Creosote is an oily, tar-like substance produced when wood or coal is burned incompletely. The term covers several related products: the dark, sticky residue that builds up inside chimneys, the industrial wood preservative applied to railroad ties and utility poles, and even a desert shrub that shares the name. Which type matters to you depends on context, but most people searching this term are either dealing with chimney buildup or wondering about the dark coating on treated wood they’ve encountered outdoors.
Two Main Types of Creosote
Creosote comes in two distinct chemical forms, and they’re made very differently.
Wood-tar creosote is produced by heating beechwood or similar hardwoods to 200–220°C and collecting the oily liquid that distills off. It’s mostly phenol (14.5%), p-cresol (13.6%), and compounds called guaiacols. This version was once used in medicine as a disinfectant, laxative, and cough treatment, though it’s been largely replaced by safer alternatives.
Coal-tar creosote is a byproduct of converting coal into coke at high temperatures. It’s a far more complex mixture, roughly 85% polycyclic aromatic hydrocarbons (ring-shaped carbon compounds) with a smaller fraction of phenols. This is the version used as an industrial wood preservative and the one most people encounter in everyday life. It’s the dark, pungent coating on railroad ties, utility poles, marine pilings, and highway construction timbers.
Chimney Creosote and How It Forms
When you burn wood in a fireplace or stove, the fire releases hydrocarbon gases. If those gases cool before they fully combust, they condense on the inner walls of your chimney as creosote. Wet or unseasoned wood produces more smoke and dramatically increases this buildup, which is why experts recommend burning only wood that has dried for at least six months.
Chimney creosote progresses through three distinct stages, each harder to deal with than the last:
- Stage 1: A light, dusty soot coating on the chimney walls. This is the easiest to remove with a standard chimney brush.
- Stage 2: Tar-like, crunchy flakes that cling to the chimney interior. Harder to brush away but still manageable with mechanical sweeping.
- Stage 3: A thick, glossy coating that resembles dripping tar. When heated, it softens to a wax-like state. This is the most dangerous form and can only be removed by professional mechanical sweeping.
Why Chimney Creosote Is Dangerous
Stage 3 creosote is a serious fire hazard. Wood that has been repeatedly exposed to intense heat can ignite at temperatures as low as 212°F, far below its normal ignition point of around 500°F. A thick layer of creosote coating the inside of a flue provides fuel for a chimney fire that can reach extremely high temperatures and spread to the rest of the structure.
Chimney cleaning logs, which release chemicals designed to loosen creosote, can help with light to moderate buildup. They work by causing creosote to flake off and fall into the firebox, where it can be swept out. But they cannot remove hardened, glazed stage 3 deposits, and they won’t reveal hidden structural problems in the flue. They’re a maintenance tool, not a substitute for professional inspection and sweeping.
Reducing Creosote Buildup
The single most effective thing you can do is burn dry, seasoned firewood. Freshly cut wood contains too much moisture, which makes it burn cooler and smokier. That extra smoke is what condenses into creosote inside the chimney. Let firewood dry for a minimum of six months before burning it.
Beyond fuel choice, ensuring good airflow matters. A hot, well-ventilated fire burns more completely and sends fewer unburned gases up the flue. Smoldering, oxygen-starved fires are creosote factories. Keeping the damper open, avoiding overloading the firebox, and making sure the flue draws properly all help reduce accumulation.
Health Risks of Creosote Exposure
Coal-tar creosote contains compounds classified as probable carcinogens. Short-term skin contact can cause rashes, chemical burns, and increased sensitivity to sunlight. Workers in wood-treatment facilities who handle creosote-treated lumber regularly face the highest exposure risks. For most homeowners, the concern is more limited: brief contact with a treated railroad tie or fence post isn’t the same as occupational exposure, but you should avoid prolonged skin contact and wash it off promptly.
Breathing creosote fumes in an enclosed space can irritate the eyes, nose, and throat. Inside a home, this typically happens when a chimney is blocked or poorly ventilated, pushing combustion byproducts back into living spaces rather than up the flue.
Environmental Contamination
Coal-tar creosote doesn’t break down quickly in the environment. At sites where wood was treated with creosote industrially, contamination typically shows up in surface soils and groundwater. The compounds leach slowly from treated wood and soil into nearby water sources. Bioremediation techniques can remove 70% to 85% of creosote constituents from contaminated groundwater, but cleanup at former treatment facilities is a long process.
The Creosote Bush: A Different Thing Entirely
The creosote bush (Larrea tridentata) is a hardy desert shrub found across the American Southwest and northern Mexico. It gets its name from the tarry smell of its resin, especially after rain, but it has no chemical relationship to wood-tar or coal-tar creosote. Known as chaparral in the U.S. and gobernadora in Mexico, the plant has a long history in traditional medicine. Indigenous and folk practitioners have used it to treat arthritis, colds, diarrhea, skin conditions, inflammation, and kidney stones. It remains widely used in herbal medicine across the border region, though it’s not approved as a pharmaceutical treatment.