Spring 2023

Assessing the Subjective Impacts of Odors

By Christopher G. DesAutels, M.S.

Evaluating odor sources and their potential impacts on surrounding communities poses a challenge that a combination of complaint data, monitoring, and air modeling aims to adequately address.

Odors are an ever-present part of our environment.  While some odors may be pleasant, such as baked bread or springtime flowers, other odors can be offensive and have an impact on our daily lives.  Odors that originate from industrial or commercial sources can be a source of conflict between companies and nearby residents.  Thus, determining the source of an odor requires a detailed understanding of the chemicals emitted from a location, the process of how pollutants disperse in the environment, and how odors are perceived by members of a community.

Industry-related odors are caused by the presence of specific chemicals within the air at sufficiently high concentrations to be detectable.  Odorous chemicals include inorganic sulfur compounds (e.g., hydrogen sulfide), mercaptans, ammonia, and certain volatile organic compounds (VOCs), like benzene and formaldehyde.  Industries such as landfills, wastewater treatment facilities, large-scale animal feeding operations, oil and gas production, and pulp and paper mills have been known to be connected to odors and odor complaints.

[U]nlike the US EPA’s criteria air pollutants – particulate matter (PM2.5 and PM10), ozone (O3), nitrogen dioxide (NO2), sulfur dioxide (SO2), carbon monoxide (CO), and lead – odorous chemicals are not regulated at specific emission or concentration levels.”

Following passage of the Clean Air Act in 1965, the United States Environmental Protection Agency (US EPA) declined to set specific air quality or emission standards for odors.  As a result, unlike the US EPA’s criteria air pollutants – particulate matter (PM2.5 and PM10), ozone (O3), nitrogen dioxide (NO2), sulfur dioxide (SO2), carbon monoxide (CO), and lead – odorous chemicals are not regulated at specific emission or concentration levels and are often handled under nuisance statutes (US EPA, 1980).  This lack of specificity stemmed partially from the wide range of odor-causing chemicals and mixtures, as well as regional differences in acceptable odor levels.  Most regulatory odor requirements consist of odor management plans, adoption of specific control technology, and regulations that prohibit the production of “nuisance odors.”  The source of a nuisance odor can be challenging to determine, however, and often occurs after an industrial facility has begun operations.  In addition, the nuisance level of an odor source can be influenced by the frequency, duration, and intensity of its impacts, as well as how objectionable the odor is perceived to be by a community.

A landfill, with multiple sources that can release chemicals that contribute to odor impacts, can be used to illustrate the challenges related to nuisance odor complaints.  A landfill’s odor-contributing sources include the active portion of the landfill, management practices of leachate and landfill gases, and leaks through cover material.  Some of these sources will be controlled point sources with directly measurable emissions from stacks, while others will be “fugitives,” such as leaks through cover material and emissions from the active portion of the landfill.

Nuisance odor complaints to the property owner or local regulatory agency may be filed by individuals from specific neighborhoods, with residents stating that they have experienced odors.  These complaints typically document the nature, frequency, and severity of the odor, though the descriptions provided can often be inconsistent and subjective.  Nuisance complaints may also include a description of the odor, which can give an indication of the detected chemical.  As a first assessment, a field sampling team may make an effort to spot sample for odorous chemicals.  Spot sampling can be challenging, however, because odors are often intermittent and the sampling team simply may not arrive on-site at the proper time to measure a representative complaint.

A more methodical approach to odor identification involves compiling emissions estimates for individual sources and then using an air dispersion model to estimate pollutant concentrations.  The results of this modeling analysis can then be used in combination with nuisance odor complaints and targeted monitoring studies to develop a complete picture of odor impacts and plan mitigations for specific sources.  The first step in a modeling analysis is to compile a comprehensive list of potential sources and emission rates.  This may necessitate near-field sampling for fugitive sources that are poorly understood and stack measurement at controlled release points.  Once odor sources are identified and emissions are quantified, an air model can be developed that will estimate short- and long-term concentrations for the area around the facility.  Air dispersion models calculate the concentrations of a pollutant at defined locations under a large range of meteorological conditions.  The concentration units can either measure the mass of an odorous chemical per unit volume (e.g., µg/m3) or be expressed in odor units, which represent the number of dilutions above detection.  Typical air dispersion modeling studies will look at five years of hourly meteorology (over 45,000 hours with differing meteorology) at receptor locations spaced tens of meters apart.  Model output can be used to determine the magnitude of the peak odor impacts and the frequency of impacts.  For an example landfill (see Figure), contours shown in the figure depict peak anticipated odor concentrations.  Among these peak concentrations, odor impacts due to isolated, rare conditions can be differentiated from repeated and typical conditions, and the contribution of individual odor sources can be estimated.  Additionally, the spread of odor impacts can be determined, which can help to ensure equitable remediation of impacts for all communities, including environmental justice neighborhoods and other populations that may not have submitted complaints.

Example Concentration Levels Derived From an Air Dispersion Model

Map showing odor concentration levels using a color scale

Click to Enlarge Figure.

As odors are commonly treated as nuisance issues, it is important that industrial and commercial facilities continually evaluate their potential odor impacts and communicate with the surrounding community.  This communication is especially important when complaints arise and sources are difficult to identify.  In combination, complaint data, monitoring, and air modeling represent the state of the practice for designing a successful response effort that will gain the confidence of community residents.

Contact Info

The author can be reached at cdesautels@gradientcorp.com.


US EPA, Office of Air Quality Planning and Standards, Office of Air, Noise, and Radiation; Wahl, GH Jr. 1980. “Regulatory Options for the Control of Odors.” EPA-450/5-80-003. 82p., February.