One of the first things Alzheimer’s does is damage the part of the brain that processes the smell of lemons. So if someone can’t smell lemons, it could serve as an early warning sign.
It also keeps a person from smelling other things, most notably natural gas (which is technically odorless, but a scent is added to it for safety purposes). This was troubling to learn, because Alzheimer’s would be bad enough without increasing your risk of exploding.
Source: https://www.eurekalert.org/pub_releases/2004-12/g-sfl121004.php
Adam Rinsky 