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I have read the Wikipedia article on coal tar. I started reading the article thinking that the principal use of it was in construction, such as in roofs and sealed roads. I found out that it was first described as a medical treatment, to be applied topically:

It may be applied to the affected area to treat psoriasis and seborrheic dermatitis (dandruff).
Coal tar

and:

It is on the World Health Organization's List of Essential Medicines, the most effective and safe medicines needed in a health system.[10] Coal tar is available as a generic medication and over the counter.
Coal tar

and:

Coal tar is used in medicated shampoo, soap and ointment. It demonstrates antifungal, anti-inflammatory, anti-itch, and antiparasitic properties.[8] It may be applied topically as a treatment for dandruff and psoriasis, and to kill and repel head lice.
Coal tar: Uses

and:

According to the National Psoriasis Foundation, coal tar is a valuable, safe and inexpensive treatment option for millions of people with psoriasis and other scalp or skin conditions.
Coal tar: Safety

Then I found out it's described as a carcinogen.

The International Agency for Research on Cancer lists coal tars as Group 1 carcinogens, meaning they directly cause cancer.[28][31][32] Both the U.S. Department of Health and Human Services and the state of California list coal tars as known human carcinogens.
Coal tar: Cancer

and:

Coal tar was one of the first chemical substances proven to cause cancer from occupational exposure, during research in 1775 on the cause of chimney sweeps' carcinoma.[29] Modern studies have shown that working with coal tar pitch, such as during the paving of roads or when working on roofs, increases the risk of cancer.
Coal tar: Cancer

I'm confused, because I expected it to an occupational health hazard, yet much of the article explains that it's considered important and safe in medicinal use, even by applying it directly to the skin. Actually it says both, and I wanted to get some clarification as to these two seemingly incompatible descriptions. I'm guessing there might be a different in their effects when administered as medicine as opposed to being exposed to it generally, but it does sound strange to read this sentence about something that's considered a topical medicine:

The International Agency for Research on Cancer lists coal tars as Group 1 carcinogens, meaning they directly cause cancer.
Coal tar: Cancer

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Currently, there seems to be insufficient evidence for the carcinogenicity of coal tar for dermatological use and some evidence that coal-tar pitch fumes may increase the risk of lung cancer.

Coal tar for dermatological use

Conclusive evidence for the carcinogenicity of tar used in dermatologic practice is lacking (Dermatologic Surgery, 1995).

This study [in 13,200 patients with psoriasis and eczema followed for 21 years] has sufficient power to show that coal tar treatment is not associated with an increased risk of cancer. These results indicate that coal tar can be maintained as a safe treatment in dermatological practice (The Journal of Investigative Dermatology, 2010).

Coal-tar pitch fumes

Lung cancer:

Except for lung cancer, evidence for increased cancer risks among bitumen-exposed workers was judged to be of low certainty (Journal of Occupational and Environmental Medicine, 2018).

An increasing number of occupational studies demonstrate a positive exposure-response relationship with cumulative benzo[a]pyrene [a chemical in coal-tar fumes] exposure and lung cancer (U.S. Environmental Protection Agency, 2017).

Skin cancer:

Both coal tar and coal-tar pitch contain many chemical compounds, including carcinogens such as benzene. Occupational exposure to coal tar or coal-tar pitch increases the risk of skin cancer (National Cancer Institute, 2018).

According to a 2004 review of occupational carcinogens in Environmental Health Perspectives, there is a strong evidence that the exposure to coal tar or coal-tar pitches increases the risk of skin cancer (see Table 7).

Other:

Coal tars and coal-tar pitches are known to be human carcinogens based on sufficient evidence of carcinogenicity from studies in humans (National Toxicology Program, Department of Health and Human Services, 2010).

It may take decades of exposure for a chemical to cause cancer. This and the fact that the chemical composition of coal tar has changed over time may partially explain the differences in estimations of carcinogenicity of coal tar among studies.

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