Odor Complaints and Investigations: Applying Professional Judgment in Low‑Exposure Scenarios

Introduction

Odor complaints are common reasons industrial hygiene (IH) professionals are asked to evaluate workplaces and occupied buildings and are among the most complex issues encountered in IH. These complaints arise in offices, laboratories, healthcare settings, warehouses, and mixed‑use facilities and frequently occur at chemical concentrations below occupational exposure limits (OELs), below irritation thresholds, and sometimes below analytical detection limits.

While often classified as nuisance issues, unresolved odor complaints can drive employee concern, disrupt operations, reduce productivity, and strain relationships between building occupants, facilities staff, and environmental, health and safety (EHS) professionals. Effective resolution requires more than confirming compliance with exposure limits.  It requires applying professional judgment informed by exposure science, building dynamics, and human perception.

Understanding Odor Thresholds & Odor Perception

Odor thresholds represent the concentration at which approximately 50 percent of a test population can detect a compound. They are detection-based values, not health-based limits, and they should not be interpreted as indicators of toxicity or irritation.

Many odor‑active compounds are detectable at concentrations well below those associated with adverse health effects. People experience odors very differently Factors such as age, sex, smoking history, health status, prior exposures, and even recent viral infections can significantly influence detection sensitivity. Adaptation, odor mixtures, and psychological context further influence perception. As a result, it is entirely possible for occupants to notice an odor even when exposure levels present no toxicological concern.

Odor thresholds are useful for providing context during investigations, but they represent only one piece of a much larger evaluation. It is also important to distinguish between odor detection and irritation. Odor perception is mediated by the olfactory system, while irritation involves activation of the trigeminal nerve. A chemical may be clearly detectable by smell without triggering irritation or physiological harm. This distinction is central to interpreting odor complaints accurately.

Odors and the Occupant Experience

Even when exposures are well below any level of health concern, odors can still affect how people experience their environment. Persistent or unpleasant odors may increase stress, create distraction, and heighten concern about unseen hazards. For some occupants, this can lead to frustration or a sense that their concerns are not being addressed. For IH professionals, acknowledging this experience while still relying on sound exposure science is often the key to resolving complaints and maintaining credibility.

A Structured Approach to Odor Investigations

A practical way to approach odor complaints is through a structured framework that considers multiple contributing factors. One widely used method focuses on the Four Ps:

• People – Who notices the odor, where it is detected, and whether there are consistent patterns in timing or location.
• Pathways – How air moves through the building, including HVAC systems, ceiling plenums, wall cavities, plumbing penetrations, and other routes that allow odor migration.
• Pressures – Pressure relationships created by ventilation systems, exhaust fans, temperature differences, or wind, which can draw odors into occupied spaces.
• Pollutants – Potential sources, materials, activities, or secondary sinks that may release odor‑active compounds.

In many cases, addressing airflow or pressure relationships resolves the issue without requiring full source removal or extensive chemical characterization.

Practical workflow: intake → walkthrough → evaluate →hypothesis → test → control → verify

• Step 1- Intake and pattern mapping: Use a complaint log (date/time/location/odor description/intensity/activities/HVAC status). A short occupant diary can be more valuable than a single air sample.
• Step 2- Building walkthrough (with operations and facilities): Look for obvious sources, but also document pathways: penetrations, shared plenums, door gaps, returns near loading docks, etc. Review housekeeping, waste handling, and chemical storage/use.
• Step 3- Evaluate pressures and transport: Use simple tools: pressure measurements, smoke visualization, and HVAC trending data. Many “mystery odors” are explained by negative pressure pulling air from a corridor, dock, or interstitial space.
• Step 4- Form a hypothesis: Example: “Odor occurs when Dock Door 3 opens and the adjacent office suite is negative to corridor; exhaust fan EF‑2 is over‑”
• Step 5- Test the hypothesis (sampling only when it changes decisions): Sampling is most effective when it’s designed to confirm/deny a specific mechanism or source, or to verify a control. Sampling that is not tied to a hypothesis often produces inconclusive results.
• Step 6- Implement controls: Prioritize engineering and building‑operation controls where feasible: pressure balancing, source containment, ventilation changes, scheduling, and pathway sealing.
• Step 7- Verify and document: Re-check pressure relationships, repeat targeted measurements, and confirm occupant experience improves. Document what changed and evidence supporting closure.

“Usually, Citadel is contacted because the challenge isn’t easy to solve.  On a project in Los Angeles, California, odors were perceived by workers inside of a building used for office space and art conservation.  Citadel determined that the odors were infiltrating the building from neighboring tar pits.  You don’t see that every day.  Finding the source is like solving a mystery, and it feels great.” – Scott Brehmer, MPH, CIH, CSP

Final Thoughts

Anticipate – Recognize – Evaluate

Anticipate odor complaints in facilities with variable airflow, complex HVAC systems, or mixed uses. Recognize that odor detection does not imply hazard, and that inter‑individual variability is expected. Evaluate beyond concentration by examining airflow pathways, pressure relationships, building materials, and complaint patterns.

Air Sampling Limitations

Air sampling can be a helpful tool, but it is rarely sufficient on its own. Odor complaints may involve short‑duration events, intermittent releases, or compounds with detection thresholds lower than analytical reporting limits. As a result, laboratory data may not align neatly with occupant observations. Sampling is most effective when used to test hypotheses, evaluate mitigation measures, or support communication, but not as a stand‑alone solution.

Communication

Clear communication is essential during odor investigations. Explaining the difference between odor detection and health risk, documenting observations and decisions, and sharing findings in plain language all help prevent misunderstandings and reduce concern. Successful investigations often depend as much on how information is conveyed as on the data itself.

Checklist

• Have odor thresholds been interpreted correctly?
• Has nuisance been distinguished from hazard?
• Have airflow and pressure been evaluated?
• Has communication been incorporated into the control strategy?

A Thoughtful, Collaborative Approach

Odor complaints exist at the intersection of exposure science, building performance, and human perception. Resolving them requires a methodical approach and an appreciation for both technical and human factors. When approached thoughtfully, odor complaints can become an opportunity to strengthen trust and improve indoor environmental quality overall.

If you have questions, please contact our team.