside the home can cause many problems such as condensation on the windows, the premature deterioration of the home and the proliferation of mold which is harmful to your health.
Another aspect of inadequate or insufficient ventilation is the increased concentration of (chemical and biological) contaminants contained in the air. For example, carrying out extensive home projects, stripping furniture, using gas-fired equipment or engaging in any other similar activity quickly degrades indoor air quality when there is a lack of ventilation.
Note that you can confront the problem of contaminated indoor air by limiting the source of indoor pollutants, such as refraining from storing large quantities of paints or cleaning products, which are easily avoidable sources of contamination.
Source: Énergie Ressources naturelles Québec