[_Algernon_]

Who is to say they don't? Most of those areas have a less developed health infrastructure, so I'd expect the statistics on dementia diagnoses to systemically underreport the issue.

The average age in developing countries is also lower (due to both lower life expectancy and higher birth rate) which is a further confounding value to skew the numbers. Dementia is an old people game, even if it is true that air pollution worsens it.