Recently, the team behind air monitoring product IAQ — which can be placed in homes, offices, labs, or any place with a power source and immediately access a full accounting of potential pollutants in the air — dove into what U.S. cities have the worst indoor air pollution.

There are a number of factors that affect indoor air pollution within a city: mold, chemicals used in certain paints, vapors, etc. However, it’s also common knowledge that outdoor pollutants affect the air quality within a building.

“While it would be unrealistic to identify all indoor emissions and sources of indoor air quality across the nation, it is possible to identify local areas with a combination of conditions likely to create higher risk of low indoor air quality,” said Gideon Rubin, CEO and co-founder of IAQ.

The company started with looking at the 100 largest MSAs (metropolitan statistical areas) as defined by the 2020 U.S. Census. From there, they analyzed as much data as was accessible to the public in order to pinpoint the MSAs with high exposure to pollutants — then compiled a list of the cities where conditions for bad indoor air quality are most prevalent.

The analysis included factors such as humidity, forest fire risk, allergies, cars per household/vehicle exhaust, industrial pollution, and particulate matter. IAQ also took into account growth rates of each city and development practices in order to account for probability of overall pollution.

“Unsurprisingly, results tend to show that cities with high growth rates and a lot of sunshine, primarily in the south and southwest, run higher risk of encountering air pollution. But there were a few unexpected findings too,” Rubin said.

Following are highlights of the company’s findings.

Top 10 Cities with the Worst Indoor Air Pollution

There are many reasons why people choose to move to a spot like Cape Coral, Florida: almost constant sunshine, beautiful beaches, not to mention no state income tax. However, like with most things, there are also cons — which led to Cape Coral landing on the list. In this case, the cons are seasonal allergies, sometimes suffocating humidity, and being in an area that attracts a lot of tourists, which leads to more pollution.

Orlando is similar to Cape Coral in the way in which it’s a place people want to be — it attracts tourists, which ultimately creates rapid growth within the city. IAQ’s analysis showed that though the cars per household is significantly lower than what was found in Cape Coral’s data (50% versus Cape Coral’s 90%), it’s offset by the annual high humidity and allergy season. In the past few years, forest fire risk has also increased in the state of Florida, meaning additional indoor air quality concerns are enhanced through the smoke and particulate matter created from the fires.

The Dallas-Fort Worth (DFW) metro area is known for a few common ingredients for air quality issues: wide freeways, an energy-driven economy, and Texas hot summers.

“A high pollen count and extremely elevated levels of humidity are two more of them,” Rubin said. “On the positive side, DFW has made strides toward reducing pollution levels in recent years.”

While Stockton has a relatively low humidity and allergy season compared to the three cities aforementioned, its particulate matter levels are one of the highest in the country, according to IAQ. This has, in turn, exacerbated the wildfires that curse this part of California. Plus, the Stockton metro area encompasses a lush agricultural area that brings in a lot of traffic to the city, which contribute to its high level of pollution.

California has more cities than any other state on IAQ’s list, and its residents encounter numerous common air quality challenges: forest fires, air pollution from traffic and industrial buildings, and a “mountainous geography” that often traps containments within its basins and valleys.

And, “sitting at the foothills of a large mountain range, Bakersfield exemplifies these issues, as it boasts the highest car ownership rate in our dataset, has experienced significant impacts from forest fires, and relies on agricultural and industrial production as pillars of its economy,” Rubin said.

The unpredictable weather in Tulsa contributes to rapid changes in temperature and humidity, which in turn makes Tulsa no stranger to drought or forest fires, as well the allergies and mold growth invited by its excessive wetness.

“Tulsa also has strong industrial manufacturing and energy sectors, which create an ongoing risk of chemical contamination,” Rubin said.

Phoenix is known for a few things: its extremely hot desert climate, characterized by its near constant dryness and hot temperatures throughout the year, and its vast size, resulting in most residents needing to own a vehicle. As a result, Phoenix has a higher level of particulate matter in the air. While allergies aren’t a huge concern for residents, “forest fires and smog are common in the valley of the sun,” Rubin said.

Rubin said that McAllen and its suburbs are prone to low air quality due to the severe seasonal pollen allergies present in this area, which is often a concern for its residents’ respiratory health.

“Located in a highly trafficked area along the Mexican border, McAllen is a hub of manufacturing and transportation, which lead to congestion and significant particulate matter emissions,” Rubin said.

Oklahoma City scored on the higher end in almost every category IAQ looked at. It’s rapidly growing and has a thriving economy, which comes with a high car ownership rate, tough allergy season, and high levels of particulate matter.

“The city’s average annual humidity is a bright spot compared to other cities in the dataset,” Rubin said. “But the other side of that coin is Oklahoma is in the top quartile in the nation in terms of wildfire risk.”

Texas has found itself on this list a few times due to its elevated humidity levels, busy freeways, and seasonal allergies from pollen. Houston in particular, however, finds itself in the first position of IAQ’s list for all these reasons but also because of the city’s high concentration of industrial jobs.

“With a high concentration of industrial jobs, Houston serves as a vital economic center that has experienced rapid growth over the past five years,” Rubin said. “As an energy capital, living in Houston comes with the ever-present risk of high pollution and particulate matter in the air.”

**Appearance on this list does not mean residents in this city necessarily face indoor air pollution, IAQ noted. Indoor air quality is highly variable and is based on factors beyond what’s possible to rank using data.