WHO_HTM_TB_2009_419_8

Well-designed, maintained and operated fans (mixed-mode ventilation) can help to obtain adequate dilution when natural
ventilation alone cannot provide sufficient ventilation rates.
In some settings, mechanical ventilation (with or without climate control) will be needed. This may be the case, for example,
where natural or mixed-mode ventilation systems cannot be implemented effectively, or where such systems are inadequate given local conditions (e.g. building structure, climate, regulations, culture, cost and outdoor air quality).