The Right Chemistry: Causation, correlation, COVID and copper


While associations cannot prove a cause-and-effect relationship, they can serve as a springboard for further investigation.

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Should you stop brushing your teeth? Statistics show that 98 per cent of Canadians who develop COVID symptoms brushed their teeth two days before the onset of symptoms. Should you avoid saunas? Finland has one of the highest rates of heart disease in the world and Finns own more saunas per capita than any other nation.

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Actually, studies show that frequent saunas reduce the incidence of cardiovascular disease. So, what’s with the Finns? They consume roughly 80 grams of fat a day, far more than the World Health Organization’s recommended daily intake of 50 grams. They also love sugar, consuming close to double the 50 grams per day stipulated by the WHO. Obviously, their high cardiovascular disease rate is more likely due to a poor diet than their love of saunas.

Although investigating such associations is at the very heart of science, the process can obviously be treacherous. The observation that the sun rises in the morning and sets at night led many to conclude that it circled the Earth until Galileo and Copernicus came along. Since the 1950s, both obesity and levels of carbon dioxide in the air have risen significantly. Based on this association, one might therefore conclude that an increase in inhaled carbon dioxide causes obesity. However, science tells us that obesity is caused by an increase in calorie intake, not carbon dioxide inhalation.

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There are many other similar examples. Does the shaking of tree branches cause wind, or does the wind cause trees to shake? If you live in the jungle, this may not be easy to answer. Wind and shaking trees always go together. Of course, sailors will know that there is wind in the middle of the ocean where there are no trees to cause it.

While associations cannot prove a cause-and-effect relationship, they can serve as a springboard for further investigation. The observation that lung cancer was seen more frequently in smokers spawned studies that proved smoking did indeed cause the disease. The observation that workers involved in the production of polyvinyl chloride (PVC) from vinyl chloride had an unusually high incidence of a rare type of liver cancer led to studies that clearly demonstrated the carcinogenicity of vinyl chloride.

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In 1731, Italian physician Bernardino Ramazzini published a book, Diseases of Workers, which is regarded as the first systematic investigation of occupational hazards. In a chapter entitled Diseases of Cleaners and Privies he described how these workers often suffered from a painful inflammation of the eyes and also noted reports of copper or silver coins in their pockets turning black. Ramazzini concluded that as the workers disturbed the excrement, some vapour that caused the eye irritation and also turned the coins black was released. A French commission set up to study the problem produced a report in 1885 that concluded, as Ramazzini had, that sewage emitted some sort of toxic gas inhalation of which could even be lethal. Victor Hugo was apparently unaware of this danger: in his classic, Les Misérables, Jean Valjean trudges unaffected through the Paris sewage system as he carries the injured Marius to safety.

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In 1772, Swedish apothecary Carl Wilhelm Scheele, who had developed a keen interest in chemistry, became the first person to isolate oxygen by heating mercuric oxide. Unfortunately for him, he didn’t publish his work until 1777 in Chemical Observations and Experiments on Air and Fire, by which time Joseph Priestley had published a paper describing essentially the same experiment performed in 1774. This earned Scheele the nickname Hard Luck Scheele. Neither he nor Priestley recognized that the “air” they produced was an element. It was Antoine Lavoisier who correctly interpreted Priestley’s experiment and is commonly recognized as the discoverer of oxygen. The credit really should be shared by the three men.

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Scheele was also the first to produce hydrogen cyanide by heating potassium ferrocyanide with sulphuric acid and noted the almond-like odour of the gas. This time Hard Luck Scheele became Lucky Scheele, because he escaped being poisoned by the cyanide. His luck persisted when he heated ferrous sulphide (fool’s gold) with an acid and produced a gas he described as having a fetid smell. Scheele had made hydrogen sulphide, a gas that is more toxic than hydrogen cyanide but its scent is so potent that Scheele likely left the premises and avoided poisoning. He did not identify the gas as hydrogen sulphide, that was determined by the French chemist Claude Louis Berthollet in 1776, and it was Baron Guillaume Dupuytren who subsequently showed that this was the gas that caused the problems in the sewers of Paris.

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How does this nasty gas form? Mostly by the reaction of sulphates with “sulphate reducing bacteria” present in sewage. Sulphates in turn form when microbes act upon sulphur-containing proteins in decomposing organic matter, such as in sewage. As far as the coins are concerned, hydrogen sulphide reacts with silver or copper to form the corresponding sulphides that appear as a black deposit on the metal.

This phenomenon was also noted on copper air-conditioning coils in some Florida homes and in houses rebuilt after Hurricane Katrina. In both these cases, supplies of domestic drywall were in short supply and drywall was imported from China. Drywall is made of gypsum, or calcium sulphate, and it seems that under humid conditions the Chinese drywall was manufactured without proper preservatives and became contaminated with microbes that produced hydrogen sulphide from the sulphate. Many people complained of the smell of rotten eggs in their homes. Indeed, when eggs spoil, bacteria break down their protein components and release hydrogen sulphide. The amounts are way too small to cause poisoning, but they will remind you of a sewer.

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One final observation. The vast majority of patients requiring intensive care in hospitals are unvaccinated. A spurious association? I think not. Oh, and keep brushing your teeth. That link with COVID is certainly bogus. Remember that correlation is not the same as causation.

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Joe Schwarcz is director of McGill University’s Office for Science & Society ( He hosts The Dr. Joe Show on CJAD Radio 800 AM every Sunday from 3 to 4 p.m.

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