Skip to content

Ozone Season is upon us, understanding it and how to protect yourself

Ozone Action Day Alerts can be triggered year-round, whenever ozone levels are expected to exceed the 70 parts per billion federal standard.
Hazy Front Range From Golden Ponds (1 of 1)
The Front Range shrouded in haze, as seen from Longmont's Golden Ponds Park. Ozone measured 81.8 at nearby Vance Brand Municipal Airport, nearly 12 points over federal limits.

Mid-July, the dog days of summer, also marks the midpoint of Colorado’s “ozone season.”

Ozone Action Day Alerts can be triggered year-round, whenever ozone levels are expected to exceed the 70 parts per billion (federal) standard. Colorado’s “ozone season” officially runs from June 1 to Aug. 31, according to Andrew Bare, communications and outreach specialist with the Air Pollution Control Division of the Colorado Department of Public Health.

It may not be a household word, but ozone season is worth paying attention to, especially for those who suffer from respiratory conditions and those who exercise or labor outdoors. (Ozone here refers to ground-level, not atmospheric, ozone.)

As of Thursday, the CDPHE issued a total of 25 Ozone Action Day Alerts for the Front Range this year, compared with 43 in 2020, 32 in 2019, and 52 in 2018. 

“It’s concerning that we have this many alerts already,” Bill Hayes, air quality coordinator for Boulder County Public Health, said. 

The Environmental Protection Agency has designated the Denver Metro/North Front Range area as an ozone nonattainment area, meaning the region exceeds ozone standards. Based on last summer’s high ozone readings, the metro area is at risk of being reclassified as “severe.”

Ozone: The Hidden Air Pollutant

Ozone and inhalable particles known as particulate matter are the two most common sources of ambient air pollution along the Front Range, according to Hayes.

“Ozone is typically the bigger concern for us, but we also have days with concerning levels of particulates. Wildfire contributes to that but is not the only culprit,” Hayes said.

Unlike particulates — which can cause obvious visual impacts such as haze  — ozone is an odorless and colorless gas generated by chemical reactions between sunlight and two pollutants, volatile organic compounds and nitrogen oxide.

Wind and cloud cover reduce the potential for ozone formation, Hayes said, while clear skies and warm, sunny days increase it. Time of day is also a factor. 

“With sunlight driving the reaction the level of ozone builds as the daylight hours go on, so your highest ozone levels are in the late afternoon/early evening time frame. Ozone then starts to break down overnight, but high levels of ozone can persist well into the wee hours of the next morning,” he said.

Ozone’s Potential Health Impacts

The Environmental Protection Agency and the Centers for Disease Control and Prevention describe a number of potential health impacts from ozone as well identifying who is most at risk.

Health Effects

Ozone exposure can aggravate existing respiratory conditions such as asthma, emphysema and chronic bronchitis. It can also cause coughing, pain when taking a deep breath, sore or scratchy throat, breathing issues, inflamed or damaged airways and increased susceptibility to lung infection. Ozone is suspected as one potential cause of asthma development.

The EPA reports that some studies have found an association between ozone levels and increased symptoms and use of medication in children with asthma, while other studies describe an association between higher ozone and increased emergency room visits and hospitalizations for asthma. A few studies have also found a relationship between ozone levels and increased hospital admissions for chronic obstructive pulmonary disease (COPD). 

Recent research highlights the potential for more serious and widespread impacts. A 2020 study found higher death rates from respiratory causes in areas with high ozone levels, as reported by Reuters

Sensitive Groups

Ozone’s health impacts depend on the degree of exposure as well as individual risk factors. In addition to those with respiratory conditions, babies, children and older adults are also more vulnerable. Nutritional issues, such as reduced vitamin C and E intake, may increase sensitivity to ozone impacts as can certain genetic factors.

For more in-depth information about the health impacts of ozone on vulnerable individuals, please visit the EPA’s “Health Effects of Ozone in Patients with Asthma and Other Chronic Respiratory Disease” webpage and the American Lung Association’s “Ozone” page.

How to Stay Safer

The starting point for minimizing the health impacts of ozone is tracking local ozone levels, according to the EPA, the CDC and local medical experts, Todd Bull, MD, pulmonologist and director of the Comprehensive Lung and Breathing Program at the University of Colorado Hospital and Nate Little, MD, pulmonary critical care physician at Centura-Longmont United Hospital.

The CDC advises limiting exposure to elevated ozone by spending more time indoors, going outside when ozone levels are lower — typically morning and evening — and by limiting outdoor activities to those that are easier and don’t require breathing as hard.  

It’s essential, however, to take individual health status into account when deciding how to best reduce the risks of ozone exposure, Bull said. 

“There’s not one set level. It kind of depends on what your underlying risks are, what your age is, what your underlying comorbidities or disease states are,” he said, advising people to consult with their health care providers for guidance about what air pollution they need to start paying more attention and being more concerned. 

However, when the air quality index (described below) goes into orange, red and certainly purple levels, then everyone is at risk, he said.

Combinations of ozone plus particulate matter can be additive in terms of risk, Bull said. If someone needs to be outdoors then masks, in particular N95 masks, will provide protection against particulates—but they don’t protect against ozone.

In terms of indoor air quality, Bull recommended the use of air filters, particularly HEPA filters. On the other hand, Nate Little, MD, pulmonary critical care physician at Centura-Longmont United Hospital, said he hasn’t seen much patient benefit from them.

“I don’t routinely recommend them. That’s my personal opinion, but if a patient chooses to use one, I wouldn’t discourage that,” he said.

“Most of it is truly being attuned to your health, your respiratory status. If you’re perceiving any of those symptoms don’t neglect them, make sure you’re listening to your body and taking the right course of action to remove yourself from the environment—especially on those days that are suggested to be high pollutant exposure,” Little said.

We also encourage patients to remain diligent on their medication regimens to maintain control of asthma and COPD during times of increased pollutants, he said. 

Air Quality Index: A Convenient But General Tool

The AQI, or air quality index that Bull referred to is a user-friendly tool that’s gained traction on websites and apps such as The Weather Channel and Weather Underground. A color-coded tiered scale, the AQI ranges from 0-500 and from green (good) to maroon (hazardous). Each tier provides a general assessment of air pollution impacts on both “sensitive” and healthy individuals.

“Once we get over 100 AQI … that is when we start seeing upticks in the number of asthma patients showing up at the emergency rooms,” Hayes said. 

It should be noted that the AQI is a combined index based on standards for five major pollutants, including ozone and fine particulates. While elevated AQI often corresponds with elevated ozone locally, there are better ways to stay on top of ozone levels, according to Detlev Helmig, principal of Boulder A.I.R., which is contracted by the city of Longmont for air quality monitoring. The AQI is calculated based on data from a 24-hour period and is not a current reading, he said.

How To Stay Informed About Ozone Specifically 

Real-Time Ozone Levels

  • Real-time ozone levels at Longmont Municipal Airport and Union Reservoir are available 24/7 at Boulder A.I.R.’s Longmont Air Quality Now webpage.(There is no email notification option available.)
  • Real-time ozone levels collected by CDPHE at various monitors across the state are available here.
    • Go to "Air Quality by Parameter (all sites)"
    • Select "Ozone" from the drop-down menu
    • Select the date
    • Click "Display"
    • Hover over the monitor abbreviations for location information
    • BOUR is the Boulder Reservoir

Ozone Forecasts

  • Subscribe to CDPHE air quality alerts by emailing and requesting “Front Range ozone alerts.” Winter and statewide air quality alerts also are available. These CDPHE ozone alerts are based on monitors across the Front Range, including one at Boulder Reservoir, according to Bare. Alerts are issued as forecasts for the coming 24 hours.
  • This CDPHE air quality web page lists current Front Range alerts, including ozone-specific alerts.
  • Airnow, sponsored by several government agencies, delivers both AQI and primary pollutant information by zip code on the website. Airnow readings are based on the EPA’s NowCast, which uses algorithms to relate hourly data to the AQI for more current information. (Please note: Airnow’s EnviroFlash email notification service distributes AQI only, without primary pollutant information.)

Update: The link to the real-time ozone data has been updated with information on how to locate the latest data.