The Bigger Picture – The Trouble with Chimneys: From Percival Potts to Archibald Leitch

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Links between occupational risks and cancer

Percival Potts entered epidemiological folklore for providing one of the first links between occupational exposures and specific cancers…namely scrotal cancer in chimney sweeps. And, to be clear, it was squamous cell carcinoma of the skin of the scrotum, not testicular cancer. And what was both striking and shocking about his observations was the young age of many patients.

The problem has a long history. Chimney sweeps practiced their trade in Shakespeare’s day. “To look like him, they are black chimney sweeps,” he wrote in The labor of love is lost.

Correct diagnosis of cancers

But it was the introduction of tall, narrow chimneys, in response to the Great Fire of London in 1665, that marked a turning point. Unfortunately, these chimneys tended to become clogged with soot: the profession of chimney sweep resumed its development. Young boys were co-opted to do the dirty work and a consequence for them was a legacy of poor health – lung disease, dermatitis, skin warts (known in the trade as soot wart) and scrotal skin cancer .

Except that before Pott’s scrotal exam, cancerous growths were thought to be venereal and best treated with mercurials. He provided not only a correct diagnosis but a great deal of sympathy for his young patients.

Hence Sir Percy’s lament: “The lot of these people seems singularly hard; in their infancy, they are most often treated with great brutality, and almost famished with cold and hunger; they are pushed into narrow and sometimes hot (sic) chimneys where they are bruised and almost suffocated: and when they reach puberty, become peculiarly subject to a most noxious, painful and deadly disease.

A typical English disease

This cancer continued throughout the 19and century and was recognized by surgeons and sweeps as a pledge of trade. But there was an anomaly. Scrotal skin cancer seemed to be a typically English disease. Sir Henry Butlin, also in Barts, sent assistants to scour the chimneys of Europe to Germany, Holland, Belgium and France to look for clues as to how foreigners escaped the soot penalty.

And the answer? Well, for the Germans it was obvious. They were much better organized, washed more regularly and wore protective clothing. And looked cool guys too. But when it came to the French, Butlin did not hold back, confidently proclaiming that it could not just be about cleanliness “which is utterly inconceivable to anyone familiar with the general habits of the French lower classes”.

Other variables considered important by Butlin were the structure of the chimneys and the quantities of soot as well as the use, in England, of what is called coal while elsewhere in Europe, coke, wood and coal. of wood were used, producing less soot residue.

The link between soot and skin cancer

Which brings us to the question: what’s in soot that causes skin cancer? This will take us to the early days of the ICR at its labs in Chelsea – but before we get to that, here’s another puzzle. Given that chimney sweeps, even in my youth, were covered in soot everywhere, why was the scrotum the cancer hotspot?

At the end of the 19and century, it began to appear that there were associations between occupational exposure to shale oils, pitch and tar and skin cancer, including scrotal cancer. There was clearly something very fishy about the carboniferous combustion. In 1922, an examination of 141 men in Manchester with scrotal skin cancer made the startling discovery that only one was a sweep, but 69 worked on spinning machines or mules in the cotton industry of Lancashire. Only men had this job and many started as young as eight years old. The mules were lubricated with crude oil and the workers, scantily clad in cotton trousers in hot environments, leaning against the mules became encrusted with oil “between the upper thighs and the abdomen”. And in that cavalier English way, there were no sanitary facilities.

Why scrotal cancer?

Now we are getting closer. It’s time to call on Archibald Leitch, director of the Research Institute at the Royal Cancer Hospital Chelsea, aka the ICR. He had calculated the risks of raccoons or muleteers dying from scrotal cancer at 1 in 1,400 and 1 in 2,000 per year respectively. But why the scrotum? Leitch suggested it was a combination of heat and the dissolving properties of sebaceous gland secretion, making the skin more absorbent and vulnerable to harmful chemicals in soot, oils and tar. I will buy this as an answer.

The discovery of benzo(a)pyrene

So where does this sooty tar trail lead us? Well, actually, in 20and experimental cancer sciences of the century and the first mechanistic insights into cancer. Leitch’s successors and the first inhabitants of the ICR laboratories in Chelsea were organic chemists and their lead came from Japanese scientists – who in 1915 caused skin cancer on the ears of rabbits by painting on tar of coal. This provided a test to ask the question: what in soot and tar causes skin cancer? The answer given by Ernest Kennaway and his colleagues at the ICR in the early 1930s was a class of chemicals called polycyclic aromatic hydrocarbons, or PAHs, one of which, benzo(a)pyrene, was particularly potent. Which led some 30 years later to the landmark discovery by Peter Brookes and Philip Lawley at the ICR that PAHs like benzo(a)pyrene caused cancer by binding directly to DNA and mutating it. .

It could, and perhaps should, have been even more revealing. Kennaway and his colleagues, observing that cigarette tar allegedly contains benzo(a)pyrene, asked if this could explain lung cancer, the incidence of which was beginning to rise in the 1930s. Here’s the weird thing; no one seems to have noticed. When Richard Doll began his landmark epidemiological studies of lung cancer in the 1950s, I highly doubt he had heard of benzo(a)pyrene; he suspected that car exhaust was the cause of lung cancer. That’s not to diminish Doll’s accomplishments, but it does illustrate how easily opportunities can be missed. But then why didn’t the ICR scientists run with this idea?

Santa Claus walking on a roof towards a chimney when it's snowing carrying a bag of parcels

The soot trail

Fast forward to 21st century and genomic sequencing reveals distinctive signatures in cancer cell DNA of chemical carcinogens like benzo(a)pyrene. The reasoned file is closed. The soot trail, however, is largely forgotten. But we are indebted to Percival Potts and his successors, including Archibald.

And let’s hope Santa wears really thick pants this Christmas.

NOTE Part of the piece above was excerpted from my book “Cancer the evolutionary legacy”. Oxford University Press 2000.

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