Bradford Hill at Work: How Science Establishes Causation in Occupational Disease

In 1965, Sir Austin Bradford Hill set out a series of viewpoints to help decide when an observed association between an exposure and a disease should be regarded as causal. Six decades on, they remain the most widely used framework for reasoning about causation in occupational and environmental health — and a structure that courts and experts return to when weighing the evidence.

The viewpoints — a structure, not a checklist

Bradford Hill was explicit that these were considerations to weigh, not boxes to tick. Only temporality is strictly necessary; the rest build or weaken confidence.

• Strength — how large is the association between exposure and disease?
• Consistency — is it observed repeatedly, across settings and studies?
• Specificity — is the exposure linked to a particular outcome?
• Temporality — did the exposure precede the disease? (Essential.)
• Biological gradient — does more exposure produce more disease (dose–response)?
• Plausibility — is there a credible biological mechanism?
• Coherence — does the link fit what else is known about the disease?
• Experiment — does removing the exposure reduce the disease?
• Analogy — do comparable exposures cause comparable effects?

Where the disciplines come in

No single profession satisfies these viewpoints alone. Exposure science supplies temporality and the dose–response gradient; toxicology provides plausibility and mechanism; occupational medicine establishes the disease and its coherence with the exposure history; occupational hygiene documents that the exposure was real and quantifiable. Assembled together, they form the weight-of-evidence case that an exposure did — or did not — cause harm.

Causation is rarely proven by one study or one expert. It is built, viewpoint by viewpoint, from evidence that several disciplines can each defend.

That is why credible, well-documented exposure data matters so much. Without a defensible dose–response gradient and a clear exposure timeline, half of the Bradford Hill structure simply cannot be argued — for either side.