Creosote is a complex, oily substance derived from the high-temperature treatment of organic materials. This dark, viscous mixture of hundreds of chemical compounds is created during the destructive distillation process. Its primary commercial purpose is as a long-lasting wood preservative, protecting timber from decay and infestation.
The Two Distinct Forms of Creosote
The substance known broadly as creosote is not a single product but refers to two chemically distinct categories: coal-tar creosote and wood-tar creosote. Coal-tar creosote, which is the industrial standard, is a byproduct of high-temperature carbonization of bituminous coal during the production of coke and natural gas.
Wood-tar creosote is produced from the dry distillation of wood, most commonly beechwood. This form has a significantly different chemical makeup, consisting primarily of phenolic compounds like guaiacol and cresol, and is a colorless to yellowish liquid with a smoky odor.
The industrial form, coal-tar creosote, is a thicker, amber to black liquid whose composition makes it far more effective as a preservative. The modern understanding of creosote largely focuses on this variant due to its widespread industrial application.
Detailed Chemical Composition of Coal-Tar Creosote
The chemical makeup of coal-tar creosote is a complex and variable cocktail of numerous organic compounds, numbering more than 250 in total. The composition depends heavily on the temperature at which the coal was carbonized, leading to differences in the concentration of individual chemicals. This complex mixture is comprised mainly of aromatic hydrocarbons, which constitute the substance’s preservative and toxic properties.
Polycyclic Aromatic Hydrocarbons, or PAHs, are the dominant components, often accounting for at least 75% of the total mixture. These hydrocarbons feature multiple fused benzene rings and include specific chemicals such as naphthalene, phenanthrene, and anthracene.
The mixture also contains a smaller percentage of phenolic compounds, like phenol, cresols, and xylenols, which typically make up 2–17% of the total. Additionally, heterocyclic compounds containing nitrogen, sulfur, and oxygen are present, including quinolines and carbazoles. Because of this complex and toxic composition, coal-tar creosote is regulated in the United States by the Environmental Protection Agency (EPA) as a restricted-use pesticide under the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA).
Common Industrial and Consumer Applications
Coal-tar creosote’s primary use stems from its potent ability to deter wood-destroying organisms, including fungi, insects, and marine borers. The substance is applied to wood using an industrial vacuum and pressure treatment process to ensure deep penetration and long-term protection. This process is conducted in specialized facilities by certified applicators, not for general consumer use.
The great majority of creosote-treated wood is used in large-scale infrastructure projects that require extreme durability. These applications include railroad ties, utility poles, and marine pilings, which are constantly exposed to moisture, pests, and mechanical wear. Creosote-treated wood offers an exceptional service life, with utility poles often lasting 60 years or more.
While industrial use is dominant, the wood-tar variant had historical consumer and medical applications. For example, a derivative of wood-tar creosote, guaiacol, was once used in cough medicines, and the substance itself was used for smoking meats to preserve and flavor them. However, the use of creosote-treated wood in consumer settings like playgrounds, residential interiors, or garden furniture is generally restricted due to health concerns.
Health and Safety Implications of Exposure
The health implications of creosote exposure are directly tied to its highly complex chemical makeup, particularly the concentration of PAHs. Exposure can occur through skin contact, inhalation of vapors, or accidental ingestion.
Acute exposure, which is immediate and short-lived, can cause severe skin irritation, chemical burns, and damage to the eyes, including corneal damage. Breathing creosote vapors can irritate the respiratory tract, leading to symptoms like coughing and throat pain.
The most significant concern lies with chronic, long-term exposure, which is primarily associated with the carcinogenic nature of its PAH components. The International Agency for Research on Cancer (IARC) has determined that coal tar creosote is probably carcinogenic to humans. Long-term dermal contact, particularly in occupational settings, has been linked to an increased risk of skin cancer. Chronic exposure may also lead to systemic effects, including damage to the liver and kidneys.