Exposure to radon has been correlated with AD death rates in the US.

To view the research article titled: "Association of Radon Background and Total Background Ionizing Radiation with Alzheimer's Disease Deaths in U.S. States", go to:
Journal of Alzheimer's Disease (http://content.iospress.com/journals/journal-of-alzheimers-disease), vol. 59, no. 2, pp. 737-741, 2017.

Excerpts:
1. "Results: Radon background ionizing radiation was significantly correlated with AD death rate in 50 states and the District of Columbia (r=0.467, p= 0.001)."

2. "Conclusion: Our findings, like other studies, suggest that ionizing radiation is a risk factor for AD. Intranasal inhalation of radon gas could subject the rhinencephalon and hippocampus to damaging radiation that initiates AD. The damage would accumulate over time, causing age to be a powerful risk factor."

3. "Radon and its daughters accrue preferentially in AD brain proteins, and are higher in AD subjects than in healthy controls [18]".

------------------------------------

The chance to improve their odds of preventing both dementia and lung cancer may motivate a substantial percentage of Americans to have their indoor radon level no higher than the outdoor level, a goal established by our federal government in 1988. At my house, we accomplished that goal in two stages. First, we installed a fresh air supply system with a filter to remove particles, some of which had radon progeny attached due to their electrostatic charge. The ventilation system made it safe for us to make the building envelope much tighter. Our indoor radon level dropped from 24 pCi/L down to 2 pCi/L. Then we installed an SSD system that dropped the indoor radon level to 0.5 pCi/l. Filtering the incoming fresh air reduced our exposure to allergenic, irritating, and toxic particles including soot and magnetite, that can damage immune, cardiovascular, respiratory and central nervous systems. Side benefits included: Lower utility bills, improved comfort, and fewer allergy symptoms.

The EPA recommends using ventilation systems to dilute and flush-out airborne pollutants, including radon, in homes, offices, etc.