A Systematic and Scientific Approach to IAQ Odor Complaints

Do you smell that?

• By David L. Zeidner
• Sep 01, 2013

Odor complaints are the most challenging problems I am asked to solve as an indoor air quality professional. Often, only one or two of the building's occupants out of 50 or 100 can detect the odor. What if the odor is only observed when entering a suite or office and then it quickly disappears, only to reappear later in the day or week? How can the source of the odor be identified if it smells like different things to different people? Is there a potential occupational exposure issue because the odor is present in the work environment? These are only some of the challenges encountered while performing an odor investigation.

Sense of smell is extremely varied among individuals. There are many factors, such as age, health, gender, genetics, pregnancy, and previous experience that affect odor perception. Some studies suggest that common odors, such as coffee, vanilla, and tar, are rarely identified correctly more than half the time.

Individuals may react differently to the same odor; what may be a comforting smell or benign to one person may cause alarm in another. Odors also can be desensitizing, meaning they quickly become non-perceptible after observing them and then just disappear. One's own sense of smell can be the greatest obstacle to overcome when performing an odor investigation.

When we smell something, a complex array of smell receptors is activated by the odorant, the actual chemical compound observed by our sense of smell. In order for us to smell an odor, the odorant needs to be volatile and dispersed into the air. Many odorants composed of carbon atoms (such as fragrances and solvents, etc.) also can be considered volatile organic compounds, or VOCs. Many VOCs have been studied to determine their odor threshold -- the minimum concentration at which 50 percent of observers can detect an odor. Odor threshold concentrations can range greatly for each individual VOC.

For example, naphthalene (mothballs) has a very low odor threshold of 15 parts per billion (ppb), while formaldehyde has an odor threshold of 871 ppb. According to OSHA, the occupational exposure limit for naphthalene is 1,000 times greater than its odor threshold, or 15,000 ppb. Conversely, formaldehyde has an occupational exposure limit of 750 ppb, lower than its odor threshold. If we can smell formaldehyde in the workplace, then we are likely at risk for an occupational overexposure.

In general, there is no correlation between odor thresholds and occupational exposure limits. We characterize odors in the workplace that are below the occupational exposure limit as "nuisance odors."

Elements of Successful Odor Investigations
Because our sense of smell may be unreliable, a systematic approach to odor investigations must be taken. There are many important components to performing a successful odor investigation, including gathering background information, performing a site inspection, surveying with a VOC meter, and possibly collecting samples.

Before visiting the building, I gather background information about the odor complaint to identify how many individuals smell the odor, where it is located, what is the frequency and duration of the odor, the time of its first appearance, and a description of the smell. Answers to these questions may greatly assist in determining the source, location, and pathway of the odorant.

Often I am told the odor is always present, but frequently when I arrive at the building to perform the investigation, the odor is not observable to the individuals who reported it. This may be due to an odor being desensitizing or possibly transient. Pathways such as the HVAC system, portions of the building affected by pressure changes, stairwells, and elevator shafts can change the perception of the odor.

Many times, the site assessment is performed as a process of elimination (i.e., identifying where the odor is not observed). While I am on site, my ability to perceive the odor is critical in determining its source and pathway. This may not be possible in instances when the odor is transient, faint, or unobserved.

If I cannot rely on my sense of smell alone and the description of the odorant indicates it is a VOC, I may perform a survey with a field instrument called a photoionization detector, or PID. This instrument may be used to survey locations/materials within an area to determine a potential odor source by measuring the relative concentration of VOCs in the air. There are several factors that may prevent the source of the odorant from being detected using a PID. These include a concentration of the odorant below the detection limit of the PID, no distinguishable difference in VOC levels from location to location, or the presence of an odorant being not detectable due to its ionization potential.  Despite these limitations, PIDs are often used and useful in odor investigations.

Using Gas Chromatography/Mass Spectroscopy
When limitations of one's sense of smell and field instruments prevent the determination of the source of the odorant, there is another approach we utilize in performing odor investigations. It is the use of gas chromatography/mass spectroscopy (GC/MS) to analyze the VOC components in the air. In addition to possibly identifying the source of the odorant, this technique may provide information in determining whether the odor presents an occupational exposure or a nuisance odor issue. GC/MS is a powerful combination of analytical instruments capable of definitively identifying VOCs present that may be beyond our sense of smell.

Air samples are collected in an evacuated canister with or without a timed regulator. If the odor is faint, it may be best to take a grab sample to ensure that the sample is not further diluted. If the odor is constant or there is a need to know the average concentration over time of the odorant, a 4-8 hour regulator can be used. The limit of detection can be as low as tenths of ppb. As with any sample collection, a sampling strategy (sample locations, number of samples, reference samples, evaluation criteria, etc.) must be developed to ensure the right questions are being answered.

When a suspected odorant is identified, a sample of the material (wall covering, mastic, paint, etc.) also may be analyzed by GC/MS. This may be a recently installed building material or chemical solution used in the building. The off-gassing of the material is collected in a controlled environment. Its chemical fingerprint is then compared to the chemical fingerprint of air in the building to confirm or negate the building material as the odor source.

In summary, a systematic approach to an odor investigation will increase the chances of identifying the odorant, its source, and its concentration. A systematic scientific and humanistic approach also serves well to appease building occupants and restore confidence and productivity in the workplace.

This article originally appeared in the September 2013 issue of Occupational Health & Safety.