Tar liquid is a thick, dark, often viscous substance produced when organic material like coal, wood, or plant matter is heated in the absence of oxygen, a process called pyrolysis. There is no single “tar,” though. The word covers a family of chemically distinct liquids whose properties, uses, and hazards vary enormously depending on what was burned and how. Coal tar, wood tar, pine tar, birch bark tar, and even the residue in cigarette smoke all carry the name, yet they differ enough that lumping them together leads to real confusion about safety, toxicity, and practical applications.
How Tar Liquids Form
The basic chemistry behind all tar liquids is the same: you heat carbon-rich material without letting it burn in open air. Instead of combusting into ash and carbon dioxide, the material breaks down into gases, a watery condensate, solid char, and a dark oily liquid. That liquid is the tar. What ends up in it depends heavily on the starting material.
Coal tar is generated during the production of metallurgical coke, which is coal heated in sealed ovens to drive off volatile compounds. The carbonization of coal in slot-type recovery ovens is the main industrial source of coal tar today.1New Trends in Coal Conversion. Coal tar: a by-product in cokemaking and an essential raw material in carbochemistry Coal tar is extraordinarily complex, containing thousands of compounds including polycyclic aromatic hydrocarbons (PAHs), phenols, and nitrogen-containing molecules. It is denser than water, pungent, and ranges from a thin, runny liquid to a near-solid pitch depending on how much of the lighter fractions have been distilled off.
Wood tar and pine tar, by contrast, come from heating wood or resinous tree parts in earth pits, kilns, or retorts. The resulting liquid is generally lighter and less toxic than coal tar, though still a complicated mixture. Pine tar in particular has a long folk-medicine history and a distinctive smoky smell familiar to anyone who has used it on leather or wooden boats. Bio-oils produced from fast pyrolysis of agricultural waste like rice husks represent the modern, renewable cousin of these older wood tars, though they tend to be more acidic and less stable.2Journal of Cleaner Production. Bio-oil from biomass fast pyrolysis: Yields, related properties and energy consumption analysis of the pyrolysis system
Tar in the Ancient World
Humans have been making and using tar for tens of thousands of years. Some of the oldest known adhesives are birch bark tars found at Neanderthal sites, used to glue stone tools to wooden handles. Researchers have tried to figure out exactly how Palaeolithic people produced these tars without ceramic vessels, testing methods like ash-mound heating and condensation techniques. The chemistry turns out to be tricky to reconstruct: biomarker signatures that distinguish tar-production methods in later periods with pottery do not reliably identify the aceramic techniques used in deep prehistory.3Nature / Scientific Reports. Identifying Palaeolithic birch tar production techniques: challenges from an experimental biomolecular approach
In ancient Greece and across the Mediterranean, resinous substances obtained by tapping live pine trees for resin, or by pyrolyzing resinous wood to produce tar, served a wide range of functions: waterproofing ships, preserving wood, treating rope, and even flavoring wine.4Acta Scientiarum Polonorum Silvarum Colendarum Ratio et Industria Lignaria. Resin and pine tar in Greece: Ancient techniques and traditional practices Pine tar remained a vital naval stores product well into the age of sail. Scandinavian countries exported vast quantities of it to keep wooden hulls watertight, and the product retains niche use today for maintaining traditional wooden boats, treating horse hooves, and conditioning leather.
Coal Tar as a Skin Treatment
One of the more surprising modern roles for tar liquid is in dermatology. Coal tar preparations have been used to treat psoriasis and eczema for well over a century, and they remain part of the treatment toolkit even in an era of biologics and targeted therapies. The mechanism is now better understood than the folk practice that preceded it.
Coal tar activates a receptor in skin cells called the aryl hydrocarbon receptor (AHR). In a study using skin models built from cells of patients with atopic dermatitis, coal tar triggered this receptor and drove the cells to differentiate properly, essentially repairing the skin barrier that is defective in eczema. When the receptor was knocked out experimentally, the beneficial effect disappeared entirely.5PubMed Central. Coal tar induces AHR-dependent skin barrier repair in atopic dermatitis This is a genuinely elegant piece of pharmacology hiding inside a messy, ancient remedy.
For psoriasis specifically, the compounds thought to do most of the therapeutic work are PAHs within the tar, with carbazole singled out as the most-studied candidate.6PubMed. Review of the mechanism of action of coal tar in psoriasis Pine tar has its own, partly overlapping, dermatological profile. It has been shown to have anti-itch, anti-inflammatory, antibacterial, and antifungal properties, and it shows up in over-the-counter shampoos and soaps marketed for dandruff and scalp psoriasis.7PubMed Central. Topical pine tar: History, properties and use as a treatment for common skin conditions
Coal tar treatments are not glamorous. They smell strong, they stain clothing and bedding, and they can make skin more sensitive to sunlight. Many patients and even some dermatologists have moved away from them in favor of cleaner-feeling modern drugs. But for people who do not respond well to other options, or who want to avoid long-term steroid use, tar preparations remain a surprisingly effective tool.
Does Medical Coal Tar Cause Cancer?
This is the question that hangs over every discussion of coal tar therapy, and the answer is more reassuring than most people expect. Coal tar contains known carcinogens, including benzo[a]pyrene and other PAHs. Industrial exposure to coal tar at high concentrations unquestionably increases cancer risk. So it seems reasonable to worry about smearing the stuff on your skin. But the evidence from dermatological use tells a different story.
A large cohort study followed over 13,000 patients with psoriasis or eczema and compared those treated with coal tar to those treated with corticosteroid creams. The median coal tar exposure was about six months, though some patients had used it for as long as 25 years. Coal tar treatment was not associated with an increased risk of non-skin cancers or skin cancer. The hazard ratios were close to 1.0 for both categories, meaning the coal tar group fared no differently from the steroid group.8PubMed. No increased risk of cancer after coal tar treatment in patients with psoriasis or eczema Earlier reviews of the literature reached the same general conclusion: while animal studies and occupational-exposure data raise flags, human epidemiological studies have not definitively shown an increase in skin cancer from therapeutic use of tar.9PubMed. Is dermatologic usage of coal tar carcinogenic? A review of the literature10PubMed. Coal tar therapy. Is it carcinogenic?
The distinction matters: concentration, duration, and route of exposure change risk dramatically. Workers in coke ovens or aluminum smelters breathe in coal tar pitch volatiles for decades at levels far exceeding what any skin patient encounters. Rubbing a thin layer of diluted coal tar on a patch of psoriasis a few times a week is a fundamentally different exposure profile from standing over an open coking oven for a career. The dose makes the poison, and in this case the therapeutic dose appears to fall below the danger threshold.
Occupational Tar Exposure and Serious Health Risks
Where tar liquid does become dangerous is in industrial settings. Workers in aluminum smelting, coke production, roofing, and road paving can be exposed to coal tar pitch volatiles over years. The risks are real and well-documented.
In aluminum smelting, particularly in older Soderberg-type potrooms where coal tar pitch is used as a binder in anodes, workers face elevated bladder cancer risk. A study of these workers found that the risk of bladder cancer increased with cumulative exposure to benzo[a]pyrene. For each year of exposure at a concentration of one microgram per cubic meter, risk rose by about 1.7 percent. Over a 40-year career at the threshold limit value for coal tar pitch volatiles, the estimated relative risk reached roughly 2.2, meaning the workers’ chance of bladder cancer was more than double that of unexposed individuals.11PubMed. Estimation of risk of developing bladder cancer among workers exposed to coal tar pitch volatiles in the primary aluminum industry
Lung cancer, skin cancer, and cancers of the urinary tract have all been linked to heavy occupational tar exposure in various industries. Modern workplace protections, ventilation requirements, and substitution of coal tar with petroleum-based alternatives in some applications have reduced these risks, but they have not eliminated them. Workers in developing countries with less stringent regulations remain especially vulnerable.
Cigarette “Tar” Is Not the Same Thing
When cigarette packs list “tar” content, they are referring to a very different substance than coal tar or wood tar. Cigarette tar is formally called total particulate matter minus nicotine and water. It is the sticky residue left when tobacco smoke condensate is collected on a filter. It contains thousands of compounds, many of them toxic or carcinogenic, but its chemical profile is distinct from coal tar.
Cigarette smoke condensate causes measurable DNA damage and oxidative stress in human cells. Research on bladder epithelial cells exposed to tobacco smoke condensate found time- and concentration-dependent cell death, oxidative stress, lipid damage, and the formation of DNA lesions.12PubMed Central. DNA damage and oxidative stress of tobacco smoke condensate in human bladder epithelial cells Counterintuitively, so-called “low tar” cigarettes are not necessarily less harmful. When cytotoxicity results are adjusted for the amount of nicotine delivered, lower-tar cigarettes have been found to be the most toxic to human respiratory cells.13Regulatory Toxicology and Pharmacology. Cytotoxicity of eight cigarette smoke condensates in three test systems: Comparisons between assays and condensates Smokers compensate for reduced nicotine delivery by inhaling more deeply or smoking more cigarettes, and the remaining compounds in the condensate turn out to be at least as harmful per unit of nicotine as those in full-flavor brands.
The shared name causes confusion. People sometimes assume coal tar shampoo must be as dangerous as smoking, or conversely that cigarette “tar” is just the same harmless stuff used on boats. Neither is true. The chemistry, the exposure route, and the health consequences are all different.
Coal Tar as an Environmental Contaminant
Former manufactured-gas plants, coking facilities, and wood-treatment operations left behind some of the most stubborn contamination problems in modern environmental science. Coal tar released at these sites does not dissolve easily in water, is denser than water, and sinks through soil and aquifer material to settle deep below the water table. This class of contaminant, known as a dense non-aqueous phase liquid, can persist for decades, slowly releasing PAHs into surrounding groundwater and sediment.14Environment Agency. An illustrated handbook of DNAPL transport and fate in the subsurface
The Sydney Tar Ponds in Nova Scotia became one of Canada’s most notorious contaminated sites, a legacy of over a century of steel and coke production. PAH fingerprinting of the sediments showed that the contamination pattern matched coal tar from the former coking operations, while surrounding urban soils carried a different PAH signature linked to coal combustion and coal handling.15PubMed. Forensic assessment of polycyclic aromatic hydrocarbons at the former Sydney Tar Ponds and surrounding environment using fingerprint techniques That kind of chemical detective work helps regulators figure out who is responsible for cleanup and which areas need the most urgent attention.
A less obvious source of coal tar contamination sits in plain sight: parking lots. Coal-tar-based pavement sealcoat, widely used in parts of the United States to give asphalt a fresh black appearance, is a major source of PAHs in urban stormwater. Research has shown that even a small amount of sealcoated pavement can be the dominant source of PAHs entering retention ponds, and contaminated sediment from those ponds can be extremely expensive to dispose of properly.16PubMed Central. Coal-tar-based pavement sealcoat and PAHs: implications for the environment, human health, and stormwater management A two-year monitoring study found that sealed parking lots released PAHs in stormwater at more than 10 times the rate of an unsealed control lot.17PubMed. Polycyclic aromatic hydrocarbons in stormwater runoff from sealcoated pavements Several U.S. states and cities have banned coal-tar sealants in response to these findings, though they remain legal and common in many areas.
Cleaning Up Tar-Contaminated Soil
Remediating coal tar contamination is slow, expensive, and technically demanding. Excavation and incineration work but cost a fortune. Increasingly, researchers and cleanup crews have turned to bioremediation, using bacteria and fungi to break down PAHs in place.
Bioremediation can be remarkably effective for the lighter PAH compounds. In one study of coal tar-contaminated soil, six months of biological treatment reduced concentrations of two-ring and three-ring PAHs by about 98 and 97 percent respectively, and four-ring PAHs by about 82 percent. The bacteria responsible belonged mainly to the Enterobacteria and Pseudomonas genera.18PubMed. Evolution of bacterial community during bioremediation of PAHs in a coal tar contaminated soil Larger PAH molecules with five or six rings are harder for microbes to attack, so they tend to persist longer in the soil.
Fungi can also contribute. White-rot fungi, known for their ability to break down complex organic molecules like lignin, have been tested on coal tar-contaminated soil. Interestingly, in at least one study, the greatest PAH losses occurred in control samples where native soil microorganisms were simply given an organic food source (straw) to colonize, without any added fungi. The native microbial community degraded PAHs as a byproduct of their normal metabolism.19PubMed. Biodegradation of polycyclic aromatic hydrocarbons (PAHs) by native microflora and combinations of white-rot fungi in a coal-tar contaminated soil This is a useful reminder that sometimes the most effective cleanup strategy is to give the organisms already in the ground the conditions they need to do their work, rather than importing exotic solutions.
Modern Bio-Oils and the Renewable Angle
Fast pyrolysis of agricultural and forestry waste produces a liquid that is, chemically speaking, a tar. But the bio-oil community tends to avoid that word for marketing reasons. Bio-oil from rice husks, corn stover, or wood chips is being developed as a renewable fuel and chemical feedstock. The optimal temperature for maximizing bio-oil yield from rice husks in a fluidized-bed system, for instance, sits around 500°C.20Journal of Cleaner Production. Bio-oil from biomass fast pyrolysis: Yields, related properties and energy consumption analysis of the pyrolysis system
Raw bio-oil is not ready to pour into an engine or a chemical plant. It is acidic, unstable, and contains a lot of oxygen-rich compounds that reduce its energy density and cause it to degrade over time. Upgrading it requires catalytic hydrotreatment, a process that removes oxygen, nitrogen, and sulfur at moderate temperatures but high pressures. The oxygen leaves as water, the nitrogen as ammonia, and the sulfur as hydrogen sulfide.21Industrial & Engineering Chemistry Research. Progress and Perspectives in the Catalytic Hydrotreatment of Bio-Oils: Effect of the Nature of the Metal Catalyst Getting this process to work economically at scale is one of the big unsolved problems in renewable fuels. The chemistry is understood; the engineering and economics are not there yet.
In the oil sands industry, the naturally occurring bitumen that is extracted from Canadian tar sands is another substance that wears the “tar” label loosely. Bitumen is not a pyrolysis product but rather a heavy, viscous form of petroleum. Extraction methods like steam-assisted gravity drainage reduce its viscosity using heat, while newer solvent-assisted approaches use mass transfer and heat together, causing heavy asphaltene molecules to precipitate out and partially upgrading the bitumen underground.22PubMed Central. Experimental Comparative Investigation of Hot Solvent/Steam-Assisted Gravity Drainage in Oil Sand Reservoirs Calling this “tar” is a geological colloquialism, not a chemical description, but it contributes to the word’s sprawling ambiguity.
Why “Tar” Means So Many Things
Part of the reason conversations about tar get confusing is that the word has been applied to any dark, sticky liquid for centuries, regardless of its source or chemistry. Coal tar, pine tar, birch bark tar, cigarette tar, bitumen from tar sands, bio-oil from pyrolysis: these are all “tar” in casual usage, yet they share little beyond appearance and a vague association with heat. Coal tar and cigarette tar both contain PAHs, but in very different concentrations and mixtures. Pine tar is relatively benign compared to coal tar. Bitumen is petroleum, not a pyrolysis product at all.
For practical purposes, the source material matters far more than the color or texture. If you are buying a pine tar soap for your scalp, you are dealing with a substance that has anti-inflammatory and antifungal properties and a long safety record. If you are considering whether to seal your driveway with a coal-tar-based product, you are looking at a material that will leach carcinogenic PAHs into stormwater for years. If you are a worker in an aluminum smelter, your concern is airborne coal tar pitch volatiles and their cumulative effect on your bladder and lungs. And if you are a patient with psoriasis whose doctor suggests coal tar ointment, the evidence says your cancer risk is not meaningfully increased by the treatment. The word is the same; the substances and the stakes are not.