What causes inversions in Salt Lake City? And what’s behind the pollution?

Salt Lake Valley residents can make a mountain disappear. It’s not a magic trick — it’s called inversion.

Inversion is a weather phenomenon where warmer air at higher altitudes traps colder air — and pollution — below it, according to the Utah Department of Environmental Quality. This “lid” of warm air then concentrates pollution, which often obscures the Wasatch and Oquirrh mountains, typically from November through February each winter.

The buildup of pollution in the valley is decreasing, scientists say. But they are learning that the consequences of this trapped smog could be far worse than a hazy horizon.

Inversions are natural — but pollution isn’t

Inversion is a natural occurrence. It’s the buildup of particle pollution that worsens visibility during an inversion event.

Heather Holmes, a chemical engineering associate professor at the University of Utah, is studying how this trapped valley pollution can affect human health.

Holmes and researchers from three other institutions received a five-year, $2.6 million grant from the National Institute of Environmental Health Sciences in 2021 for the study. She hopes the project can pinpoint the main sources of pollution during local inversion events and identify their respiratory and cardiovascular effects.

A recent study by Brigham Young University found that air pollution throughout the state shortens the average Utahn’s life expectancy by two years. The study found that such pollution, often trapped by inversions, also costs Utah’s economy around $1.8 billion annually, when accounting for factors like health care expenses, lost earning potential and loss of tourism.

“We primarily have particle pollution here in the wintertime, and when you breathe in those particles, a lot of those particles will end up going into your lungs,” Dr. Kevin Perry, an associate professor of atmospheric science at the University of Utah, said.

Once inhaled, any particles that are water soluble dissolve into the bloodstream, he said.

Breathing in this polluted air can contribute to heart disease, preterm births, gastrointestinal problems and an increased risk of stroke, Perry said. Particle pollution also has been linked to aggravated asthma, decreased lung function, and increased respiratory symptoms such as coughing or difficulty breathing.

These health impacts cause emergency room visits to increase across the Wasatch Front during events of high pollution, according to the Division of Air Quality.

What’s causing all that pollution?

Some days can be more “inverted” than others, but there isn’t an official measurement for inversions, said Bo Call, air monitoring center manager at the Division of Air Quality. The division also doesn’t track inversion days, but its data on pollution concentration shows the effects of the phenomenon.

The level of fine particulate matter — also called PM2.5 — is a main indicator of an inversion’s impact in trapping pollution.

Transportation emissions account for about 48% of PM2.5 pollution in a local inversion event, according to the Department of Environmental Quality. That includes emissions from vehicles, planes, trains, and other mobile sources.

Smaller, stationary sources like wood burning and home heating account for 39% of PM2.5 emissions during a typical inversion event. Emissions from larger industrial facilities account for about 13 percent.

“The biggest kind of misinformation that I see is that it’s all it’s all major industry, and it really isn’t,” Call said. “Since cars are the biggest single problem, doing whatever we can do to drive newer cars that pollute less — good hybrid cars, electric cars — that’s all going to be great.”

According to this winter’s data from a local air monitoring station, Salt Lake City has only exceeded the Environmental Protection Agency standard for unhealthy PM2.5 pollution four times between November 2021 and Feb. 1. From November 2000 through February 2001, the city exceeded that standard 29 times.

Perry said the reduction in particle pollution is likely due to improvements in transportation, since that’s one of the biggest sources of emissions in the valley.

“Over the last 30 years, there have been a lot of changes that have been made to vehicles to make them pollute less,” Perry said. “So even though we have more people and more vehicle miles being driven than 30 years ago, they actually put out less pollution than they did 30 years ago. What has changed is our understanding of how it affects human health and how dangerous it actually is.”

Inversions are experienced all over the western U.S., but levels of particle pollution are worse in some places than in others.

“If you go up into other parts of the state in the mountains where they have valleys, they have inversions just the same — it’s just there’s not as many people there, or any people there, so there’s no pollution that kicks in,” Call said.

“The pollution is not driving the inversions,” Call continued. “The inversions are driving the pollution.”

Stopping the smog

When the Division of Air Quality sees the potential for unhealthy levels of pollution combined with an inversion, it issues an action forecast. These voluntary and mandatory action days warn residents of bad air quality forecasts.

The DEQ recommends individuals use public transit, carpool or bike to reduce emissions during inversion days. When driving can’t be avoided, Tier 3 gasoline — a cleaner fuel that’s available at gas stations across the state — can also help reduce pollution in vehicles by up to 80%, according to the DEQ’s website.

“What’s really critical to reducing the air pollution during the inversion is not putting pollution into the atmosphere,” Holmes said. “That’s why you get things like the warning signs on the highway to drive less and carpool, because if you aren’t driving, then you’re not putting the pollutants into the air that then stick around because the inversion is there.”

Since inversion is a meteorological phenomenon, it can’t be avoided. But people can avoid exacerbating the effects pollution tend to have during inversion days — at least until another weather event clears out the haze, and makes the mountains reappear.