“Stay inside” isn’t a unique direction to receive in the summer of the coronavirus, but it was the recommendation to Front Range residents the first week of September for a reason that’s becoming too common: smoke. At one testing site in Longmont on Sunday, September 6, the Air Quality Index (AQI)—a standardized rating system that analyses the health of the air—clicked up into the 130s for wildfire smoke alone. Anything above 100 is unsafe for sensitive groups and above 150 is near universally unhealthy. But with the sky turned gray, the mountains invisible, and a dusting of ash coating vehicles, most probably didn’t need an alert from the National Weather Service to tell them as much. 

The episode, which corresponded with record temperatures and strong winds giving a boost to nearby wildfires, was a dramatic climax (so far) to Colorado’s record wildfire season. Data from the Environmental Protection Agency (EPA) shows spikes in AQI for Front Range counties well above September averages—the result of a monster year for wildfires that has included two of the state’s top five largest blazes in recorded history, according to data from the Denver Post

On top of that, the closer-than-normal proximity of fires to the Front Range, a flood of fires across the Western United States, and the perfect air currents and winds to carry it all over Colorado’s most populous region led to an unprecedented period of poor air quality. “Preliminary estimates show that this was likely one of the worst wildfire smoke seasons for Colorado in recent memory,” says Scott J. Landes, an air quality meteorologist for the Colorado Department of Public Health and Environment (CDPHE). “Particularly the month of August with PM2.5 and ozone levels well above average.”

PM2.5 is the fine particulate matter in smoke, specifically that about 3 percent the width of a human hair, or 2.5 micrometers. This is the stuff that scares scientists like Landes, who works to forecast smoke across the state. “The biggest issue with PM2.5 is that it is inhaled into your respiratory system, but much of it is not exhaled,” Landes says. “That makes it more difficult to breathe, but also PM2.5 can get absorbed into your bloodstream.” The result is not only a problem for breathing, but it can also cause heart issues. According to math worked out in a study by Berkeley Earth, an AQI of 130 for 24 hours leads to roughly the same PM2.5 intake as smoking two cigarettes. 

“We normally suggest everyone to stay inside as much as possible with the windows closed during the worst of the wildfire smoke episodes, especially older adults, children, and people who suffer from heart or lung ailments,” says Landes. 

But it may also be beneficial for Front Range residents to invest in a portable air filter (a HEPA filter can remove particulate as fine as PM2.5) and isolate it in one room running continuously, using this “clean air room” during the worst smoke. Anyone who needs to head outside—unfortunately the cloth masks we have been using to limit the spread of respiratory droplets can’t stop the microscopic PM2.5 particles—should limit strenuous activity, use their vehicle’s air conditioner, and recirculate the cabin air as opposed to bringing in outside air. 

While this summer has shown to be unique compared to previous years, it may only prove to be the beginning of a new normal. Jennifer Balch is the director of Earth Lab, a data-aggregating climate research group at the University of Colorado Boulder, and leads a project analyzing humans’ impact on wildfire, and according to her, as wildfires inevitably intensify, so will the smoke. “There are tens of millions of people right now choking on climate change,” she says. “And that trend is not going to change any time soon.”

Aside from seeing more fires thanks to climate change, warmer and drier weather leads to fires that both burn hotter and longer, sending more smoke aloft. And as more people move to the edges of wildlands, they are likely to be affected by poor air quality from smoke. Balch calls the Cameron Peak fire—the Larimer County blaze that has been the Front Range’s primary pollutant this year—a good example of a destructive fire close to a populated area. 

 “I hate to say it but could there be a Paradise in Colorado?” Balch asks, referring to the California town destroyed by 2018’s Camp Fire. “Yes. Climate change is only making it more likely, and we’re loading the dice.”