House Fires and Wildfires Produce Different Air Toxics - and the Gap Is Narrower Than Expected

When wildfire burns through a neighborhood, the smoke tells a complicated chemical story. The houses themselves are not simply kindling; they contain plastics, insulation, synthetic carpets, and dozens of other petroleum-derived materials that release different compounds when they ignite than a stand of Douglas fir does. Knowing exactly which compounds are elevated - and by how much - matters for the millions of people now living at the expanding wildland-urban interface.

A laboratory study from the University of Colorado Boulder's Cooperative Institute for Research in Environmental Sciences (CIRES), working with Colorado State University (CSU), has now produced the most detailed picture yet of how building materials change the chemistry of fire smoke. The results, published in ACS Environmental Science and Technology, contain a surprise: the gap between a house fire and a vegetation fire is real but narrower than many researchers expected, because synthetic materials represent a small fraction of any home's total mass.

What 18 materials revealed under laboratory conditions

CSU researchers burned 18 building materials individually inside a fume hood while CIRES researchers measured the volatile organic compounds (VOCs) released. VOCs are a broad family of carbon-based gases that affect air quality and human health; some, like benzene and styrene, are classified carcinogens.

Synthetic materials - plastics, foam insulation, nylon-based components - produced clearly elevated VOC levels compared with natural materials. Burning plastics and insulation released concentrations of benzene and styrene that stood out markedly. The experiments also detected less-understood molecular fragments derived from nylon polymers that warrant further investigation.

"There's a need to better understand what these emissions look like for structures that contain a complex mixture of different synthetic materials that have unique emission profiles," said lead author William Dresser, a CIRES research scientist.

The whole-house calculation complicates the story

Individual material results, however, do not translate directly to real-world fire scenarios. A house is mostly wood. Timber, plywood, engineered lumber panels, and other cellulosic materials constitute the overwhelming majority of a home's mass; synthetic materials account for only a modest fraction. When Dresser and colleagues built a whole-house emission estimate using standard construction data, that proportion mattered enormously.

Comparing simulated whole-house fire emissions against two reference scenarios - a structural wood fire representative of framing lumber, and a natural Douglas fir forest fire including bark, needles, and wood - the researchers found that benzene and styrene remained elevated in the house fire scenario. Other VOCs, though, did not separate clearly from structural wood or forest fire baselines.

"Since synthetic materials are a pretty small part of a home, we found their impact can be muted," Dresser said. "But it was in part surprising to see the nuanced picture."

The finding does not minimize the health risk. Urban fires are growing in frequency and scale - the Marshall Fire in Colorado, the Lahaina fires in Hawaii, and the 2025 Los Angeles fires together destroyed thousands of structures - and even modest elevations in carcinogen levels in dense smoke have health significance for exposed residents and firefighters. But the data suggest blanket assumptions about house-fire smoke being dramatically more toxic than wildfire smoke require qualification.

What was not measured

The study focused on airborne VOCs during combustion. It did not address ash chemistry, which can carry a different suite of toxic compounds including heavy metals and persistent organic pollutants that leach into soil and waterways after a fire. That gap is significant; post-fire ash contamination has emerged as a major environmental problem following large structure fires.

"Future work is focused on the compounds that end up in ash or can be leached from ash into waterways," said CIRES Fellow Joost de Gouw, a co-author on the study.

The laboratory setup also burned materials in isolation rather than in the chaotic, interacting conditions of an actual structure fire, where flame temperatures, oxygen availability, and material proximity all influence emission chemistry. Real-world measurements from instrumented structure burns or actual wildland-urban interface fires would help calibrate how well laboratory data predict field conditions.

Implications for air quality management

For air quality agencies and public health officials responding to structure fires, the study offers two practical points. Benzene and styrene - and potentially novel nylon-derived compounds - should be monitored specifically near burning structures, as they distinguish house fires from vegetation fires. But treating all house-fire smoke as uniformly and dramatically more toxic than wildfire smoke would misrepresent what the chemistry shows. The nuance matters for communicating risk to the public and for designing monitoring protocols at the wildland-urban interface.