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A Tale of Benzene Poisoning and a Snake Snaring Its Own Tail

August Kekule had been puzzling over the structure of benzene and sank into half-sleep in which he had a daydream of a snake, and the idea of a ring-shaped molecule was born.

This article was first published in The Montreal Gazette.

Professor Carl Santesson was intrigued.

In 1897, four women working in a Swedish tire-manufacturing plant exhibited similar, serious blood-clotting problems. Today, the condition would be recognized as aplastic anemia, the inability of the bone marrow to make proper blood cells. As a professor of pharmacology, Dr. Santesson had studied various toxins and he suspected the women’s ailment may be due to exposure to some chemical. On investigating the tire-manufacturing process, he concluded that the culprit was benzene, a solvent commonly used to dissolve rubber. Santesson’s paper published on the subject was the earliest report of chronic benzene poisoning. It certainly wasn’t to be the last.

During the first few decades of the 20th century, reports emerged detailing risks associated with benzene exposure in the printing, petrochemical refining, plastic manufacturing and leatherworking industries. Besides aplastic anemia, there was suspicion that benzene also causes leukemia. This was confirmed through the elegant work of Turkish hematologist professor Muzaffer Aksoy, who observed an unusually high incidence of leukemia among patients at his clinic. He recalled the advice of 17th century physician Bernardo Ramazzini, the “Father of Occupational Medicine,” to ask ill patients about their occupation. Dr. Aksoy found that leukemia was especially prevalent among shoemakers and leather manufacturers.

On learning that the previously used petroleum-based glues had been replaced by cheaper benzene-based ones, Dr. Aksoy launched an epidemiological investigation and found that the incidence of leukemia in shoemakers was almost four times that seen in the general population. His work prompted legislation governing occupational exposure to benzene with risk being attributed to levels in the air greater than one part per million.

Michael Faraday could have never guessed that the compound he discovered in 1825 would cause such commotion in the future. At the time, gas lighting was beginning to replace candles, with compressed gas delivered to public buildings and homes in pressurized vessels, much like propane tanks for barbecues today. The illuminating gas was made by heating whale or cod oil in a hot furnace and collecting the gas formed. When this gas was compressed, an oily liquid also formed, and it was this liquid that interested Faraday. Careful distillation of the oil allowed him to separate its components by virtue of different boiling points. One of these compounds he named “bi-carburet of hydrogen,” having determined that it was composed only of carbon and hydrogen. It was renamed “benzene” in 1833 by German chemist Eilhard Mitscherich, who correctly determined that it was composed of six atoms of carbon and six of hydrogen.

An explanation of how these atoms are joined to form benzene would have to wait until August Kekule’s epic publication in 1865 in which he described the structure of benzene as being six carbons in a ring with a hydrogen attached to each one. Then in 1890, at an elaborate ceremony in Berlin organized to celebrate the 25th anniversary of his landmark publication, the so-called “Benzolfest,” Kekule regaled the audience with a story that has become chemistry lore, retold in numerous articles and textbooks. He described how he had been puzzling over the structure of benzene and sank into half-sleep in which he had a daydream of a snake snaring its own tail. Presto! The idea of a ring-shaped molecule was born.

There is controversy about this account, since there is no record of Kekule ever mentioning it before “Benzolfest.” Some have argued that the story was made up to fortify his claim of being the first to propose a ring structure for benzene, since a couple of other chemists, Archibald Couper and Johann Loschmidt, had suggested cyclic structures for molecules earlier, although not the same as proposed by Kekule, which turned out to be correct. We will never know if the daydream story was dreamed up, but Kekule is widely regarded as the founder of the theory of chemical structure.

Arguments about how atoms combine to form molecules are long in the past. Now, with nuclear magnetic resonance (NMR) instruments and X-ray crystallography equipment, exact three-dimensional molecular structures are routinely determined. Today’s benzene controversies extend beyond occupational exposure, which is still an issue since benzene is needed for the production of numerous substances ranging from nylon and polystyrene to acetone and epoxy resins. These days, given that the International Agency for Research on Cancer (IARC) has listed benzene in Category 1, restricted to substances that are known to be human carcinogens, we worry about exposure to amounts much smaller than those encountered by workers.

The first issue occurred in 1990 when traces of benzene were detected in Perrier water. Media reports of finding a carcinogen in Perrier caused public panic and resulted in a marketing nightmare for the company. Benzene can occur naturally in some underground waters and trace amounts did make their way into the spring from where Perrier drew its water. The benzene had always been filtered out, but a worker forgot to change the filter, which meant that a large run of bottles was contaminated to the extent of about 15 parts per billion. This is just a little more than the amount allowed in drinking water, which has a hundred-fold safety factor built in. Someone would have had to drink dozens of litres of Perrier a day for years to be at risk.

Legitimate concern is warranted in breathing cigarette smoke, which is loaded with benzene. Air that contains benzene from automobile exhaust is also an issue. A recent study in the U.K. that involved measuring benzene concentrations in the air and monitoring disease patterns in close to 400,000 people revealed that both heart disease and cancer increase with increasing concentration of benzene in the air. A further worrisome aspect is that elevated risk was found even at concentrations less than one part per million, the amount thought to be safe in occupational environments.

Recently, worry has been raised about aerosol deodorants, sunscreens and dry shampoos, some of which were found to contain benzene as an inadvertent contaminant. Measures have since been taken to ensure removal of benzene. There is also concern about acne products containing benzoyl peroxide since this chemical can decompose and release benzene. Benzene does not easily pass through the skin, but the study did determine that such products when heated can release benzene into the air from where it can be inhaled.

This may an issue, albeit a minor one, if someone gets into a hot car in which such an acne cream has been left. However, I think conjuring up a risk for this is stretching the science. The real issue when it comes to benzene is long-term inhalation of air tainted with gasoline vapours and cigarette smoke. Unfortunately, stopping to breathe is not an option, and undoubtedly some cases of leukemia are caused by benzene in ambient air.


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