Coal tar is officially classified as a Group 1 carcinogen, meaning there is sufficient evidence that it causes cancer in humans. But that classification is based largely on heavy, prolonged occupational exposure, not the low-concentration products sold for psoriasis and dandruff. The distinction between industrial exposure and medical use matters enormously here, and understanding it is the key to making sense of the conflicting information you’ll find online.
Why Coal Tar Is Classified as Carcinogenic
The International Agency for Research on Cancer (IARC) places coal tar in its highest risk category, Group 1, alongside substances like asbestos and tobacco smoke. This classification is built on decades of evidence from workers in coal gasification plants, coke production facilities, and tar distilleries. Multiple epidemiological studies found excess rates of lung cancer among workers exposed to coal tar fumes. Case reports documented increased skin cancer, including scrotal cancer, in patent-fuel workers and others handling coal tar regularly. Smaller excesses of bladder cancer, kidney cancer, leukemia, and digestive organ cancers have also appeared in occupational studies.
These workers were breathing in coal tar fumes or getting concentrated coal tar on their skin for years, often without protective equipment. The exposure levels in these settings are orders of magnitude higher than what you’d encounter using a medicated shampoo a few times a week.
How Coal Tar Damages DNA
Coal tar contains hundreds of chemical compounds, but the ones responsible for its cancer risk are polycyclic aromatic hydrocarbons, or PAHs. The most well-studied is benzo[a]pyrene, classified as highly genotoxic. When your body processes PAHs, enzymes convert them into reactive molecules that can physically attach to your DNA, forming what scientists call “adducts.” These adducts distort the DNA structure, and if your cells can’t repair the damage before dividing, the errors can accumulate into mutations that eventually drive cancer.
PAHs primarily target specific building blocks of DNA (guanine and, to a lesser extent, adenine). The damage follows multiple chemical pathways: some create stable attachments that persist in the DNA, while others create unstable bonds that break away and leave gaps in the genetic code. Either way, the result is the same: a higher chance of mutations with repeated, significant exposure.
What the Evidence Says About Medical Use
Here’s where the picture shifts. A 25-year follow-up study tracked patients with psoriasis who were treated with coal tar ointments, sometimes combined with UV radiation (a common treatment approach called the Goeckerman regimen). The study found that skin cancer incidence in these patients was not appreciably increased above what you’d expect in the general population. The researchers concluded that coal tar treatment carried minimal skin cancer risk for psoriasis patients.
The American Academy of Dermatology notes that dermatologists have prescribed coal tar for more than 100 years and consider it safe for long-term use. That said, the body does absorb PAHs through the skin. One study measuring a PAH breakdown product in urine found that patients applying coal tar topically actually had higher levels of that marker than some occupational workers. The critical difference is that medical use is intermittent and covers limited skin area, while occupational exposure is continuous and affects the whole body, including the lungs.
Concentration Limits and Regulations
In the United States, the FDA allows coal tar in over-the-counter products for dandruff, seborrheic dermatitis, and psoriasis at concentrations between 0.5 and 5 percent. Most shampoos and ointments you’ll find on store shelves fall within this range. Industrial workers, by contrast, handled crude or concentrated coal tar with far higher PAH content.
The European Union takes a stricter approach. Crude and refined coal tars are listed on Annex II of the EU cosmetics regulation, which is the list of substances completely banned from cosmetic products. This means coal tar shampoos marketed as cosmetics cannot be sold in the EU, though medicated products prescribed by a doctor may still be available in some member states under pharmaceutical regulations. The difference in regulatory philosophy reflects a precautionary stance rather than evidence of harm at low concentrations.
Occupational Risk vs. Bathroom Cabinet Risk
The gap between industrial and therapeutic exposure is the single most important factor in evaluating your personal risk. A coke oven worker in the mid-20th century might have inhaled coal tar fumes for 8 hours a day over decades. A person using coal tar shampoo might apply a 2 percent solution to their scalp for 5 to 10 minutes, three times a week. The total PAH dose, the route of exposure, and the duration are fundamentally different.
That doesn’t mean therapeutic coal tar is completely inert. Your skin absorbs PAHs, and those compounds do interact with your DNA. But the long-term clinical data available so far has not shown a meaningful increase in cancer rates among people using these products as directed. The risk appears to be very low at the concentrations and frequencies typical of home use, which is why U.S. regulators and dermatology organizations continue to support its availability.
Practical Considerations for Coal Tar Products
If you’re using a coal tar shampoo or ointment, a few things are worth keeping in mind. Coal tar can make your skin more sensitive to sunlight, so protecting treated areas from UV exposure reduces any additive risk. Using the product at the recommended concentration and frequency, rather than applying more in hopes of faster results, keeps your PAH exposure in the range that long-term studies have found to be low risk. Rinse-off products like shampoos leave less residue on the skin than leave-on ointments, which means lower overall absorption.
For people with psoriasis or stubborn dandruff who benefit from coal tar, the practical reality is that it remains one of the oldest and most affordable treatment options available. The cancer risk that earned coal tar its Group 1 classification is real, but it reflects a world of industrial fumes and unprotected handling, not a medicated shampoo used a few times a week.