The Harvard Six Cities Study was published in 1993. Critics argued the findings could be explained by socioeconomic factors, lifestyle differences or other confounders.
The problem with that argument is that the evidence didn’t stop in 1993.
Over the last 30 years, and especially in the last 10–15 years, there has been an explosion in air pollution research driven by improved exposure modelling, satellite observations, biomonitoring, toxicology, imaging and access to massive health databases.
Instead of the evidence weakening, it has become stronger.
A 2024 Danish phenome-wide study followed 3.1 million people for up to 18 years and found associations between air pollution and more than 700 disease categories spanning cardiovascular, respiratory, neurological, endocrine and many other systems.
We now have large epidemiological studies, natural experiments, controlled exposure studies, toxicology studies and mechanistic evidence showing particles reaching the bloodstream, placenta and brain.
If air pollution epidemiology was simply socioeconomic confounding, you’d expect the signal to disappear as methods improved.
Instead, after 30 years of increasingly sophisticated research, the evidence base has expanded dramatically and the findings remain remarkably consistent.