Creosote is produced by heating wood or coal in the absence of oxygen, then collecting and distilling the oily liquid that condenses from the resulting vapors. There are two fundamentally different types, wood creosote and coal tar creosote, and they’re made through different processes with very different chemical profiles. Understanding which type you’re dealing with matters, because coal tar creosote is classified as a probable human carcinogen and is restricted to certified industrial use in the United States.
Wood Creosote vs. Coal Tar Creosote
These two substances share a name but are chemically distinct. Wood creosote is made from hardwoods, primarily beechwood, and consists mostly of phenols, natural compounds that give it antiseptic properties. Four components alone (phenol, p-cresol, guaiacol, and 4-methylguaiacol) make up more than two-thirds of beechwood creosote’s composition. Guaiacol is the single largest component at nearly 24%, followed by 4-methylguaiacol at 19% and phenol and p-cresol each around 14%.
Coal tar creosote is a different animal entirely. It’s distilled from coal tar, a byproduct of converting coal into coke or natural gas. At least 75% of coal tar creosote consists of polycyclic aromatic hydrocarbons (PAHs), a class of chemicals strongly linked to cancer. This is the dark, pungent creosote used to treat railroad ties and utility poles, and it’s the type most people picture when they hear the word.
How Wood Creosote Is Made
Wood creosote is produced through a process called destructive distillation. Wood, traditionally beechwood, is sealed in a container and heated without oxygen. Instead of burning, the wood breaks down thermally into charcoal, gases, and a condensable liquid mixture historically called pyroligneous acid. This liquid separates into layers: a watery portion containing acetic acid and methanol, and an oily portion that includes creosote.
The recovery of these volatile byproducts from hardwood carbonization began around 1812, when operators first started collecting and condensing the gases from beehive kilns. Before that, the vapors were simply lost to the atmosphere. Even as late as 1878, one observer noted that “scarcely a thought has been had as to the saving of the volatile products of carbonization.” The yield is modest: roughly 3 gallons of creosote oil per ton of dry wood processed.
The raw liquid collected from the kiln is then refined through further distillation. Different compounds boil off at different temperatures, allowing the creosote fraction to be separated from the lighter methanol and acetic acid. The resulting product is a yellowish, oily liquid with a smoky smell. In the past, wood creosote was used as a disinfectant, a laxative, and a cough treatment, though these uses have largely been abandoned.
How Coal Tar Creosote Is Made
Coal tar creosote starts with coal tar itself, which is generated as a byproduct when coal is heated in sealed ovens to produce coke for steelmaking. The coal tar is a thick, black liquid containing hundreds of chemical compounds. To produce creosote, the tar is fed into distillation columns where it’s heated progressively. Different fractions condense at different temperature ranges, and the creosote fraction, sometimes called “creosote oil,” is collected from the middle portion of the distillation.
The resulting product is a dark brown to black oily liquid with a strong, sharp odor. Its chemical makeup is dominated by aromatic hydrocarbons like naphthalene, anthracene, and phenanthrene. This complexity is part of what makes it effective as a wood preservative: the mixture of chemicals is toxic to fungi, insects, and marine organisms that would otherwise break down wood.
Industrial Pressure Treatment
The most common use of coal tar creosote today is pressure-treating wood, particularly railroad ties, utility poles, bridge timbers, and marine pilings. This process forces the preservative deep into the cellular structure of the wood, extending its service life to 20 to 25 years even in moist conditions.
Before treatment, wood is typically dried and debarked. For dense timber, sharp steel teeth are pressed into the surface in a process called incising, which creates channels for the creosote to penetrate more deeply. The prepared wood is then loaded into a horizontal steel cylinder that can be up to 150 feet long and 7 feet in diameter.
Two main methods are used. In the full-cell process, a vacuum first pulls air out of the wood’s cells. The cylinder is then flooded with creosote and pressurized to 140 to 150 psi for several hours, forcing the chemical into both the cell walls and the open spaces within. A final vacuum removes excess creosote from the surface. In the empty-cell process, compressed air at 35 to 40 psi is first pushed into the wood before the creosote is introduced under the same high pressure. When the pressure is released, the trapped air expands and pushes excess creosote back out of the cell cavities, leaving preservative primarily in the cell walls. The empty-cell method uses less chemical while still providing protection.
Health Risks of Creosote Exposure
Coal tar creosote is classified as a known human carcinogen by the National Toxicology Program and as probably carcinogenic to humans by both the EPA and the International Agency for Research on Cancer. The cancer risk comes primarily from the high concentration of PAHs in the mixture.
Short-term exposure can cause dizziness, altered vision, and headaches. Prolonged occupational exposure has been linked to decreased lung function, and severe kidney damage including renal failure has been reported following chronic inhalation of coal tar fumes. Skin contact with coal tar creosote can cause burns, rashes, and increased sensitivity to sunlight.
Wood creosote, with its phenol-based chemistry, carries different risks. Phenol itself is toxic in concentrated form and can cause chemical burns on contact, but wood creosote does not carry the same carcinogenic classification as its coal tar counterpart.
Regulatory Restrictions
In the United States, coal tar creosote is a restricted-use pesticide. It can only be applied using high-pressure equipment in wood-preserving facilities by certified pesticide applicators. Creosote products are not available to homeowners, and you cannot legally purchase them for personal wood treatment projects.
The EPA has not found health risks of concern for the general public or for workers who handle already-treated wood after application. However, creosote may pose risks to fish and aquatic invertebrates when treated wood is used in water. Reuse of creosote-treated wood, such as repurposing old railroad ties for landscaping, is not regulated under federal pesticide law, but the EPA advises against burning creosote-treated wood in residential settings to avoid inhaling toxic chemicals in the smoke and ash.