Smoking a pack a day the deadly threshold
Expert determined the ‘critical dose’ of cigarettes that could cause disease
How much cigarette smoking causes lung cancer?
Turns out that a pack a day for five years is the critical tipping point where smokers are twice as likely as non-smokers to get lung, throat and larynx cancers.
This precedent-setting calculation figured prominently in Canada’s biggest class-action settlement, handed down Monday, awarding more than $15 billion in damages to smokers.
It is based on the expertise of Université de Montréal epidemiologist Jack Siemiatycki, internationally known for research on occupational causes of cancer, who invented a way of measuring “critical dose” specifically for this case.
Monday’s judgment by Quebec Superior Court Justice Brian Riordan found three Canadian cigarette-makers liable for selling a harmful product they knew was dangerous while hiding those ill effects on health from consumers.
But that fault alone does not make a direct link between cigarettes and each individual smoker’s sickness.
The tobacco industry would have preferred that each member of the class action suit go to court to prove damages. But a million potential victims cannot individually parade through Riordan’s court, and that’s where Siemiatycki’s epidemiological analysis of probabilities came in.
A Centre de recherche du Centre hospitalier de l’Université de Montréal (CRCHUM) researcher who holds the Guzzo-cancer research society chair in environment and cancer, Siemiatycki established a link between smoking and four diseases — emphysema, and lung, throat and larynx cancers — in his report.
It looked at several questions, including:
What is the risk of the disease among smokers compared with non-smokers?
What is the dose-response relationship between smoking and the disease?
At what level of smoking does the balance of probabilities become greater than 50 per cent that smoking played a key role in an individual’s disease?
Among all smokers who got the respiratory diseases in Quebec since 1995, for how many did the balance of probabilities of causation exceed 50 per cent?
According to the scientific literature, if no one smoked, 98 per cent of lung cancer would be eliminated, and up to 90 per cent of throat cancers, too.
But how much cigarette smoke does it take to reach a “probability threshold” for each disease?
“My job in this case was to estimate where on the scale you have to be in order to get across the threshold of doubling the risk, and how many people in Quebec who get those diseases actually smoke that amount,” Siemiatycki said.
Turns out that “five pack years” is a critical dose, although the average smoker has more than 20 years of cigarette history.
A “pack year” is the equivalent of smoking one pack of 20 cigarettes a day over one year: 365 days times 20 equals 7,300 cigarettes, and spread over five years, 36,500 cigarettes smoked.
My job in this case was to estimate where on the scale you have to be in order to get across the threshold.
It could be any combination of the five pack years; for example, 25 cigarettes per day for four years or 10 cigarettes per day for 10 years.
Siemiatycki turned to Statistics Canada surveys on smokers in Quebec for the combination of people who smoke and dipped into the Registre des tumeurs du Quebec for a list of people with cancer; he then used an algebraic formula to calculate the critical carcinogen dose of “pack years” and relative risk of disease for the probability of smokers getting sick from smoking. (Relative risk here is the ratio of cancer among non smokers versus cancer among smokers.)
While Siemiatycki’s critical dose is five-pack years of smoking, the judge set the limit at 12 pack years as the cut-off point to be considered in the litigation.
“However, we know from our own cancer studies, and others’ studies, that smoking patterns are predictable and most people don’t stray from the average,” Siemiatycki said. The average smoker starts the cigarette habit between ages 15 and 20, and typically smokes for 20 years, he added, before starting to think about quitting.
The experts hired by the tobacco industry tried to discredit Siemiatycki’s research as flawed but Justice Riordan found it “reliable and convincing.”
For his part, Siemiatycki says he’s grateful to have been a critical part of the puzzle of this landmark trial.
The three tobacco companies, JTI-Macdonald, Imperial Tobacco and Rothmans, Benson & Hedges, said they’ll appeal the ruling.