Indoor Air Quality can be improved, in decreasing order of effectiveness, by

• A. Source control; Improved ventilation; Air cleaners
• B. Air cleaners; Improved ventilation; Source control
• C. Improved ventilation; Source control; Air cleaners
• D. Source control; Air cleaners; Improved ventilation

Answer: (A)

Controlling emissions at the source provides the greatest improvement in indoor air quality because it stops pollutants from entering or forming in the indoor environment in the first place. When you eliminate or reduce the source—such as using low-emitting materials, proper storage of chemicals, and sealing leak paths—you achieve the largest, most consistent drop in pollutant concentrations for occupants.

Following that, improving ventilation helps by diluting remaining contaminants with outdoor air and exhausting indoor air. A well-designed ventilation strategy reduces concentrations over time, but its effectiveness depends on outdoor air quality, energy costs, and proper balance with filtration.

Air cleaners address pollutants already present in the air, but their impact is typically smaller and they don’t prevent generation at the source or account for all pollutant types. They also require maintenance and correct sizing and placement to be effective.

So the decreasing order of effectiveness is source control, then improved ventilation, then air cleaners.