“Smog Lake City”: The History of Utah’s Poor Air Quality

Utah is known for its natural beauty. Thanks in part to the splendor of places like the Wasatch Mountains, Moab, and Canyonlands National Park, tourism has grown exponentially in the past several decades with people learning what Utah has to offer. What many may not know, however, is that Salt Lake City is sometimes referred to as “Smog Lake City.” On bad air days, there is a visible blanket of smog that is filled with particulate matter and contaminants.


The atmosphere has changed over time as Salt Lake County and Utah County have grown in population significantly since 2020. According to an article by the Utah Deseret News, “July 1, 2023, estimates produced by the Utah Population Committee show a total Utah population of 3,456,482, adding 55,989 residents to the state since July 1, 2022. The midyear reference date reflects a post COVID-19 era, with rising inflation, increasing interest rates, and a moderation of high growth from the last few years.” With increased travel, newly built homes, and more people across the state, the air quality was bound to change. Let’s dive into the science behind air quality and how polluted air gets trapped. 

Photo: Kendal Ryle


The Science


What causes this to happen? Is it natural or man-made? Or both?


To develop a better understanding, we have to know what air quality is and how it is affected. Air quality is a measure of how clear or polluted the air is. In the Salt Lake Valley, and other areas, inversion is the main component that traps the pollutants within the atmosphere. Inversion is a natural phenomenon where the air pressure and temperature changes traps cold air under warm air. When pollutants are released into the atmosphere, this causes the layer of inversion to contain them. Some examples of pollutants include emissions from vehicles, home heating, factories and industrial processes, and wildfires. It can also be caused by soil disturbance from construction sites. Poor air quality is the result of inversion and pollutants mixing within the atmosphere. 


There are two air quality components that are studied and monitored: Ozone (O3) and PM2.5. Ozone is a gas composed of three atoms of oxygen. Ozone occurs both in the Earth’s upper atmosphere and at ground level, which can be good or bad depending on the elevation. At ground level, O3 is a harmful air pollutant created by chemical reactions and is the main ingredient in “smog.” This happens when pollutants emitted by cars, power plants, industrial boilers, refineries, chemical plants, and other sources chemically react in the presence of sunlight. PM2.5 is particulate matter, fine inhalable particles with diameters that are generally 2.5 micrometers and smaller. Some are emitted directly from a source (e.g. construction sites, unpaved roads, fields, fires). Most particles form in the atmosphere as a result of complex reactions of chemicals such as sulfur dioxide and nitrogen oxides, which are pollutants emitted from power plants, industries and automobiles. Both Ozone and PM2.5 are analyzed when comparing air quality and public health.



Air quality is measured with the Air Quality Index (AQI). The index value ranges from 0 to 301 and higher, which determines the amount of pollution in the air. Each section has a defining color that signifies the quality (e.g. orange means the air is unhealthy for sensitive groups, 101- 150 index value). AQI tracks five major air pollutants: ground level ozone, carbon monoxide, sulfur dioxide, nitrogen dioxide and airborne particles, or aerosols. Ground level ozone and airborne particles are the two air pollutants that pose the greatest risk to human health in the United States.









Where does Utah air quality stand in 2024?


Studying the AQI data from the Central Wasatch Commission’s Environmental Dashboard, it appears that not all has changed for the worse. Figure 1 shows the average amount of O3 (ppm) in the Salt Lake Valley atmosphere on a monthly basis. The top graph shows a bell-curve trend where ozone levels are higher during the summer months. These results are due to an increase in sunlight that reacts to ground level O3 pollutants. The bottom graph shows a gradual increase in average annual O3 ppm since 2006, which is the first year we had year-round data available. Before 2006, we only had summer data available, which accounts for the higher average levels of O3. This change is likely explained by the increase in population growth, tourism activity, and industrial expansion.

Figure 1. The average amount O3 (ppm) in the Salt Lake Valley atmosphere on a monthly basis. The graph on the top compares the years 2006 and 2022. The bottom graph describes the historic O3 in the region on a yearly basis. 


The graph in Figure 2 describes the shifts in the average PM2.5 over the past 23 years. The top section compares the daily PM2.5 average in 2000 (blue) and 2023 (red). Initially you may notice there is significantly more PM2.5 in December and January of 2000, compared to 2023, aligning with typical inversion months. This is also the case in the other early spring months and August (potentially due to wildfires). Historically, the average annual PM2.5 levels are split between two decades of annual average levels. From 1999 to 2010, the annual average consistently stayed above 9.000 PM2.5 (μg/m2); whereas, from 2011 to 2023, the average levels were almost always below this level and even reached as low as 6.454 PM2.5 (μg/m2) in 2019. Although we see similar month-to-month air quality trends aligning with our yearly inversion, the graphs show a general improvement in PM2.5 levels over the past two decades. 


Figure 2. The average amount PM2.5 (μg/m2) in the Salt Lake Valley atmosphere on a daily basis. The graph on the top compares the years 2000 and 2023. The bottom graph describes the historic PM2.5 in the region on a yearly basis. 


Based on the provided data, there is evidence of improvement but room to grow. Thousands of Utahns are pressuring state officials to work towards improving air quality and holding mass polluters accountable. Organizations such as O2 Utah, Heal Utah, and Ucair are working towards a healthier and more sustainable future for everyone affected by the “smog”.


If you are asking yourself, “what can I do to help reduce my air quality impact?”


Here are some ways to reduce pollutants created by Salt Lake City Sustainability Department:


  • Avoid wood burning
  • Don’t idle your car
  • Drive less and take public transportation, walk, or bike
  • Avoid volatile organic compounds, also known as VOCs (e.g. paints, aerosol sprays, air fresheners, hobby supplies, etc.)
  • Be conscious about what you spray in the air!


Keep you, your family, and your community safe by being considerate, and advocating for a healthier place to live. 


Learn more about air quality on the CWC’s Environmental Dashboard and explore the interactive graphs shown above. 






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