To dilute any kind of contaminant below a concentration level that may adversely affect human health, a specific amount of outdoor air must be provided at the positions of the building occupants. The amount of outdoor air needed depends on the source strength of the contaminant. With aeration by windows opening and closing (if it is possible), there may not be a method to control the amount of, or the distribution of, the outdoor air inside the space. Control of the amount and distribution of outdoor air is instead achievable with mechanical ventilation systems with different efficacy depending on their specific design. In addition, a mechanical ventilation system (usually a part of some HVAC systems) will facilitate the filtration of the outdoor air, which is usually performed to remove dust, particulate matters, and other airborne contaminants.