Creosote is an oily substance produced from tar, but the word actually refers to several different things depending on context. It can mean an industrial wood preservative made from coal tar, a medicinal compound distilled from wood tar, a sticky buildup inside chimneys, or even a desert shrub. These are all distinct things that share a name, and understanding which one someone is talking about matters because their properties and risks are very different.
Coal Tar Creosote: The Industrial Preservative
The most common use of the word “creosote” refers to coal tar creosote, a thick, dark liquid distilled from coal tar at high temperatures. It’s roughly 85% polycyclic aromatic hydrocarbons (PAHs), a class of chemicals that make it exceptionally good at repelling insects, fungi, and moisture from wood. This is the creosote you see soaked into railroad ties, utility poles, and marine pilings.
The EPA classifies coal tar creosote as a restricted-use pesticide. It has no registered residential uses. You cannot legally buy it for home projects, and indoor applications are specifically prohibited, along with any use on wood that will contact food, animal feed, or drinking water. Only certified commercial applicators can work with it. If you’ve ever walked past a stack of dark, pungent railroad ties at a lumberyard, that sharp, tarry smell is coal tar creosote.
Wood Tar Creosote: A Different Chemistry
Wood tar creosote comes from burning beechwood or other hardwoods and collecting the tar that drips out. Chemically, it’s a very different substance from the coal version. Instead of being dominated by aromatic hydrocarbons, wood creosote is mostly phenolic compounds, particularly phenol, cresols, and guaiacol.
This version has a long history in medicine. First prepared in Germany in 1830, wood creosote was once listed in the German, American, and Japanese pharmacopoeias as an antibacterial agent used to treat pulmonary tuberculosis, diarrhea, and external wounds. Western countries have since removed it from their official drug references, but it remains in the Japanese Pharmacopoeia, where it’s still used to treat diarrhea. A popular Japanese over-the-counter remedy called Seiro-gan combines wood creosote with herbal ingredients for digestive trouble, including food poisoning.
Chimney Creosote: The Fire Hazard
When people talk about “creosote in the chimney,” they’re referring to a sticky, flammable residue that forms when wood smoke cools on its way up and out. As smoke, vapor, gases, and unburned wood particles rise through a chimney flue, they condense on the cooler surfaces higher up. Over time, this condensation builds into a coating that progresses through three stages:
- Stage 1: A fine, soot-like powder that brushes away easily during routine cleaning.
- Stage 2: A hard, crusty tar layer that’s significantly harder to remove.
- Stage 3: A dense, glossy, hardened glaze that is extremely difficult to scrape off and highly flammable. This is the stage that causes chimney fires.
Several factors accelerate creosote buildup. Burning wet or unseasoned wood is the biggest culprit because it produces more moisture and smoke, both of which feed condensation. Low, smoldering fires create incomplete combustion, which sends more unburned particles into the flue. Poor ventilation and restricted airflow compound the problem by slowing the movement of gases, giving them more time to cool and deposit on chimney walls. Burning dry, seasoned hardwood with good airflow and a hot fire is the simplest way to slow creosote formation.
The Creosote Bush
Creosote bush (Larrea tridentata) is a shrub native to the deserts of the American Southwest and Mexico. It belongs to the caltrop family and earned its common name because its resinous leaves produce a strong, tarry smell, especially after rain, that resembles the scent of coal tar creosote. The plant itself has no chemical relationship to the industrial product.
Creosote bush is remarkably hardy. Its leaves contain compounds that inhibit the growth of certain bacteria and yeast, which likely helps the plant resist infection in harsh conditions. The resinous coating also makes the foliage highly flammable, a notable trait in fire-prone desert ecosystems. Some creosote bush root systems are among the oldest living organisms on Earth, with clonal colonies estimated to be thousands of years old.
Health Risks of Coal Tar Creosote
Coal tar creosote is the version that poses serious health concerns. Direct skin contact causes rashes, severe irritation, and chemical burns. Prolonged or repeated exposure, even at low levels, can increase your skin’s sensitivity to sunlight and damage the cornea. Workers who breathe coal tar fumes report respiratory irritation and breathing difficulties.
The cancer risk is well established. The International Agency for Research on Cancer (IARC) classifies creosotes as “probably carcinogenic to humans.” Long-term occupational exposure has been linked to cancers of the skin, lung, bladder, kidney, pancreas, prostate, rectum, and central nervous system. IARC goes further for specific industrial settings: exposure during coke production, coal gasification, aluminum production, coal tar distillation, and roofing or paving work is classified as outright carcinogenic to humans, not just probably.
This is why coal tar creosote is restricted to professional use. Casual handling of creosote-treated wood, like cutting up old railroad ties for garden borders, can produce skin contact and dust exposure that carries real risk over time.
Environmental Persistence
When creosote-treated wood sits in soil or water, its chemical components leach into the surrounding environment. The EPA notes that creosote-treated wood used in aquatic and railroad structures may pose risks to fish and invertebrates. Creosote contaminants bind to soil and break down slowly, making old wood treatment sites a long-term cleanup challenge.
Contaminated sites can be restored through bioremediation, which uses specialized microorganisms to break down creosote compounds. Techniques include land farming (spreading and tilling contaminated soil to encourage microbial activity), bioreactors, trickling filters, and direct inoculation of soil with pollutant-degrading bacteria. In one study, introducing specific bacteria to contaminated soil reduced the half-life of one key creosote component from two weeks to less than a day. Another project using a bioreactor removed 95% of monitored PAHs from soil and over 99% of a toxic compound from groundwater. These approaches work, but cleanup of the roughly 700 former wood treatment sites across the U.S. remains a significant undertaking.