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The worst U.S. cities and states for ozone pollution

Updated on January 26, 2024
Written by
Amparo Lopez
Amparo is a staff writer for HouseFresh, having joined us in 2023. She is a sustainability advocate, holding degrees in communications, human-centered design and environmental policy. Her work has a focus on air pollution and climate change.

Ozone pollution continues to be a significant concern for air quality, both in the United States and worldwide, despite recent gradual decreases in ozone levels. 

Cities in the Western and Southwestern regions of the United States continue to dominate the list of most ozone-polluted cities. 

Notably, California maintains its historical record as the state with the most cities on the list. States-wise, Nevada tops the list with the worst ozone pollution in the U.S.

Ozone remains one of the most widespread pollutants, affecting air quality in the United States and worldwide. 

The U.S. Environmental Protection Agency (EPA) has taken significant measures under the Clean Air Act to reduce air pollution by promoting the replacement of polluting engines, fuels, and processes across the nation. 

As a result, positive changes have been observed in this year’s State of the Air report by the American Lung Association, with reduced ozone levels in some states and cities.

During the COVID-19 pandemic, reduced economic activity led to lower fossil fuel burning and emissions, temporarily improving ozone levels. However, as the world economy rebounds and activities return to full swing, ozone pollution is expected to rise again, according to NASA studies.

Despite this progress, ozone remains a significant threat to public health, especially in western U.S. cities. Your location in the country can significantly impact your health, so understanding the detailed breakdown of ozone levels in your area is crucial for informed decision-making about your surroundings.

Across the United States, more than 2,000 ambient outdoor air monitors are actively measuring concentrations of harmful pollutants, such as inhalable particulate matter, toxic gasses, and ozone.

To determine the states with the worst ozone levels, we ranked them based on the Air Quality Index (AQI).

Gathering data from the U.S. Environmental Protection Agency (EPA), we aggregated the latest annual AQI data from county to state levels. For city data, we cross-referenced county-level data with county seat assignments. 

Key findings 

  • Nevada has the worst overall ozone pollution in the U.S., with a concentration of 0.076 ppm. However, the state’s most polluted city, Reno, only ranked #30 among the most polluted cities, according to the EPA’s Air Quality Statistics Report.
  • California is the fourth most affected state while maintaining its historical trend of having the highest number of urban areas on the list of most polluted cities. 
  • The five most polluted cities are located in California — San Bernardino, Riverside, Los Angeles, Visalia and Bakersfield.
  • Western and Southwestern cities dominate the most ozone-polluted list. While efforts to control ozone precursor pollutants have been effective to some extent, higher temperatures, dry and sunny skies, and draughts produced by climate change increase the number of unhealthy ozone days in the region.
  • Connecticut ranks as the second worst state for ozone pollution in the U.S., claiming the title of the worst state in the East. Often dubbed the “tailpipe of the nation,” Connecticut receives significant transboundary pollution from neighboring states to the west and south.
  • The state with the lowest ozone levels is Hawaii, with 0.047 ppm.

The worst U.S. states for ozone pollution are Nevada and California

Nevada ranks as the state with higher levels of ozone pollution. Smog formation is more prevalent in hot, dry weather and with changing climate conditions, including increased frequency of warm weather and drought, smog formation can accelerate. 

Being one of the driest states in the U.S., Nevada’s scarce rainfall limits the potential for rain to mitigate ozone concentrations.

However, city-wise, the situation changes. Neighboring California, the fourth most polluted state, leads the list of most polluted cities, with ozone levels increasing toward the south. The busy warehouses and facilities like oil and gas refineries and power plants in the region serve as the primary sources of emissions.

On the other end of the spectrum, Hawaii claims the title for the state with the lowest ozone levels. The Pacific Ocean islands benefit from abundant wind and rain, which helps cleanse the air, while their relatively low population contributes to maintaining cleaner air quality. 

Additionally, Hawaii’s lack of heavy industry prevents significant atmospheric pollutant emissions, as observed in other parts of the nation.

The 5 worst U.S. cities for ozone pollution 

California is home to the five most polluted cities in the U.S. Interestingly, the state’s major urban centers aren’t as smoggy as the Inland Empire, with levels consistently rising towards the south. 

Nevertheless, ozone pollution remains a concern throughout the state.

#1. San Bernardino, California (0.11ppm)

Image Source: KVCR / 2023, Ron Reiring

San Bernardino belongs to the expansive Inland Empire region. It is strategically located close to two bustling US ports in Los Angeles and Long Beach and is well connected through a strong network of highways and rail lines. As a result, over the past decade, warehouses for online retailers, logistics and distribution companies have reshaped southern California’s landscape.

In 2022, the number of warehouses in San Bernardino County surpassed 3,000. These warehouses play a crucial role in the booming online retail industry, serving as storage and distribution hubs for multiple products and brands. 

However, there’s a downside to this growth. Most of these warehouses heavily rely on trucks to move goods in and out of their facilities, trucks that burn fuel and contribute to traffic congestion on the streets and highways of San Bernardino. As a result, these areas have earned names like “diesel death zones” from environmental activists and the media.

The situation worsened during the COVID-19 pandemic as online shopping reached new heights, leading to more truck traffic and cargo moving through the region. With the pandemic behind us, the trend of increased online shopping and truck transportation continues to impact the city’s air quality.


Your risk of being affected by ozone pollution rises the longer you stay active outdoors and the more intense the physical activity. To minimize exposure, exercise during cooler hours or indoors with proper ventilation on high-ozone days.

If you’re in an area where traffic pollution is an issue, check out our selection of the best air purifiers for traffic pollution.

#2. Riverside, California (0.1ppm)

Image Source: Wikimedia

Southern California’s “SMOG Belt” is another name for the Inland Empire. Similar to San Bernardino, transportation and logistics are key to Riverside’s economy, with vast warehouses sprawling over a billion square feet of land. 

Yet, the environmental impact is significant, with Riverside ranking as the second most ozone-polluted city in the United States. The distribution activities conducted through inland ports and the heavy reliance on trucks to transport goods to and from warehouses contribute significantly to air pollution, especially when port congestion increases, leading to a dramatic rise in smog.

Southern California’s distinct geography, which compounds the issue, allows air currents to flow through mountain passes, carrying pollutants from the Los Angeles basin into the Inland Empire, including Riverside. The encircling mountains in the North and East only worsen the problem by trapping the pollution within the region.


Avoid spending too much time outside during the afternoon and early evening when air pollution is usually at its highest. And if there’s an alert, it’s even more important to take it easy. Less physical activity during those times is a good idea because you breathe in more polluted air when you’re active.

Air purifiers can effectively reduce indoor ozone pollution. However, choosing purifiers that do not generate additional ozone as a by-product is crucial to avoid worsening indoor air quality. Here’s our list of the best ozone-free purifiers.

#3. Los Angeles, California (0.1ppm)

Image Source: Wikimedia

Los Angeles is renowned for its smog, and the bustling shipping industry is a major contributor to this issue. The Port of Los Angeles is one of the busiest ports in the United States. Together with the nearby Port of Long Beach, they handle over 40% of all inbound containers for the country. 

As many port operations heavily rely on fossil fuel or diesel to power ships, trucks and other transportation, they produce great air pollution in the Los Angeles Basin. In 2022, they were responsible for approximately 10% of the basin’s total NOx emissions.

The city’s large population is another factor. With four million people residing in Los Angeles and another six million in the surrounding Los Angeles county, heavy vehicular emissions and traffic congestion have significantly contributed to the ozone levels.

To make matters worse, Los Angeles’s geographical location doesn’t help dissipate the air pollution. The city sits in a basin surrounded by mountains. During the warm months, temperature inversions trap the smog at the ground level. 

This phenomenon occurs when relatively warm air from the South Coast Basin or inland Los Angeles traps cooler ocean air close to the Earth’s surface, preventing polluted air from rising and dispersing.


When driving, especially on the freeway, harmful pollutants can enter your car and affect your air. Keep your car windows rolled up to protect yourself, and use the re-circulate air setting on your vehicle’s ventilation instead of drawing it in from the outside.

#4. Visalia, California (0.09ppm)

Image Source: Wikimedia

Visalia is no stranger to air pollution due to its geography, especially high smog concentrations. The entire San Joaquin Valley, including Tulare County’s main city, deals with atmospheric inversions that trap exhaust from vehicles, airplanes, manufacturing and more. Unfortunately, Visalia’s low annual rainfall (11.03 inches) allows smog to accumulate over consecutive days.

The local economy thrives on agriculture, particularly as the nation’s largest dairy-producing county. Cattle production in concentrated animal feeding operation (CAFO) systems releases volatile organic compounds (VOCs) into the air, reacting with Visalia’s semi-arid weather to form ozone. 

On the other hand, goods movement in the San Joaquin Valley relies heavily on trucking, fueled by burning fossil fuels, which adds to the formation and high levels of ozone in Visalia.


The San Joaquin Valley Air Pollution Control has developed a helpful app called Valley Air, designed to make it super simple to monitor air quality in the region, receive real-time updates, and save important locations for quick reference.

Keep outdoor ozone from affecting indoor air quality by using an efficient air purifier at home. At HouseFresh, we have tested and reviewed the latest air purifiers and compiled a list of the best air purifier brands.

#5. Bakersfield, California (0.09ppm)

Image Source: Rawpixel

Bakersfield is a bustling energy production, agriculture, and distribution hub. The city’s outskirts are filled with large warehouses and distribution centers that operate day and night, accompanied by frequent diesel truck traffic. 

Sources like farm machinery, crop burning, industrial activity powered by fossil fuels and pesticides contribute to harmful gases like nitrogen dioxide, which transform into ozone in Bakersfield’s hot weather.

On the other hand, Kern County, of which Bakersfield is the county seat, has been an oil and gas powerhouse since the 1890s. It is the most productive oil-producing county in the state of California, covering an area of 10,750 acres. 

The extraction and refining processes for gasoline and diesel fuel demand significant energy consumption. Once it’s produced, freight trains transport the oil, adding to the fossil fuel emission in the city. 


Keep an eye on the air quality conditions as they can change quickly. It’s a good habit to check the air quality before planning to reduce your ozone exposure. The color code for the Air Quality Index (AQI) is explained here.

What is ozone pollution?

Ozone, known as smog, is a gas of three oxygen atoms bonded together (thus O3). According to the American Lung Association, it is one of the least effectively controlled pollutants in the United States and one of the most dangerous. 

However, not all ozone is bad. We find the “good” ozone up high in the atmosphere and in the stratosphere. This ozone layer acts as a shield, protecting us from the sun’s harmful ultraviolet rays. 

But at ground level, we encounter the “bad” ozone, also called tropospheric ozone. This type is not directly emitted into the air but forms when certain gases, namely oxides of nitrogen (NOx) and volatile organic compounds (VOC), react with sunlight. 

These gases are produced primarily when fossil fuels are burned, such as in motor vehicles, power plants and factories, or when certain chemicals from refineries, factories, chemical plants, gas stations, or even paint evaporate. Because the reaction occurs in the atmosphere, ozone can be transported by wind, far from its original emission sources. 

Breathing in ground-level ozone can spark various health issues, especially on scorching, sunlit days when ozone levels tend to rise. 

Ozone also makes the lungs more susceptible to infections and aggravates lung diseases such as asthma. Studies indicate that long-term exposure to ozone may even contribute to the development of asthma. But that’s not all. Elevated ozone exposure extends beyond our cities and impacts vegetation and ecosystems.

Here are some helpful tips to prevent or minimize the release of pollutants indoors and avoid outdoor ozone from entering your home:

1. Keep outdoor ozone out of your home  

If you live in an area with high air pollution, there is not much to do to control ozone external sources. However, you can minimize outdoor ozone from entering your home by keeping the windows closed, especially on warm and sunny days with little or no wind.

2. Check your home appliances

Some appliances produce ozone intentionally or as a by-product. Reduce or restrict the use of ozone-generating appliances, such as ozone generators, laundry water treatment systems, ozone fruit and vegetable washers, laser printers, photocopiers and ozone hair and facial tools.

3. Avoid high-VOC cleaning and personal care products 

Volatile organic compounds can react with sunlight or heat and turn into ozone. Many everyday cleaning products can release harmful VOCs, even when unused. Choose no- or low-VOC products without fragrances, irritants and flammable ingredients if possible.

4. Be mindful of VOCs in high-emitting chemicals

When using paints, solvents, adhesives and caulks, only buy what you need, as unused chemicals stored in the home can release VOCs into the air. Read our article compiling over 100 sources of VOCs in the home.

5. Use an air purifier with activated carbon

While HEPA filter air purifiers effectively remove other pollutants, activated carbon filters are necessary to absorb and break down ozone molecules. These filters trap airborne gases, ensuring cleaner and healthier indoor air.

Here are the ones we recommend for ozone: 

Levoit EverestAirLevoit Core 300Alen BreatheSmart 75i
AIR CLEANING SPEED ⚡13 minutes40 minutes24 minutes
CADR 👩‍🔬360 CFM145 CFM347 CFM
FILTER TECHNOLOGY 💨3-Stage Filtration (Pre-filter, H13 HEPA and activated carbon)H13 True HEPA filterPre-filter, B7 Pure filter, H13 True HEPA filter
MAX ROOM SIZE 📏558 sq. ft.219 sq. ft.1,300 sq. ft.
WEIGHT ⚖️20.7 lbs7.48 lbs27 lbs
OUR REVIEW 🔍EverestAir review Core 300 reviewAlen 75i review
PRICE 💵$599.98$99.99$748.99

Final thoughts

During the early months of 2020, the COVID-19 pandemic caused a significant global slowdown in commerce, resulting in a considerable 2% decrease in ozone levels worldwide. Although we can still observe some positive effects of lockdowns on air pollution data, the economy has since rebounded, leading to a return of usual emissions.

The EPA, acting under the Clean Air Act, has encouraged states and policymakers to implement substantial changes, and the good news is that tangible results are being seen. Numerous states and cities across the U.S. have made notable strides in reducing ozone pollution and improving overall air quality.

However, despite these efforts, climate change remains a pressing concern and progress in addressing air pollution is uneven. Over 30% of the nation’s population is still affected by ozone pollution in areas with unhealthy air quality. Now, more than ever, a deeper understanding of air pollution, its sources and effects, is crucial to drive effective solutions and protect public health and the environment for future generations.


Last update on 2024-04-21 / Affiliate links / Images from Amazon Product Advertising API

About the author

Amparo Lopez

Amparo is a staff writer for HouseFresh, having joined us in 2023. She is a sustainability advocate, holding degrees in communications, human-centered design and environmental policy. Her work has a focus on air pollution and climate change.