The limbic system is a primitive region of the brain associated with instinctual responses and behavior. For example, it records the pleasant value most people assign to the smell of new-mowed grass or bread baking. The limbic region's role, if any, in chemical sensitivity is unknown, but is becoming the subject of research. No one knows. One theory is that many people suffer from chemical sensitivity but may not recognize it.
Humans have an enormous capacity to adapt to many substances.
Examples are nicotine and alcohol. Many report that their illness began with flu-like symptoms. If they avoided exposures, either intentionally or unintentionally, they noticed their symptoms improved. No one knows. Dr. Miller said there are many theories.