Winter Inversions

Wintertime inversions are a common event in Utah, occurring primarily during the months of December through February. Prolonged inversions can lead to the high levels of fine particulate pollution, or PM2.5. These high pollutant levels raise significant health and air quality concerns, particularly on days when the pollutant concentrations exceed the national health standards.

Utah residents may be surprised to discover that vehicles and urban “area sources” contribute the largest proportion of the emissions responsible for the formation of fine particulates. Whether it’s driving less or being more energy efficient, personal behavior changes that reduce emissions have a beneficial impact on air quality, not only during inversion periods but year round.

What Is an Inversion?

What is an Inversion?Inversions occur during the winter months when normal atmospheric conditions (cool air above, warm air below) become inverted. Inversions trap a dense layer of cold air under a layer of warm air. The warm layer acts much like a lid, trapping pollutants in the cold air near the valley floor. The Wasatch Front valleys and their surrounding mountains act like a bowl, keeping this cold air in the valleys. The snow-covered valley floors reflect rather than absorb the heat from the sun, preventing the normal vertical mixing of warm and cold air. Fog exacerbates the problem, facilitating chemical reactions that create even more particles and higher pollutant concentrations. The longer the inversion lasts, the higher the levels of pollution trapped under it. The warm inversion air layer is usually displaced by a strong storm system which restores air quality to healthy levels.

In the Salt Lake Valley, inversions typically occur following a snowstorm. The new snow pack enhances colder temperatures near the surface. At the same time, clear skies lead to warmer temperatures above. Fog and freezing rain can also occur during inversion periods. The lowered visibility that accompanies inversions doesn’t necessarily signal high pollution levels because visibility often deteriorates well in advance of harmful concentrations of pollutants.

What Is Fine Particulate Pollution (PM2.5)?

What is a Fine Particulate Pollution.Utah’s mountain valleys and wintertime temperature inversions provide ideal conditions for the formation of fine particulates, or PM 2.5. Concentrations of PM2.5 build as temperature inversions persist. Utah’s unique geography and weather, when combined with emissions, creates unusual chemical and photochemical conditions that lead to the formation of PM2.5.

Fine particulates are particles that are less than or equal to 2.5 micrometers in diameter. They are measured in micrograms per cubic meter or µg/m3. The health-based standard of 35 µg/m3 can be likened to one grain of table salt in a one liter bottle filled with air.

PM2.5 is categorized as either primary or secondary. Primary PM2.5 is emitted directly as a particle and enters the atmosphere as soot from roadways or tailpipe emissions. Secondary particulates form when precursor emissions react in the atmosphere and combine to create PM2.5. Twenty-five percent of Utah’s PM2.5 is considered primary, with the majority (seventy-five percent) is considered secondary, or the product of chemical reactions. Both categories of PM2.5 are the products of fuel combustion.

What Are the Health Concerns?

Fine particulate matter poses serious health concerns because it can pass through the nose and throat, lodge deeply in the lungs, and pass across the lungs into the cardiovascular system. Particles can aggravate lung diseases such as asthma and bronchitis, and increase respiratory symptoms such as coughing, wheezing, or shortness of breath. PM2.5 can aggravate heart conditions, including congestive heart failure and coronary artery disease.

People of all ages face health risks from PM2.5 pollution, but some groups are more susceptible than others. Children are at high risk because they spend more time playing outdoors, their bodies are still developing, and they breathe more rapidly than adults, inhaling more air per pound of body weight. The elderly and those with acute or chronic respiratory problems are also at high risk. Even active adults who exercise outdoors face an increased risk from fine particulates because PM2.5 penetrates deeper into the parts of the lungs that are more vulnerable to injury.

What Are the Sources of PM2.5?

While many people assume that industry contributes the majority of the emissions that form PM2.5, mobile and area sources are by far the greatest sources of fine particulate pollution. The chart below, based on Division of Air Quality (DAQ) estimated emissions for 2010 for the four urbanized Wasatch Front counties (Salt Lake, Davis, Utah, and Weber) illustrates how mobile sources and area sources combined account for 89 percent of the emissions that contribute to fine particulate pollution along the Wasatch Front.

PM2.5 sourcesBecause DAQ has been actively working to reduce larger particulates (PM 10) in the state in recent years, the Division has already implemented a number of emissions control strategies. These strategies have resulted in significant particulate reductions. Historically, the primary reduction strategies have been directed at industrial emissions. But with PM 2.5, industry currently contributes about eleven percent of the pollution. This leaves reductions in mobile and area sources as the critical focus areas for reducing the emissions that form PM 2.5. Mobile sources are on-road vehicles. Area sources are small industrial and commercial sources that emit less than 100 tons per year of pollution and activities generally associated with urban living, including gas and wood stoves, dry cleaning, gas stations, and water treatment facilities.

As part of its pollution reduction planning for the PM2.5 State Implementation Plan (SIP), DAQ proposed rules to reduce emissions from these mobile and area sources. Because the chemical reactions forming the majority of fine particulate pollution are complex and the emissions come from a wide variety of sources, finding solutions to the problem of high PM 2.5 in Utah’s airsheds has proven complicated. It will take a concerted effort on the part of the public and small business to find ways to collectively reduce these emissions. A combination of small, individual reductions will be needed to achieve the large scale reductions necessary to address the state’s PM2.5 challenge.

What You Can Do

While it may seem like individual efforts won’t make a difference in reducing high particulate levels during inversions, if Utah residents reduced the emissions produced in the course of their daily lives, the collective impact on air quality would be significant.

Drive Less, Drive Smarter

Department of Motor Vehicles (DMV records show that there over 1.4 million registered vehicles along the Wasatch Front. If every driver committed to driving 10 percent less every day, the emissions levels from mobile sources would be reduced by 10 percent. This might mean one less trip per day, not idling, carpooling, trip chaining, or delaying unnecessary travel. Since mobile sources account for 41 percent of the emissions along the Wasatch Front, a 10 percent decrease in emissions could have a sizeable impact.

DEQ’s Choose Clean Air site contains other tips for reducing mobile and area source emissions at home, at work, or on the road.

Watch for Division of Air Quality Action Alerts

AlertsThe Division of Air Quality (DAQ) issues action alerts when pollution levels are approaching levels that could impact health. These alerts are designed to be proactive, notifying residents in advance of pollution build-up so they can begin to reduce their emissions before fine particulate concentrations reach unhealthy levels. When pollution levels reach 15 µg/m3 for PM2.5, DAQ issues a ‘yellow’ or voluntary action day, urging Utah residents to drive less and take other pollution reduction measures. At 25 µg/m3, 10 µg/m3 below the EPA health standard, DAQ issues a “red” or mandatory advisory prohibiting burning of wood and coal stoves or fireplaces. Calling a “red” no-burn action before pollution levels reach the national health standard is designed to preemptively reduce air emissions before they build up to unhealthy levels.

Check the Air Quality Index

DAQ provides information to residents concerned about the health impacts of particulate levels through the Air Quality Index.

The Air Quality Index (AQI) is a color coded tool that helps residents understand the health impacts of local air quality. It is divided into six categories that correspond to different levels of health impacts based on EPA numerical values for five major pollutants regulated by the Clean Air Act: ground-level ozone, particulate matter, carbon monoxide, sulfur dioxide, and nitrogen dioxide. For each of these pollutants, EPA has established national air quality standards to protect public health. Particle pollution is one of two top pollutants that pose the greatest threat to human health in the U.S.

The color codes correspond to air quality conditions. Green is good, yellow is moderate, orange is unhealthy for sensitive groups, red is unhealthy for all groups. Purple and maroon signal very unhealthy and hazardous air quality conditions, respectively.

Participate in Air Quality Planning

DAQ implemented an unprecedented public involvement process for the development of the PM2.5 SIP, meeting with over one hundred participants from counties that are currently in nonattainment for EPA PM2.5 standards. These stakeholders formed working groups tasked with recommending emission control strategies to reduce PM2.5 levels. These recommendations formed the basis for recent DAQ rulemaking for area sources. DAQ will continue to work in the coming year on additional reductions and welcomes public input into the process.

The Utah Clean Air Partnership (UCAIR) partners with DEQ and other organizations working to improve air quality in Utah. To become more involved with UCAIR or any of its clean air partners, visit the UCAIR web site for more information.

Winter Inversions– information provided by

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Utah Department of Environmental Quality

Office of Planning and Public Affairs

Phone: 801-536-4000 | E-mail

Today’s Air Quality

Tooele County Health Department

151 North Main Street
Tooele, Utah 84074
(435) 277-2300

Monday-Thursday 8am - 6pm
Friday 8 am to noon
Closed holidays.

920 East Wendover Boulevard, Wendover Utah 84083
Call (435)277-2326 or 277-2327 for available services and hours.

To report an emergency after hours call (435) 882-5600.