Whether or not a specific chemical appears neurotoxic to humans depends upon many factors, including the route of entry, the ability of the chemical to enter the nervous system, the concentration of the chemical within the nervous system, the toxicity of the metabolic products of the chemical, and the rate that the chemical can be excreted from the body. The dose of the exposure is important, because certain chemicals at one dose may be harmless, or even necessary for human functioning, while at other doses they may become potent neurotoxins. Pyridoxine, for example, is an essential vitamin required for human life. When administered in high doses, however, it causes profound sensory loss. A history of having been exposed to a chemical followed by complaints of central nervous system dysfunction is not sufficient to substantiate a cause-effect relationship.
Simple association is insufficient to establish a toxic cause.
Neurological symptoms are common in the general population, often appearing as a consequence of normal physiologic variation. Normal events such as aging profoundly affect bodily systems with consequent changes, e.g., arteriosclerosis, affects the blood vessels so that blood supply to the brain is reduced with age. The resultant effects range from strokes to senility. Standard medical and scientific practices require interpretation of such symptoms in the context of other findings in order to establish whether the symptoms have any clinical significance. There are many symptoms that may mimic organic neurologic disease which are actually associated with abnormal stress states, such as hyperventilation syndrome, muscle contraction headache, generalized fatigue, and feelings of chronic exhaustion.