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In Denver, people of color and those with lower household incomes are more likely to live in neighborhoods near facilities that emit bad smells, a new CU ����Vlog�ƽ��-led study reported.

For many Denver residents, the city’s crisp mountain air is a perk of living near the Rocky Mountains. But every so often, a gust of wind can carry less welcome scents, like the pungent odor of pet food factories or the skunky smell of marijuana grow houses.

In a paper�� Feb. 22 in the Journal of Exposure Science & Environmental Epidemiology, scientists revealed inequality in exposure to Denver’s��odor. The research found that while racial minorities and socioeconomically marginalized communities are more likely to be exposed to bad smells, residents in gentrifying neighborhoods file more odor complaints.

“Our study suggests that there's potentially an underreporting of odor in certain neighborhoods,” said first author��, a researcher with CU ����Vlog�ƽ��’s Institute for Behavioral Science and assistant professor at the University of Colorado Denver.�� “Some residents in these communities that are disproportionally affected by odor may not feel that their voices are being heard.”

Before 2016, Denver, like many cities across the U.S., used an inefficient and archaic way to address odor issues, according to��Shelly Miller, the paper’s senior author and professor in the Department of Mechanical Engineering at CU ����Vlog�ƽ��.

Traditionally, when the city received complaints about an odor issue, an odor inspector would go to the site and��, but these inspections often found nothing. Because odor is very transient, by the time the inspector arrived, the smell might be long gone.

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A 1938 map illustrates the redlining of neighborhoods in the City and County of Denver. (Credit: Home Owners' Loan Corporation/Wikimedia Commons)

“Odor is a complex issue. It’s very hard to measure, and it’s subjective. You probably smell things differently than I do, but that doesn’t mean what you’re smelling is any less valid than what I’m smelling,” Miller said.

Miller’s�� in odor and��air quality, as well as concerns from local communities, prompted the Denver Department of Public Health and Environment to�� its air pollution control ordinance in 2016. A new rule requires odor emitting facilities, including pet food factories and marijuana growers, to submit odor control plans if they receive five or more complaints within a 30-day period.

“This updated rule is one of the most innovative odor ordinances of all major cities in the U.S. It’s willing to take action based on community complaints,” deSouza said.

The team mapped out all 265 facilities in Denver that had to submit an odor control plan as of 2023. More than 96% of these facilities were marijuana growers, processors and manufacturers. The rest included pet food manufacturing, oil refining and construction.

They found that malodorous facilities are more often located in historically redlined neighborhoods. After the Great Depression, the U.S. government implemented a racist and discriminatory policy that designated neighborhoods with racial and ethnic minority residents as�� high-risk, or “red” for mortgage lenders.

In Denver, these neighborhoods are located along the city’s two major interstate highways, I-70 and I-25, and still host a higher percentage of people of color. A revealed that residents in these historically redlined areas are exposed to higher levels of air pollution.

When the team compared the odor facility map to an odor complaint map, they found that neighborhoods more susceptible to odor did not file more complaints than other neighborhoods. Instead, more complaints per population came from older neighborhoods that are transitioning into more modern, wealthier neighborhoods, like River North in Denver’s city center.

It is unclear why gentrification has led to more odor complaints, but it could be that the more privileged feel more empowered to file complaints, the authors said. New residents could also be more sensitive to the city’s smells compared to long-term residents.

“Given that odor complaints are the primary driver for Denver to take action on odor control, I hope our findings could encourage the city to continue working��on rebuilding trust with these communities and empower them to use the tool,” deSouza said.��