I was going to say something similar. Obviously I was just taking a visual look and not doing any rigorous analysis, but California certainly sticks out like a sore thumb. It has many areas in the Central Valley and Southern California with very high PM2.5 levels, but no "purple" areas with high dementia levels in the state. I also just read the article and not the study but I'd hope they give an explanation for that glaring outlier.

I'd be interested to compare the disease map with a map of average income, because at first glance the disease data looks to be correlated with wealth, and we already have tons of research that shows that wealth is one of the biggest determinants of health outcomes in the US.

I noticed this too. I do wonder what it would look like if you controlled for relocation (moving to another region and then developing symptoms), disease onset (both maps are for the same time ranges), and the types of pm 2.5 exposure.