God gave human beings the ability to smell not just so that we could enjoy the fragrance of fresh flowers or the scent of freshly baked bread, but also as a warning system against harmful chemicals and fires, and the presence of decay.

Our knowledge of the neuroanatomy and physiology of smell has expanded greatly in recent years. It is now known that the smell (olfactory) nerves within the nasal cavity connect to neurons that can enter the brain at the site of memory generation, behavioral elaboration, and endocrine regulation (the hippocampus). From there, the neurons connect to critical areas in the prefrontal cortex that control higher brain functions such as thinking, social control, behavior, and executive brain function. These are the parts of the brain first affected by Alzheimer’s disease.

Normally, the brain protects itself from unwelcome substances in the bloodstream by means of a sort of filter called the blood-brain barrier. But this barrier is inadequate in the olfactory tracts, thus allowing things we smell to directly enter the brain.

Studies have shown that many substances can enter the brain by this route, including:

• Toxic metals

• Pesticides, herbicides, and fungicides

• Many pharmaceutical drugs

• Engine exhaust particles

• Aerosolized chemicals

The first changes we see in cases of Alzheimer’s disease occur at the entry point of the olfactory tract into the brain — which is called the entorhinal cortex. Mercury and aluminum can easily enter the brain by this route.

Studies have shown that the highest level of aluminum in a brain with Alzheimer’s occurs in this area. And aluminum is very toxic to the brain.

Because so many chemicals can enter the brain by this route, pharmaceutical companies have begun experimenting with developing nasal sprays that contain drugs used to treat brain disorders. For example, an insulin spray allows insulin direct access to the hippocampus. Studies on animal models of Alzheimer’s disease suggest that insulin nasal sprays can reverse many of the pathological and behavioral features of Alzheimer’s disease, including neuritic plaques and memory loss.

Spraying the insulin into the nose can also prevent the severe drop in blood sugar that occurs when giving insulin by injection.

Because of this direct link between what we smell and the brain, people have to be very careful breathing toxic compounds and working in enclosed areas with toxic fumes. You should also be cautious about medications delivered as nasal sprays, including nasal decongestants and anti-allergy sprays.

In addition, some saline sprays may contain harmful compounds. If you use a nasal saline spray, make sure it contains only saline.

Jogging, walking, or biking along the side of a roadway — especially if it is heavily traveled — is also hazardous. Studies have shown that inhaled micro- and nanoparticles from the exhaust fumes of automobiles can cause prolonged brain inflammation and lead to neurodegenerative damage.

I have also seen joggers and cyclists going through clouds of pesticides that are being sprayed by government authorities. This allows high concentrations of these brain-damaging chemicals to enter the most vulnerable parts of the brain.

High resolution air filters are a good idea for the home. But you should avoid ionizers.