Jennifer McPartland, Ph.D., is a Health Scientist.
A study published online in the Annals of Neurology last week, “Solvent Exposures and Parkinson Disease Risk in Twins,” adds to scientific evidence linking exposure to the solvent trichloroethylene, or TCE, and other common solvents with onset of Parkinson disease. Parkinson disease is a debilitating condition well known for symptoms of trembling but can also include slowed motion, impaired posture and balance, and loss of automatic movements (e.g. blinking, arm swaying when walking). Most unfortunately, it has no cure.
According to the authors, this new twin study is the first confirmation in a population-based study of a significant association between exposure to TCE and incidence of Parkinson disease.
What we already know about the ill effects of TCE exposure
TCE was first commercially produced in the 1920s and has served various purposes since that time. It is estimated that 80−90% of global TCE production is used for degreasing metals.
Commonly associated health effects associated with inhalation exposure to TCE include lung irritation, dizziness, poor coordination and difficulty concentrating. EPA’s Integrated Risk Information System (IRIS) toxicological review of TCE (finally issued in September 2011) concludes that TCE poses “potential human health hazard for…toxicity to the central nervous system, kidney, liver, immune system, male reproductive system, and the developing fetus.” And it further states that the scientific evidence supports the conclusion that TCE is “carcinogenic in humans by all routes of exposure.”
The IRIS review did include some discussion on TCE exposure and Parkinson disease, but could not give it much consideration owing to limited data. The review notes, “If indeed TCE can target dopamine neurons, it would be anticipated that human exposure to this agent would result in elevated rates of parkinsonism. There are no systematic studies of this potential relationship in humans….” This new twin study would have helped EPA evaluate the weight of evidence for an association for TCE exposure and Parkinson disease.
What the new study tells us
In this study, ninety-nine pairs of twin males, one of whom has Parkinson where the other does not, were assessed for their exposure to solvent chemicals that have been associated with Parkinson disease either “anecdotally” or in other studies. By using twins, the investigators sought to reduce the influence of confounding factors such as differences in genetics, demographics, and lifestyle.
In addition to TCE, the investigators examined associations between onset of Parkinson disease and exposure to the solvents n-hexane, xylene, toluene, carbon tetrachloride, and PERC (perchloroethylene). The researchers found that exposure to TCE resulted in a six-fold increase of risk for developing Parkinson disease. In addition, they found that exposures to PERC and carbon tetrachloride, “tended toward significantly increased risk” of developing the disease. A statistically significant association was not identified for the other solvents.
The methodology of the study was quite interesting: The investigators used detailed questionnaires to evaluate exposure of the twin subjects to the six solvent chemicals, including both occupational and hobby sources of exposure. For any particular occupation, having been “ever” exposed to a solvent was defined as exposure for at least 2% of work time or 1 hour per week. The researchers also collected information on lifetime history of smoking and head injury, which are other factors considered to influence the onset of Parkinson disease.
Evidence of widespread exposure to TCE
If it isn’t alarming enough that exposure to a chemical increases the risk of developing Parkinson disease, the sirens really go off when we consider just how widespread exposure to the implicated chemicals are. For TCE:
- The occupations most frequently associated with exposure include electrician, dry cleaner, industrial machinery repairer, and health worker.
- The Executive Summary of EPA’s Integrated Risk Information System (IRIS) Final Toxicological Review of TCE states, “There is substantial potential for human exposure to trichloroethylene (TCE), as it has a widespread presence in ambient air, indoor air, soil, and groundwater.”
- The IRIS review also cites data summarized by the U.S. Agency for Toxic Substances and Disease Registry (ATSDR) showing that TCE is the most frequent organic contaminant in groundwater, finding that a whopping 9% to 34% of the U.S. drinking water supply sources may have some TCE contamination.
- For 2011, ATSDR has ranked TCE as the 16th highest priority substance for Superfund sites listed on the national priority list (NPL), a ranking that takes into consideration a chemical’s frequency, toxicity, and potential for human exposure at the sites.
- According to the EPA Toxics Release Inventory (TRI), more than 2.4 million pounds of TCE was released into the environment in 2010.
- In 2007, the Government Accountability Office published a report which described exposure to TCE as “pervasive in the environment” and noted that the CDC National Health and Nutrition Examination Survey (NHANES) survey indicates that approximately 10% of the U.S. population has detectable levels of this chemical in their blood.
This evidence of extremely widespread potential for exposure to TCE, coupled with clear evidence of the harmful effects of TCE exposure – and now the emerging evidence associating such exposure with higher risk for developing Parkinson disease – demands that far more decisive action be taken by officials charged with protecting public health.