Dr Lawrence Whalley of the University of Aberdeen, who led the research, said the explanation could be that smoking causes oxidative stress - cumulative damage caused by molecules called "free radicals" - to organs including the brain.

BBC: NEWS | Health | Smoking 'is bad for your brain'

Experts believe vitamins A, C, and E -- the so-called ACE antioxidants -- combat oxidative stress in the lungs that can lead to COPD.

CNN: Vitamin E may protect lungs

Moreover, the widespread occurrence of nitrotyrosine in neurons suggests that oxidative damage is not restricted to long-lived polymers such as NFTs, but instead reflects a generalized oxidative stress that is important in disease pathogenesis.

FORBES: The Lessons Of Failure: What We Can Learn From Bapineuzumab's Blowup