Investigate the simplest explanations first, then expand your study until you find the problem and its solutions.

Relative humidity not within the normally desired range of 40 to 60 percent. Typical issues include faulty humidification equipment or controls, or extremes of weather outside the building.

Repair or modify the humidification control equipment. Ask for patience when temporary extreme conditions exist that exceed the system’s ability to cope.

Improper airflow to the space; a variable air volume system is malfunctioning; ductwork is detached, damaged, or not balanced; damper controls not functioning properly; defective fan speed controls; lack of OA; poor mixing of air and distribution in the space; occupants located within the “terminal velocity line,” usually about 50 feet per minute.

Adjust airflow to the space; rebalance the ventilation system. Modify fan speed. Adjust system controls. Reattach or repair defective ductwork. Adjust OA dampers. Improve air mixing and distribution. Provide personal air mixing fans and check SA registers to be sure they are set properly.

Cold air infiltrating the exterior wall results in a thermostatic call for heat, so the interior of the building overheats. Air infiltration is often caused by a negative pressure in the building.

Rebalance HVAC systems so that the building’s interior spaces are under positive pressure in relation to the outside. Check for proper zoning of the HVAC system. Check for cracks, holes, or openings in exterior walls.

Incorrect measurements; insufficient OA; poor mixing of air in the space; too many people for the HVAC system (many buildings and spaces house more people than they were originally intended to support). In rare cases, other sources of carbon dioxide (such as dry ice or a malfunctioning gas appliance) are in the area.

Provide more OA. Improve air distribution and mixing. Use higher-velocity SA diffusers to improve air mixing in the space. Provide induction-­type diffusers. Reduce the people-load in the space. Locate then reduce or eliminate other CO2 sources. Check the calibration of monitoring equipment.

Thermostats improperly set; heating or cooling coils blocked or dirty; control system malfunction; inadequate SA volume and distribution; blocked SA registers; leaking or disconnected ductwork; individual needs vary from the norm.

Modify airflow to the space by adjusting dampers. Rebalance the ventilation system. Increase fan speed. Modify system controls. Reattach or repair broken ductwork. Open OA dampers. Keep coils clear and clean. Check SA register settings. Provide more thermostats or adjust existing ones in the space.

Ductwork is contaminated; air filters are missing or damaged, which allows particles to flow through the system to the occupied areas.

Check and repair or replace filters. Inspect and, if necessary, clean the ductwork and HVAC equipment.

High-velocity air moving through some part of the system (such as a dirty coil); loose equipment rattling in the airflow; malfunctioning or damaged fans; dampers not set properly; vibrating ductwork; air escaping somewhere in the system through a hole or crack.

Inspect the ductwork and system and make corrections.

An older HVAC system that reduces or terminates SA when the lower temperature setting is achieved; poor HVAC system design or control. Many older systems were designed to reduce air delivery when cooling or heating is not called for, sometimes to zero airflow. While useful for energy conservation, this approach has been a large cause of IAQ complaints.

Set the system to fixed minimum SA and OA delivery rates. Provide a separate supply of outside air.

Cross-contamination of air within the building (for example, chemicals used on the third floor being detected on the fifth floor). Possible reasons include inadequate control of odor emission sources, pressure differences between the two spaces causing leakage of air from one space to the other, air leakage at heat exchangers, a chimney effect in the building, or exhaust or relief stacks located too close to air intakes.

Control odor sources. Use local exhaust of air contaminants where possible. Rebalance HVAC systems. Isolate odor-generating equipment and spaces from the rest of the building. Keep odorous spaces negative to adjacent areas. Fix leaking or defective HVAC equipment. Provide additional dilution OA. Use taller exhaust and relief stacks or better-located air intakes.

Mold, slime, or decaying materials somewhere in or near the HVAC system.

Find and remove the source of the odor. Look first at the air-handling unit, especially near the cooling coils and pans and the humidification systems. Check the air intake plenum and the ductwork. Check building records to see if any flooding or water leaks have occurred in the past few months that could have infiltrated the HVAC system. Check for birds or rodents in or near OA intakes. If you’re involved in responding to occupant complaints and IAQ problems, it’s a good idea to become familiar with HVAC systems and your building’s air-handling equipment and systems. Find the guys in the coveralls who are in charge of the HVAC system and ask them to give you a tour. Review the most recent HVAC plans and specifications. Ask when the system was last balanced, checked, and cleaned; whether the plans and specifications are up to date; and what kind of complaints the HVAC crew has heard in the past. Such preparation will make it easier for you to quickly respond to complaints and solve problems before they graduate to major issues.

For more information and other examples, see my published by AIHA.

Tap on the figure to open a larger version in your browser.