It is going to be difficult if not impossible to prove what it was but either way we should be extremely careful what materials and chemicals we use especially ones that can cause hormonal changes or damage DNA. Even more we should be careful what we expose kids to before they are fully developed.
This makes me think of the way that the tobacco[1] and petroleum[2] industries knew about the harmful effects of their products long before the causes were removed or the products were phased out.
Perhaps there's an incentive problem somewhere if research like that exists and is ignored due to profit motives?
[1] - https://www.theguardian.com/news/2005/jun/02/thisweeksscienc...
Also consistent with "men had a greater reduction (24%) than women (8%)."
I'm in the camp of it is something to do with cars. The phase out of the carburetion came around that time. This was a huge win for air quality as carburetors are grossly inefficient. In off throttle situations, like coasting down a hill, they are literally dumping unspent gasoline in the atmosphere. Some of it ignites in the exhaust system (making that notable pop-ping-pop old cars have when off-throttle in low gear), but most is dumped straight in the atmosphere.
This would also explain why the decline happens in the USA & western Europe. And might also explain the gender discrepancy: with men being much more likely to work with and around automobiles as a career.
That's off in terms of timeline.
California smoking ban came online in 1996. Anti-smoking efforts were in swing quite a bit before that.
And, things like the 3 Martini lunch were on their way out by 1990 which also reflects that smoking was no longer considered "professional" and was something done by the lower classes instead.
I do suspect that the whole thing is a combination of factors: leaded gas started going away in 1992, smoking was going down by 1996, air pollution was finally getting better by 1990 (collapse of the steel industry in the 1980s, gas fired power plants, and better fuel efficiency in cars finally hitting the general public), etc.
Both types of pollution have been studied in pretty great detail and I suspect that if they were linked to dementia we'd at least suspect it by now.