When winter rolls around in the mountainous American west, Salt Lake City, Utah can become home to some of the worst air quality in the United States.

That’s because along with a few other cities like Los Angeles and Mexico City, Salt Lake City’s geography makes the city prone to temperature inversions.

How do temperature inversions harm Salt Lake City air quality?

Salt Lake City lies in the Salt Lake Valley, positioned between the Oquirrh Mountains and the Wasatch Range.

When temperature inversions occur, a layer of warm air settles above the valley, trapping colder air beneath it. Temperature inversions in Salt Lake City are intensified by the surrounding mountain ranges. The mountains form a barrier, trapping cold air and stopping it from dissipating.

When the city experiences stagnant and polluted air, the inversion layer traps pollutants close to the surface. This leads to decreased air quality and the formation of a visible layer of smog, especially during winter months when temperature inversions are more common.

Temperature inversions can worsen the health of residents, as exposure to PM2.5 – particle pollutants measuring 2.5 microns in diameter or less – can aggravate respiratory and cardiac diseases.

“The Mother of All Inversions”

In 2013, Salt Lake City suffered from what was then called “the Mother of All inversions” (1). The poor air quality in Salt Lake City was the result of a cold-weather “inversion” that led a group of Utah doctors to call for a public emergency due to air pollution.

The poor air quality caused by pollution from the growing urban activity in and around Salt Lake City – situated in a valley with mountain ranges to both the east and the west – combined with a cold air mass that moved in early in January. The cold air was trapped in place by warmer air hovering above. The U.S. Environmental Protection Agency (EPA) said Salt Lake City’s air in January was the dirtiest in the nation. While EPA considers anything above 35 micrograms of fine pollution (PM2.5) per cubic meter unhealthy, levels in Salt Lake City reached as high as 130 in January.

In response to the inversion, more than 100 doctors delivered a letter calling on the governor of Utah to declare a public emergency.

Air quality and asthma

Another major concern during temperature inversions is asthma. Research published in 2010 by the Utah Department of Health reported an increase in emergency department visits related to asthma when winter inversions occur.

It’s important to note that Salt Lake City is not the only city prone to temperature inversions that cause dangerous spikes in particle air pollution. Los Angeles, California, Denver, Colorado, Pittsburgh, Pennsylvania, Mexico City, Mexico, and Beijing, China.

What to do when an inversion affects your city

It’s important to be aware of poor air quality, which may present as smog or may be completely invisible. When you know the air quality is poor, you can take action to protect your health.

• Receive real-time air quality alerts and forecasts through a free air quality app.
• Monitor your outdoor air quality data in your neighborhood.
• Close doors and windows.
• Set your HVAC to recirculate mode.
• Avoid exercise and outdoor activity.
• If you must go outdoors, wear a KN95/FFP2 mask.
• Turn on a high-performance air purifier.