Disease causality

One of the main goals of epidemiology is to identify causal relationships between outcomes - like death, diseases, or injuries - and exposures - like smoking cigarettes, eating junk food, or drinking alcohol.

For example, nowadays, it’s widely known that smoking cigarettes causes lung cancer, or in other words, that smoking cigarettes leads to the development of lung cancer in many people. But how did we figure this out?

In the mid- 1950’s, an epidemiologist named Sir Austin Bradford Hill came up with nine guidelines for determining whether or not two things are causally related, and these are called the Bradford Hill Criteria.

The first criterion refers to the strength of association, and says that two things are more likely to be causally related if the strength of association between them is large.

For example, for people who smoke, the relative risk of lung cancer is around 25, meaning people that smoke have 25 times the risk of developing lung cancer compared to people that don’t smoke.

On the flip side, the relative risk of breast cancer for people who smoke is only about 1.5, which is much lower than the relative risk of lung cancer.

So, smoking is much more likely to cause lung cancer than it is to cause breast cancer.

The second criterion states that causal relationships are dose- dependent, meaning a person who has higher amounts of exposure will also have a higher risk of developing the outcome.

For example, the relative risk of lung cancer for people who smoke 10 cigarettes per day might be 8, the relative risk of lung cancer for people who smoke 15 cigarettes per day might be 13, and the relative risk of lung cancer for people who smoke 20 cigarettes per day might be 22.

On the other hand, if a person decreases or stops their exposure, their risk of the outcome also decreases.

For example, in people who stop smoking, the risk of dying from lung cancer decreases by half.

In a causal relationship, the exposure has to precede or come before the outcome, and this is called temporality, which is the third criterion.

For example, typically people are more likely to develop lung cancer after they have smoked, so smoking comes before lung cancer.

Oftentimes, there is a lag- time or gap between the exposure and the outcome, and the timing of this gap also has to be consistently true.

For example, it usually takes 20 years for lung cancer to develop after a person begins to smoke, since the negative effects of smoking on the body build up over time.