What Are the Dangers of Furnace Soot in the Air?

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A well-maintained heating system produces little, if any, soot.

A furnace warms your home by distributing heated air through a series of ducts or pipes. Most furnaces are fired by either oil or gas, but others are powered by wood or coal, and under certain circumstances they may emit unburned gases in the form of soot into your living space and the air you breathe. While soot on your walls and other surfaces is unsightly, the health risks it poses are far more serious.

What Soot Is

When any fuel is ignited, gases containing tiny particles of solid matter escape. These gases are produced when hot particles meet cooler air, resulting in condensation. Since they exist in the air inside your home, they are virtually invisible until they start collecting on walls and other surfaces in the form of black soot. A clear indication of this occurrence is a dark gray stain that becomes visible when you remove a wall hanging. Soot can also sometimes be visible around heat registers, across ceilings and on a furnace itself -- which is never a good sign.

Recipe for Disaster

Soot is primarily carbon, and where it's visible, there's a good chance the air contains unhealthy levels of carbon monoxide gas, a potentially deadly byproduct of soot. All combustible fuels emit carbon monoxide, a problem made worse if your furnace isn't operating properly and is dumping more of this substance into the air you breathe than it should. Indoor levels of the gas should measure the same as outdoor levels. According to the New Jersey Chimney Sweep Guild, inhaling the substance may cause flu-like symptoms while exposure to high concentrations or prolonged exposure to lower concentrations may cause permanent damage or even death.

Respiratory Concerns

If the air in your home contains a high concentration of soot, its tiny particles can get deep into your lungs and cause inflammation. Breathing sooty air for long periods of time can jeopardize your respiratory system and damage blood vessels, according to the Environmental Defense Fund. Children are particularly vulnerable as is anyone with a known respiratory or heart condition.


The danger of carbon monoxide poisoning is the primary issue when dealing with furnace soot in the air. While traces of visible soot accumulation are definitely cause for concern, it's what you don't see that matters most. Installing carbon monoxide detectors should go hand in hand with identifying and correcting the source of soot.


Rachel Lovejoy

Rachel Lovejoy has been writing professionally since 1990 and currently writes a weekly column entitled "From the Urban Wilderness" for the Journal Tribune in Biddeford, Maine, as well as short novellas for Amazon Kindle. Lovejoy graduated from the University of Southern Maine in 1996 with a Bachelor of Arts in English.