Hill's Criteria for Causality

Smoking had long been a contentious issue. Despite its increasing popularity, many had opposed smoking on moral grounds; others claimed that smoking had adverse health effects, but the evidence to support these claims was thin. There had been a remarkable increase in lung cancer in both the US and Britain during the first half of the 20th century, but the cause had not been established. Many attributed the increase to the steady increase in the use of motor vehicles, or the paving of roads, or the steady rise in industry.

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When investigators gathered data on the number of motor vehicles in use over time, they found that the increase correlated strongly with the increase in lung cancer mortality. The correlation is evidence of an association between motor vehicle use and lung cancer mortality. Does this indicate that motor vehicle use is a cause of lung cancer mortality?


Bradford Hill In 1939, a German study reported an association between smoking and lung cancer. Then, in the 1940s and 1950s there was a succession of studies that sought to examine the cause of the epidemic of lung cancer that was claiming more and more lives. Richard Doll and Austin Bradford Hill (shown on the right) conducted landmark epidemiologic studies that were important in establishing the strong association between smoking and lung cancer. The first was a case-control study conducted in London area hospitals. The cases were patients with lung cancer, and the controls were age and gender matched patients at the same hospital who had diseases other than cancer. [NOTE]

After their case-control study, Doll and Hill launched a prospective cohort study among male physicians in the UK, looking at cause of death as the primary endpoint. The initial findings were published in 1954, with a follow up report in 1958. These studies demonstrated an even stronger association between smoking and lung cancer mortality and also showed that smoking was also significantly associated with other cancers and with a variety of other non-cancerous causes of death including emphysema, chronic bronchitis, TB, atherosclerotic heart disease, stroke, hypertension, and aneurysms.

Despite the strong associations that they found, there was controversy about whether the association was causal. Out of this debate came the notion that causality could not be proven by formulaic consideration of observations; instead, a conclusion of causality was a judgment based on a body of evidence. In 1965 Hill and others proposed certain aspects of evidence that should be considered when trying to draw conclusions about causality. These were not intended to be rigid criteria.

Hill's Criteria

These reports provided the basis for the 1964 US Surgeon General's Report that concluded, "Cigarette smoking is causally related to lung cancer in men; the magnitude of the effect of cigarette smoking far outweighs all other factors." It estimated that moderate smokers had a 10-fold increase in risk of lung cancer, while heavy smokers had a 20-fold increased risk. The report backed pedaled regarding the addictive properties of nicotine concluding that the "tobacco habit should be characterized as an habituation rather than an addiction,...."

The US Surgeon General's report stirred controversy, and even physicians (many of whom were smokers) refused to accept the conclusion that smoking was a cause of lung cancer. In the video below, medical historian Alan Brandt summarizes the reaction to the report and the scientific studies upon which they were based.

Eventually, the epidemiologic studies and the Surgeon General's report did begin to have an impact on public opinion, however A 1958 survey found that only 44 percent of Americans believed smoking caused cancer, but in 1968 78 percent believed that smoking caused lung cancer.

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In addition to the tactics described by Alan Brandt in the video, the tobacco industry also steadfastly claimed that there was no proof that cigarettes caused lung cancer. In 1994 the Occupational Health and Safety Administration (OSHA) began a series of hearings regarding a proposed rule on indoor air and the potential harm of environmental tobacco smoke . Part of the hearing was aimed at the more direct question of whether active smoking causes lung cancer. The link below will bring you to a series of responses supplied by tobacco industry scientists when they were asked whether they believe that active smoking causes lung cancer. As you read their responses, consider the following questions:

  1. What distinction do they make between "risk factors" and "cause".
  2. One of the tobacco industry witnesses suggests that lung cancer is multi-factorial. Is this a reasonable possibility?
  3. And, if smoking is multifactorial in etiology, does this mean that tobacco is not a cause?
  4. Can anything be proven to have caused a given case of lung cancer?

Link t the OSHA Testimony (for more excerpts from the hearing, click here).