Smoke from the Marshall Fire as seen from Sombrero Marsh in Boulder on Dec. 30, 2021. Credit: Anthony Albidrez

Boulder County health officials worry warm and windy conditions could stir up toxic ash from the Marshall Fire, potentially posing a health risk to residents living around the burn scar and students attending nearby schools. 

On days when the ash is visible in the air, Boulder County Public Health urges people to stay inside and to wear an N95 mask. It suggests students remain indoors, too. 

To help residents understand at least some of the potential health risks on a given day, the health department launched an air monitoring program on Monday, April 11, allowing anyone to sign up for real-time alerts to get notified when the monitors indicate the air might be unhealthy to breathe. 

Since the Marshall Fire razed more than a thousand homes in the Town of Superior and the City of Louisville, leaving behind potentially harmful particles that can end up in the air, the county installed the monitors at 25 locations around homes, schools, a recreation center and open space. 

Anyone can easily sign up online for alerts at up to three of these locations using their phone number or email. 

The alert system requires users to choose the severity of harmful air pollution for which they want an alert. That severity is based on the Environmental Protection Agency’s air quality index (AQI), ranging from “good” to “hazardous.” 

An AQI of more than 150 is considered unhealthy for most people. For sensitive groups, an index above 100 can be unhealthy. On Tuesday night, amid dry and windy conditions, the county monitors were registering an index of around 6 (give or take). AirNow, a federal air quality monitoring program, was showing slightly higher levels nearby, though it has no monitors in the burn area. 

Boulder County health officials did not return requests for interviews. 

What are they measuring, and why does it matter?

The monitors are measuring PM 2.5, fine particulate matter of 2.5 microns in diameter, which can enter people’s lungs and make its way into the bloodstream. Exposure to the particles can cause “serious health problems” when inhaled, according to the Environmental Protection Agency, including premature death, heart attacks, aggravated asthma and decreased lung function. 

PM 2.5 is a pollutant often emitted from wildfire smoke. 

“For outdoor air quality, we found that PM 2.5 levels decreased very quickly after the fire because of the snow that came just after. The PM 2.5 levels have not been too high since, either,” Colleen Reid, an assistant professor of geography at CU Boulder studying the potential health effects of the Marshall Fire’s impact on air quality, told the Boulder Reporting Lab. 

The county has said that “snow and moisture can keep potentially harmful particulates on the ground and out of the air. However, air quality will fluctuate when the affected areas dry out and the wind speeds increase.”

The data is on the LoveMyAir Network, a program that uses “low-cost cutting-edge air pollution sensor technology, redeveloped with solar, battery storage and data connectivity” in Colorado municipalities. The program launched in 2018 in partnership with Denver Public Schools, as part of the Bloomberg Philanthropies U.S. Mayors Challenge

Reid and other CU Boulder researchers also have been studying the potential health impacts of volatile organic compounds (VOCs), including benzene, which can cause cancer with long-term exposure 

“There were other air pollutants of concern such as VOCs that we are still trying to analyze,” Reid said.

The Marshall Fire was Colorado’s most destructive wildfire in terms of property damage, and the VOCs are largely the result of torched buildings, automobiles and other belongings. Health risks from these chemicals have been largely understudied after wildfires. 

“From any wildfire (and this one was unique in burning so much human-made material compared to pure vegetation), there has been little studied in the way of long-term health impacts,” Reid said.  

According to the county, a preliminary analysis of outdoor air quality measurements of VOCs by the National Oceanic and Atmospheric Administration in the burn area indicate the concentration of VOCs is similar to normal urban air pollution.

Indoor air quality, other potential health effects

Researchers have been studying indoor air quality impacts, too, and have released tips on how to clean out potentially toxic smoke and ash from your home. The county health department recommended residents with homes still standing in the burn area professionally clean their HVAC systems and replace any air filters that appear soiled. 

The fire has also had an impact on local water quality, prompting hundreds of Superior residents to report unusual tastes and odors. The town said it has collected dozens of water samples and all have tested within federal health standards for the chemicals tested. The town also said it is removing ash from the banks of its water storage reservoir and installing a new filtration system to address taste and odor concerns. 

The disaster may compound long-standing air quality concerns from automobile traffic, oil and gas drilling, and wildfires. This combination of air pollution reacts with sunlight to create ground-level ozone, all-too-common across the Denver metro area, which can be harmful to breathe. On Tuesday, the EPA moved ahead with plans to downgrade Colorado’s noncompliance with federal ozone standards along the northern Front Range from “serious” to “severe.” 

Residents across the state can also sign up for unhealthy ozone alerts.

John Herrick is a reporter for Boulder Reporting Lab, covering housing, transportation, policing and local government. He previously covered the state Capitol for The Colorado Independent and environmental policy for Email: