Hong Kong's average air pollution levels can be so high - double or even triple the World Health Organization limit - that some analysts estimate the air contributes to an extra 2,000 deaths a year.

The difference is that in the case of ecological studies there is no information available about the individual members of the populations compared (e.g. comparing several states based on state-wide average air pollution and state-wide average prevalence of respiratory diseases); whereas in a cohort study the data pair exposure/health is known for each individual.