Wintertime Inversion Explained

On December 4th, 2019 Salt Lake City and surrounding areas experienced the worst air quality in the nation, with Air Quality Index (AQI) levels rising above 150. This level of air pollution is considered unhealthy for everyone. 

The early December 2019 inversion layer captured with a drone by Micah Berman.

What causes wintertime pollution?

The Salt Lake Valley is prone to temperature inversions because of the geography of the Salt Lake Valley. Most of the time, cool air rests above warm air in the valley, but during an inversion, a dense layer of cold air becomes trapped beneath a layer of warm air above, which acts much like a lid. Since the air is trapped within the bounds of a valley, there is nowhere for the air to escape. When snow falls and sticks to the ground, the valley floor reflects, rather than absorbs heat from the sun, preventing the mixing of warm and cool air. Pollutants from vehicles, refineries, buildings, and homes become trapped beneath the layer of cold air. Until a winter storm passes through the valley to clear out the inversion layer, it will persist and pollutants will continue to build under the trapped layer of cold air.  Personal vehicles, and urban “area sources”, or buildings that emit air pollutants, are the largest contributors to the Salt Lake Valley’s inversion pollution. 

What is PM 2.5?

The geography and resulting inversion in the Salt Lake Valley create perfect conditions for the accumulation of particulate matter pollution. Specifically, fine particulate matter, or PM 2.5, which measures less than 2.5 micrometers in diameter, such as ash from burning wood and vehicle emissions. Particulate matter this fine can pass through airways, settle in the lungs, and be absorbed by the bloodstream. Inversion pollution also impacts the viewshed of the Central Wasatch from the Salt Lake Valley because it lowers visibility once particulate matter builds up under the layer of cold air. 

Inversion smog in the Salt Lake Valley in December 2019. Photo by Austin Lawrence.

Health risks

So, what does inhaling particulate matter do to our bodies? Dr. Brian Moench, M.D. and Board President for Utah Physicians for a Healthy Environment (UPHE) explained that “air pollution provokes the entire spectrum of diseases associated with smoking cigarettes.” These diseases include asthma, kidney disease, arthritis, and lung diseases. Dr. Moench explained that “even if you don’t feel it in your lungs doesn’t mean you are not being harmed.”

Although there are only a handful of inversion days in the Salt Lake Valley, research has found that short term pollution exposure causes long term health effects (Miller et al. 2017). Dr. Moench explained that the “biologic consequences [of PM2.5 exposure] can go on for weeks, if not months and sometimes indefinitely” because particulate matter is absorbed by lung tissue and the bloodstream and cannot escape the body.

What can you do to help and protect your health?

Here are steps we can take to lessen our impacts on the air, our communities, and our bodies.

  • Take public transit
    • Get acquainted with public transit options around where you live. Bus schedule information can be found here. Use the UTA GoRide app to easily pay for transit and UTA’s Transit app to find your connection
    • If you have a ski pass, your pass works as a bus pass on ski buses in the Cottonwood Canyons. Ski bus schedules.
    • The CWC, UTA, member jurisdictions, ski resorts, Save Our Canyons, and the Wasatch Backcountry Alliance have increased ski bus service for the 2019/2020 ski season. Find more information here.
  • Carpool
    • Carpool with co-workers or friends to work, school, the mountains, etc.
    • Get closer parking spots at Alta Ski Area when you carpool with three or more people.
    • The R.I.D.E. app is a rideshare and carpooling incentive app that gives rewards for ridesharing to all four Cottonwood Canyon resorts. 
    • Pay less for parking when you carpool to Solitude Mountain Resort. Parking is $5 for vehicles with four or more occupants, $10 for three occupants, and $20 for one to two occupants.
  • Follow volunteer and mandatory no-burn days and do not burn wood on poor air quality days.
  • Be idle free. In 2011, the Salt Lake City Council unanimously approved the city’s first Idle Free ordinance. This ordinance banned vehicle idling for over two minutes within Salt Lake City limits.
  • Ride your bike, even in winter. Make sure to wear a pollution mask when riding on days with poor air quality.
    • Wear a PM2.5 face mask. You can find pollution masks at Iconoclad downtown and at both of Salt Lake Running Company locations.

When it comes to air pollution, we can make a difference if we are willing to change our transportation habits.


Resources for live air quality information

Purple Air

Air Visual – this can also be downloaded as an app that shows real-time pollutant levels near you

Air Now

Utah DEQ Air Quality monitor



Miller M, et al.  Inhaled Nanoparticles Accumulate at Sites of Vascular Disease.  ACS Nano. 2017 May 23; 11(5): 4542–4552.

Dr. Brian Moench


Written by Quinn Graves

One thought on “Wintertime Inversion Explained”

  1. Susie G-H says:

    Great explanations! Thanks for
    Making it simple.

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