A hood - correctly called a local exhaust hood - is the point where contaminated air is drawn into the ventilation system. The sizes and shapes of hoods are designed for specific tasks or situations. The air speed (velocity) at the hood opening and inside the hood must be enough to catch or capture and carry the air contaminants. To be most effective, the hood should surround or enclose the source of contaminant or be placed as close to the source as possible.

The required air change rate is sometimes given in ventilation regulations and ventilation design standards. For example, a flammable storage room requires six air changes per hour according to US OSHA requirements. The Canadian National Building Code (NBC) requires residential houses to mechanical ventilation system capable of providing at least one half (.) air changes per hour during the heating season to avoid chimney back drafting.

Duct systems should be designed to have air flow through the ducts with as little friction or resistance as possible. The amount of air that flows through a duct depends on the cross section area (duct opening area) of the duct and the air speed. Air moving too slowly will allow contaminants such as dusts to settle and accumulate and these particles will eventually clog the duct. Air moving too fast wastes power, can create noise problems, and may cause excessive abrasion by dust particles hitting the ducts. Recommended speed ("duct velocity") for different types of contaminants can be found in reference books on ventilation.

Duct systems typically require large amounts of air to move relatively small amounts of   contaminants. The required volume of airflow depends of the acceptable concentration of air contaminants in the inside work space. A carefully designed system can achieve the required air concentration while using the least amount of power. Other design considerations include initial capital costs, reliability, maintenance, and durability of air handling equipment.