I remember my first view of Denver’s infamous brown cloud over 40 years ago, as I drove from Loveland to a hearing on the risks of carbon monoxide pollution at altitude convened by Senator Hart for the National Commission on Air Quality. As an Albuquerque resident at the time, Denver’s dense layer of pollution was more impressive than my city’s, which stacked up as a visible brown layer against the Sandia Mountains on many winter days. Denver’s was darker and denser. Both cities continue to face some of the same challenges in controlling air pollution: strong sunlight and altitude, mountains hemming the air basins, and sprawl with heavy motor vehicle traffic. For the Front Range, there is the added challenge of emissions from the oil and gas industry. While our skies look clearer today, poor air quality remains a problem for Denver and other cities along the Front Range, with Denver receiving failing marks for ozone and particulate matter air pollution in the American Lung Association’s State of the Air report for 2022. And unfortunately, we have become numb to the pollution problem by frequent reminders that the Air Quality Index is in a range of concern.

Is air pollution along the Front Range a threat to our health and can it be improved? We have the scientific evidence to address the first question and the answer, unfortunately, is yes. The answer to the second is more complicated and depends on the impact of control efforts in progress. 

Is It a Health Risk?

Turning to the first question, we learn about the threat to public health posed by air pollution by describing the characteristics of the pollutants we inhale, studying them in the laboratory, and investigating how they affect health in the community. The last is the domain of epidemiology, my scientific field, which involves studying populations to directly assess the drivers of health and disease. Epidemiological studies of air pollution compare the health of people with higher and lower pollution exposures, or the frequency of health events on days with higher versus lower levels of air pollution in so-called time-series studies. With new sources of data, such as readings from satellites, and large-scale computer models for air pollution, we can now make exposure estimates for key pollutants for many locations around the world. Such estimates support the conduct of very large-scale studies in populations; exposures for participants in studies can be assigned based on where they live, and measures of health obtained from administrative records for health care systems. For example, researchers have used Medicare for pollution research, tracking mortality and hospitalizations in relation to exposures to ozone and particulate matter assigned based on residence location. With over 60 million participants, research based on this large cohort provides a powerful look at the risks of today’s air pollution.

For ozone and airborne particulate matter (the U.S. EPA uses PM_2.5 as the indicator, that is, particulate matter less than 2.5 microns in aerodynamic diameter), the most recent evidence shows risks to health at even the lowest levels typical of US urban environments, including Denver’s. The research shows increased risk for premature death, heart and lung disease, poor pregnancy outcomes, and reduced lung growth and risk for respiratory disease in children. The list of adverse consequences keeps growing, and now even includes  “brain aging.”

For current urban ozone concentrations, we know that breathing hard during exercise or work will temporarily reduce lung function for some people. The newer epidemiological findings challenge regulators in following the requirements of the Clean Air Act in setting National Ambient Air Quality Standards (NAAQS).  For the major or “criteria” air pollutants, the EPA Administrator is called on to set primary NAAQS “…the attainment and maintenance of which in the judgment of the Administrator, based on such criteria and allowing an adequate margin of safety, are requisite to protect the public health.” The Clean Air Act places a strong mandate on the Administrator to set NAAQS that are protective of public health. The more recent findings for ozone and particulate matter suggest that protection is not being achieved with the current NAAQS for these pollutants. In fact, this newer evidence led to the recent reduction in the annual standard for PM_2.5, which was lowered from 12 to 9 micrograms per cubic meter of air. For reference, Denver’s annual average for 2019-2021 was 7.8. 

But while we are currently in attainment for PM_2.5, we have been out of attainment for ozone for many years. For the ozone NAAQS, evidence review is just getting underway, and the extant evidence points to the need for lowering it. And, as a reminder, the NAAQS only indirectly touches on some of the specific sources of air pollution that we can see and that affect our neighborhoods, like the Suncor Energy refinery in Commerce City or highway traffic. We also know that air pollution’s risks to health fall unevenly across Denver, adding environmental justice concerns to control approaches.

So, for the first question—does air pollution along the Front Range pose a health risk—the now massive evidence on risks of air pollution, and particularly from the new studies, implies that the answer is a certain yes.

Is it Controllable?

To answer the second question—the potential for success in reducing air pollution in Denver and along the Front Range—let’s take a close look at regulatory requirements. Under the Clean Air Act, states are required to develop State Implementation Plans (SIPs), showing how they meet the NAAQS, and how air pollution will be reduced in areas that are not in attainment of the NAAQS. Denver and the North Front Range are in severe non-attainment for the 2008 NAAQS and moderate non-attainment for the 2015 standards for the 8-hour ozone standard.     

Given how ozone air pollution is generated, our nonattainment status is not surprising.¹ The chemist Arie Haagen-Smit described the photochemistry of ozone generation in a 1952 paper as the sunlight-driven chemical reactions involving nitrogen oxides (emitted by motor vehicles and other sources) and volatile organic compounds (emitted by many sources including the oil and gas industries).² With this understanding of ozone production, how can it be reduced? We have a potent driver of ozone generation in Colorado’s abundant sunlight and our mile-high altitude leads to a build-up of pollution in colder, stagnant air.  Hence, control relies on reducing emissions of precursors, which come primarily from motor vehicles and oil and gas operations. A 2017 report from the National Center for Atmospheric Research (NCAR) documented the critical contributions of the two pollution source categories with motor vehicles dominant in Denver and oil and gas operations more important to the north. Both of these source categories are trending upwards. With population growth, vehicle miles traveled increased by 40% from 2000 to 2018 in the Denver region. There are thousands of wells along the Front Range with an ever-increasing number of abandoned wells. Routine operations release nitrogen oxides, and the wells are a source of volatile organic compounds, both of which impact air quality in the region. 

Addressing these two major sources is an ongoing challenge covered in the SIP. The state projects that it will soon meet the 2008 federal standards and progress toward meeting the 2015 standards. To accelerate progress, Governor Polis has called for substantial reductions of nitrogen oxide emissions by the oil and gas industry in the nonattainment area during the ozone season (March – November).³ Colorado’s Air Conservation Commission has also implemented a rule restricting emissions from gas-powered lawn and garden equipment. While still at least five years away, a reduction of the ozone NAAQS based on the recent epidemiological findings would pose a further challenge for Colorado. 

Climate change may complicate Colorado’s efforts to control ozone pollution. Ozone generation rises with warmer temperatures, and wildfires are increasing in frequency and size. Wildfire smoke includes volatile organic compounds that are ozone precursors and that also contribute to PM_2.5. Initiatives to reduce greenhouse gas emissions will have the co-benefit of reducing ozone and particulate matter air pollution through the electrification of vehicles and the rising use of renewable energy.

I am optimistic for the long run that Denver’s air pollution problem will come under tighter control, and a suite of measures are in place to make that happen. But, be patient and remember that the “brown cloud” is not what it used to be. Everyone can take steps to reduce health risks during the summer ozone season. Check the Air Quality Index (AQI) and if the ozone level is unhealthy, follow the common-sense guidance that will protect you—staying indoors and avoiding exercise and other vigorous activities outdoors. 

Go deeper: Check out the Clearing the Air podcast on the state of air in Colorado. And read the companion article, Air Pollution and Health, by Jon Samet.

Additional References

1. Jacobs C, Kelly WJ. Smogtown. The lung-burning history of pollution in Los Angeles. The Overlook Press, Peter Mayer Publishers, Inc.; 2008.
2. Haagen-Smit AJ. Chemistry and Physiology of Los Angeles Smog. Industrial & Engineering Chemistry. 1952/06/01 1952;44(6):1342-1346. doi:10.1021/ie50510a045
3. Polis J. Memo. In: Robbins J, Hunsaker Ryan J, Oeth T, Murphy J, editors. 2023.

Welcome to our monthly column on the biggest issues facing us today in public health, written by the former dean of the Colorado School of Public Health, Jon Samet, a pulmonary physician and epidemiologist, and Professor of Epidemiology and Occupational and Environmental Health. Dr. Samet is a global health leader, shaping the science and conversation on issues ranging from tobacco control to air pollution to chronic disease prevention and more. Each month he shares expert insights on public health issues from local to global.