Neurological Disorders

Many environmental health hazards are neurotoxins. Indeed, most pesticides kill insects and other pests by disrupting the nervous system, and the warning labels that accompany most petroleum-based solvents and solvent-based products (turpentine, Stoddard solvent, gasoline, laquer thinner, etc) warn specifically about confusion and other neurologic symptoms, including death, that can result from prolonged exposure. "Glue sniffing" is popular because of the confused euphoria that it produces.

Although the effects of some neurotoxicants appear to be reversible, at least in the short-term, chronic exposure may lead to irreversible destruction of neurons and neural pathways. In many instances, the clinical manifestations of this damage may not become apparent until later life as neurons naturally die. For example, the symptoms of Parkinson's disease do not become apparent until approximately 75% of the dopaminergic neurons implicated in Parkinson's disease have died. Mercury and lead are other known neurotoxicants.

Although neurological problems caused by environmental health hazards can occur at any age, there is a tendency to talk about several general mechanisms by which neurotoxicants damage the nervous system:

neurodevelopmental damage;
reversible (?) neurotoxicity; and
neurodegenerative damage.

Neurodevelopmental Damage
Exposure to environmental health hazards, such as cigarette smoke or mercury, during pregnancy may cause neurological problems and/or may lead to underweight premature births. Premature babies are at special risk for brain damage.

Children with a learning disability may have verbal and mathematical difficulties, or may lack self-control and attention span. Attention deficit hyperactivity disorder (ADHD), dyslexia, and autism are all types of learning disorders.

According to the National Academy of Sciences in 2000, exposure to known toxic substances such as drugs, cigarette smoke, lead, PCBs and mercury cause three percent of developmental and neurological defects in children. Lead and mercury are discussed elsewhere in this website.

Reversible (?) Neurotoxicity
Short-term low-level exposure to solvents, alcoholic beverages or mood-altering drugs such as Valium and cocaine, neurotoxic pesticides such as the organophosphate (OP) pesticides (Dursban and Diazinon are familiar OP pesticides in the Houston area), and oxygen-depleting chemicals such as carbon monoxide and methylene chloride generally cause transient euphoria, sleepiness or other neurologic symptoms. Whether or not there is subtle long-lasting but asymptomatic damage to neurons or their pathways from low-level exposure is uncertain but unlikely except in susceptible individuals.

Chronic use or a single high-level exposure generally causes permanent damage, the clinical manifestations of which often intensify with age. Exposure to neurotoxic heavy metals such as lead and mercury in adulthood is less damaging than during brain development, but it is unclear whether reducing exposure and body loads completely reverses the symptoms. "Painter's Encephalopathy" and "Mad Hatter's Syndrome" (from mercury exposure) reflect occupational exposure to neurotoxic substances.

Neurodegenerative Diseases
Alzheimer's Disease, Parkinson's Disease, and Lou Gehrig's Disease are the more well-known neurodegenerative diseases, but more subtle forms of neuronal damage that lead to mild tremor in one's hands, difficulty with balance, memory loss or confusion are much more common. These changes indicate dead or dying neurons and/or disruptions in the pathways through which neurons communicate.

Among the environmental factors that have been implicated in neurodegenerative diseases are occupational exposure to strong electrical fields, lead, solvents and herbicides. Occupational exposure to lead has been associated with an approximately 3x greater risk of developing Alzheimer’s disease.

Exposure to industrial chemicals may lead to an earlier onset of Parkinson’s Disease, as well as an increase the severity of Parkinsonian symptoms. Parkinson’s disease may also be linked to pesticide exposure, and farmers who regularly use pesticides have a higher incidence of Parkinson's disease than those who use few pesticides.