Summer of smog: Colorado on track for its worst air quality in over a decade

After years of slow progress, Front Range ozone levels spike in 2020 and 2021

By: - July 30, 2021 5:00 am

The Rocky Mountains are faintly visible as air pollution blankets Denver in a thick layer of haze on July 29, 2021. (Chase Woodruff/Colorado Newsline)

Amid scorching heat waves, record highway traffic and rebounding oil and gas drilling activity, Colorado’s Front Range is on track in 2021 to experience its worst summer of air pollution in well over a decade, putting millions of people in and around the Denver metro area at greater risk of serious health impacts and premature death.

Thursday marked the 25th day in a row on which officials with the Colorado Department of Public Health and Environment issued an air quality alert for the region, warning residents, especially children and those with heart or lung disease, to avoid “prolonged or heavy outdoor exertion.” Eight-hour average concentrations of ground-level ozone, the main culprit in summertime smog, reached 85 parts per billion at the Chatfield State Park monitoring station as of 6:00 p.m. Thursday, well above the federal government’s 70 ppb health limit.

The smoggy skies are part of a clear turn for the worse for Colorado’s air quality in recent years, according to a Newsline analysis of Environmental Protection Agency data. State regulators have spent much of the last two decades attempting to bring regional air quality in line with federal standards, inching ozone levels gradually downwards. But those levels spiked in 2020 and are off to an even worse start in 2021, threatening to abruptly erase nearly 20 years of incremental progress.


Officials with CDPHE’s Air Pollution Control Division once expressed confidence that by this year, they would reduce Front Range ozone enough to comply with the health standard set by the EPA in 2008. Instead, the state appears to be further away from meeting that standard than at any point since it was enacted.

Across the metro area this summer, air-quality monitoring stations maintained by state and local health agencies have recorded their highest readings in years. An eight-hour ozone average of 102 ppb at Chatfield State Park on July 12 was that station’s highest measurement since 2003.

To account for outliers and statistical noise, regulators often evaluate air-quality trends over time using the fourth-highest reading at a given station in a given year. These "4th max" measurements, too, paint a bleak picture of recent Front Range air-quality trends. Barring drastic revisions to preliminary data released through the EPA's AirNow service, daily measurements at many regional monitoring stations already show 4th-max readings in 2021 at their worst levels in over a decade — even with a month or more remaining in the peak summer ozone season.

Despite the now-daily public health alerts, state policymakers appear to be in no hurry to respond to this summer's ozone spike with government action. A proposal that would have required large Denver-area employers to take steps to reduce single-vehicle commuting was abandoned by the APCD last week. Gov. Jared Polis addressed the summer's air quality issues for the first time in a Wednesday Facebook post, telling Coloradans that "we all have a role to play here," and reminding them to check their tire pressure and tighten their gas caps.

"We’ll have a clearer sense of what the data says about our current ozone season soon — we’re scheduled to present a report on the 2021 ozone season to the Air Quality Control Commission at its October meeting," Andrew Bare, a spokesperson for the APCD, said in a statement. "Long-term trends show a decline in ozone values, and it’s too early to tell whether the high readings seen last year and during the beginning of this summer reflect a change in that trend or are just part of the variability that we see from year to year."

Ground-level ozone — which is known to cause a wide variety of negative health impacts, including respiratory issues like asthma, heart disease and other cardiovascular conditions — is formed by a chemical reaction between sunlight and certain "precursor" pollutants, like nitrogen oxides and volatile organic compounds. Research has shown that tailpipe emissions from gas-powered vehicles and emissions from oil and gas activity each account for roughly 40% of ozone-precursor emissions along the Front Range, with the remainder coming from other industrial sources.

After ebbing somewhat due to the COVID-19 pandemic, both motor vehicle traffic and oil and gas drilling have rebounded in Colorado in 2021. In June, a Colorado Department of Transportation monitoring station at Interstate 25 and Colo. 6 recorded an average count of over 258,000 vehicles per day, an all-time record. The Colorado oil and gas industry's average monthly "rig count," which measures drilling activity, has rebounded to 12 after reaching a low of four last October, according to oilfield services company Baker Hughes.

The Front Range region faces unique challenges in tackling ozone, including "background" ozone that drifts in from other states and even from other countries, as well as higher levels of sunlight and topological factors.

"We're kind of uniquely poised for high ozone, in part because we have the sources of nitrogen oxides and VOCs, but it's also that sunlight component," Bill Hayes, air quality program coordinator for Boulder County Public Health, said during a recent briefing with reporters. "At higher elevation, we get greater solar irradiance than they do at sea level, so more energy to drive the reaction."

Notably, although many Coloradans — including state officials — are quick to blame smoke from wildfires across the West for lower air quality overall, experts say the jury is still out on whether it impacts ozone levels specifically.

"Wildfire smoke and ozone is a bit contentious, and there's a lot of study going on right now about it," Hayes said. He said that an analysis of Boulder County monitoring data recently completed by researchers at Colorado State University, which has yet to be published, found that ozone levels are "not heavily impacted" by wildfires. In some cases, wildfire smoke that is high enough in the atmosphere can actually inhibit ozone formation by reducing the amount of sunlight that reaches ground level.

Scientists say that no level of air pollution is known to be safe. After lowering its eight-hour ozone standard from 80 ppb to 75 ppb in 2008, the EPA lowered it again to 70 ppb in 2015 — over the objections of top Colorado officials like then-Gov. John Hickenlooper. The World Health Organization, meanwhile, recommends a standard of roughly 50 ppb.

Overall, air pollution is estimated to cause between 90,000 and 360,000 premature deaths in the U.S. annually. A 2019 study by the University of Washington's Institute for Health Metrics and Evaluation estimated that ambient ozone pollution, specifically, causes more than 12,000 deaths each year in the U.S., including 814 in Colorado.

That's why public health experts say that even if the spike in ozone levels over the last two years proves to be only a temporary backslide, Colorado needs to do more than it's done over the last decade to combat air pollution.

"For the past 20 years, we're seeing a very slow downward trend," said Hayes. "Certainly not the trajectory we need to see."


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Chase Woodruff
Chase Woodruff

Chase Woodruff is a senior reporter for Colorado Newsline. His beats include the environment, money in politics, and the economy.