When can relative risks provide causal estimates?

It is emphasised that for epidemiological studies where disease incidence is rare, results from conventional proportional hazards models can often correctly estimate causal associations. The well-known "backdoor criteria" from causal-inference is applied to the common epidemiological study of rare diseases with a proportional hazards model, providing an example of when and how estimates from conventional proportional hazards studies can be used. A similar study with the "frontdoor criteria", that allows studies with unmeasured confounders, finds similar results to conventional mediation analysis with measured confounders. Reasons for this are discussed.