Which type of air must be filtered according to general requirements for filters?

In mechanical systems, general requirements for filters typically emphasize the importance of ensuring that indoor air quality is maintained and that contaminants are removed effectively from the air circulating within a building. The correct choice, return air, refers to the air that is drawn back into the HVAC system after it has circulated through the occupied spaces.

Filtering return air is vital because this air contains dust, allergens, and other particles that accumulate during use. Proper filtration at this stage prevents the accumulation of pollutants within the HVAC system and maintains clean air for occupants. It also helps protect the components of the HVAC system, thus enhancing the system's overall efficiency and lifespan.

While other types of air, such as supply air, outside air, and makeup air also have important roles and may be filtered based on specific situations or requirements, the general requirement most specifically highlights the need for filtration of return air. This ensures that indoor air remains clean and minimizes the potential spread of contaminants.