A blanket of wildfire smoke swept across the Denver metro area Thursday, discoloring Colorado’s famously blue sky and raising concerns about the associated health risks — particularly for vulnerable populations like the elderly and very young.

The Air Quality Index, a system that daily measures how clean or polluted the air is, indicated unhealthy conditions for sensitive groups in Denver on Thursday, according to AccuWeather.

During unhealthy air conditions, people with respiratory or heart conditions, the young and old should limit prolonged outdoor exertion.

Denver has some of the worse air quality in the country.

A known source of pollution across the Denver metro area during the summer months is ground-level ozone and particulate matter in the winter.

“In the summer we’re getting this from wildfires and in the winter we’re getting it from fireplaces,” Colleen Reid, a geography professor at the University of Colorado – Boulder, said of PM 2.5.

PM 2.5 — also known as particulate matter 2.5 — is a type of air pollution comprised of tiny particles 2.5 micrometers or less in diameter that are so small thousands of these could fit on the period at the end of a sentence. It’s a byproduct that comes from burning such things as the exhaust from a tailpipe, a cigarette or wildfires.

The unhealthy levels of air pollution, Reid said, are “likely because of the wildfires.”

Breathing in unhealthy levels can pose an increased risk of health problems such as asthma, heart disease and low birth weights.

“They’re very small so we can inhale them deeply into our lungs,” said Sheryl Magzamen, a professor of environmental and radiological health sciences at Colorado State University.

Firefighting teams have scrambled to battle the Stone Canyon fire near Lyons, which scorched 1,548 acres and was 20% contained Thursday, as well the Quarry fire, a blaze in Jefferson County that has charred more than 340 acres with 0% containment.

“Even if you are healthy, you can experience some of the symptoms related to smoke exposure,” Magzamen said.

Magzamen added, “We don’t know what the long-term effects look like.”

The reason?

Those studies, Reid said, haven’t been done yet.

And that’s troubling — Reid and Magzamen said — in a state known for its sunny skies and draw to the outdoors.

A University of Montana study in 2020, for example, found higher average PM 2.5 concentrations during wildfire season was associated with increased influenza the following winter.

It’s not just the fine particular matter, though, that is a worry.

Thomas Borch, a professor of environmental water and soil chemistry at Colorado State, is concerned about soil contamination.

In a soil study following the 2021 Marshall Fire — which killed two people and burned nearly 1,100 structures, becoming one the most destructive fires in Colorado history — Borch found that the severity and frequency of wildfires can drive chemical changes in the soil that distress the ecosystem and pose risks to human health.

“These fires are more characteristic of the fires we’d like to see,” Borch said. “Fires are part of a healthy ecosystem.”