The skunky smell of pot smoke. Burning stenches from a pet food factory. Smoke from construction sites. These are the smells that communities of color and lower income people in Denver are disproportionately exposed to at home and at work, according to a new study.
The study, published in the Journal of Exposure Science and Environmental Epidemiology, is one of the first to examine the environmental justice dimensions of bad odors in an urban setting.
“Odors are often ignored because they’re difficult to study and regulate.”
There’s been a wealth of research in recent years showing that people of color and those with lower incomes are exposed to more air pollution, including nitrogen oxides and particulate matter. Exposure to air pollution causes or exacerbates cardiovascular and respiratory illnesses, among other health problems, and increases the overall risk of death.
Odors are more challenging to measure than other kinds of air pollution because they are chemically complex mixtures that dissipate quickly. “Odors are often ignored because they’re difficult to study and regulate,” said Arbor Quist, an environmental epidemiologist at the Ohio State University who was not involved with the research.
Though other kinds of air pollution in the United States are limited by federal laws and regulated at the state level, smells are typically regulated under local nuisance laws. Though somewhat subjective—some folks don’t mind a neighbor toking up—odors can have a big impact on how people experience their environment, and whether they feel safe. Bad smells can limit people’s enjoyment of their homes and yards, and reduce property values.
“Odor is one of the ways municipalities can take action on air pollution.”
Odors are more than a nuisance—they pose real health risks. Exposure to foul smells is associated with headache, elevated blood pressure, irritated eyes and throat, nausea, and stress, among other ills.
University of Colorado Denver urban planning researcher Priyanka deSouza said local regulation of odors gives municipalities an opportunity to intervene in environmental health. “Odor is one of the ways municipalities can take action on air pollution,” she said.
Previous research on ambient odor air pollution has focused on point sources, including chemical spills and concentrated animal-feeding operations such as industrial hog farms. DeSouza said Denver’s unusually robust odor enforcement system made it possible to study the environmental justice dimensions of smelly air over a large geographical area.
Making a Stink
The city maintains a database of odor complaints that includes a description of the smell and the address of the complaint. DeSouza’s team used machine learning to identify themes in complaints made from 2014 to 2023. They found four major clusters: smells related to a Purina pet food factory, smells from a neighbor’s property, reports of smoke from construction and other work, and complaints about marijuana and other small industrial sources.
They used the text of the odor complaints and the locations of the complaints to deduce the likely source of the odor. For instance, complaints about the pet food factory often included the words night, dog, bad, and burn. Marijuana-related complaints frequently used the words strong and fume.
They also matched complaint locations against the addresses of 265 facilities that have been required by the city to come up with odor control plans for reasons including the nature of their business, or because five or more complaints have been filed about them within 30 days. (Growing, processing, or manufacturing marijuana occurs in 257 of these facilities.)
Less privileged census blocks—those with higher percentages of non-white workers and residents, residents with less formal education, lower median incomes, and lower property values—were more likely to contain a potentially smelly facility, according to the analysis. DeSouza said this is likely due to structural racism and historical redlining in Denver.
The facilities were concentrated in a part of the city that is isolated by two major freeways. Previous research has shown that people in these neighborhoods are exposed to more traffic-related air pollution, and that people of color, particularly Hispanic and Latino populations, are more likely to live there.
Yet people living and working in those areas weren’t more likely to register a complaint about bad smells than people in other parts of the city. In fact, most of the complaints came from parts of the city that are gentrifying. DeSouza said it’s not clear why people who live or work near a stinky facility aren’t more likely to complain than people who live farther away from one.
It may be that wind is carrying smells to more affluent neighborhoods, where more privileged people are more aware of Denver’s laws and feel empowered to complain. The research team, which includes researchers from the city’s public health department, is continuing to study odors in the city. Their next step is to integrate information about wind speed and direction with the odor complaints.
Quist said the study is unique in that it factors in potential workplace exposures, where people spend a large part of their day. Workplace exposures can also have health effects that aren’t captured in research that looks only at where people live. “A lot of research has focused on residential disparities,” she said, adding that the inclusion in the analysis of facilities that have had to submit odor-monitoring plans is also significant. “This is an important paper,” she said.
DeSouza said she suspects that people who live and work near smelly facilities may not be complaining because they feel disenfranchised. “People are resigned to odors, they have been living there a long time, and they don’t feel they have a voice.” If residents in less privileged neighborhoods were able to successfully lodge an odor complaint and get results, it may make them feel more connected in general to the city government, she added.
“I’m really interested in supporting policy action,” she said. “We’re trying to get residents to be aware that they can complain.”
—Katherine Bourzac, Science Writer
