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Current air pollution levels isn’t indicative of lifetime or especially childhood exposure.

Leaded gas for example has seen a huge decline, but was still a thing when current 70+ year olds where young. Similarly, current developing economies where very different 50 years ago.

Sure, but even so it's not clear that air pollution is a factor. Some researchers recently compiled a "chronology of global air quality" which might be worth a review to see if any correlations can be uncovered. At a glance, it looks to me as though Europe and the USA cleaned up their act a long time ago -- so unless pollution exposure in early childhood is somehow especially bad, pollution still has no explanatory power: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7536029/

There's also a paper which reviews the "global incidence of young-onset dementia" -- and, again, incidence in the low-pollution USA is higher than in high-pollution developing countries. https://alz-journals.onlinelibrary.wiley.com/doi/full/10.100...

Leaded gas was a thing when current 30 year olds were young.

Surely studies could get a good control group of people who grew up in rural Montana, or the Andes. Assume we can find groups with near-zero lifetime exposure to atmospheric (and/or waterborne) lead. What's their background rate of Alzheimer's?

Not really [0], leaded gas was completely banned in the US in 1996 but really hadn’t been used since the early 80s and had start the phase out in 1996.

So you’d probably need to look at people in their 50s who really had an impact of lead from auto fuel.

[0] https://www.greencarreports.com/news/1133434_why-did-the-wor...

Where "completely" in this case implies "for road-going vehicles". General aviation (largely small, piston-engine planes) still overwhelmingly run on 100 octane low-lead (100LL).

This is a very big deal if you spend time around small aircraft (hangar floors in particular sometimes have terrifyingly high lead levels.) But if you’re not spending time close to aircraft, the amount of lead you’ll be exposed to is negligible - at least compared to auto emissions, which were ubiquitous and extremely concentrated in residential neighborhoods. Unfortunately the lead remains in the soil, so there’s still exposure to it today.

Good point. I meant “completely” for consumer vehicles in the US. That’s what was pumping out all that lead into the air in the US. The other uses were much smaller.

Leaded gas is still not banned today. I have a car which only consumes leaded race gas and while it is surely expensive it is definitely not illegal.

Many smaller, older aircraft use leaded fuel as well.

Basically most piston engine aircraft use Avgas 100LL (low lead solution). So the majority of the air traffic found at a typical municipal airport. Fortunately this is being phased out by the FAA in favor of unleaded.

On road use is banned. Also, the cost per gallon of leaded gas is at least $10/gallon now, which is a pretty large incentive enough for people not to use it :)

> really hadn’t been used since the early 80s

I had summer jobs at gas stations in the early 90s and maybe 10% of the fuel sold was leaded.

This is correct. Environmental accumulation would be highest in the areas that industrialized first.

Romans used lead everywhere, even for cooking.

Roman bones also show much higher levels of copper. It is well documented that the brains suffering from Alzheimer's lack copper. (from autopsies)

>> "Conclusions: The modern 'epidemic level' of advanced dementias was not described among ancient Greco-Roman elderly. The possible emergence of advanced ADRD in the Roman era may be associated with environmental factors of air pollution and increased exposure to lead. Further historical analysis may formulate critical hypotheses about the modernity of high ADRD prevalence."

It's not as if they were doing detailed demographic studies back then, though. There's just certain things that can't be known, and this is one of them.

You don't need demographic studies. These were people's parents, grandparents. Part of their lived life. Enough random diaries are sufficient.

Indeed - it could equally be explained by dementia/Alzheimer’s being a contagious disease, such as a prion or similar protein malformation, which quite a bit of current research seems to suggest, and it simply had not become endemic at that point.

It disappoints me that something which severely impacts the lives of perhaps one third of humans isn't better understood.

If it were a prison disease, surely with ~2 billion data points of those with/without dementia, we would have isolated exactly what molecule causes it and how it gets transmitted from person to person.

Cures are not good business. Treatments are. Treatments of minimal or dubious efficacy even moreso.

Track the incentives.

Do you have any research you can point for that, its quite interesting.

That looks like a negative correlation.

Anyway, the cause seems to be nutritional. People used to consume vastly more copper than we do today. The modern recommended intakes were hastily made up for the need in the WW2, and never seriously revised.

Asia has comparatively fewer cars and break pad dust aka asbestos.