Chlorinated solvents are a group of organic compounds defined by the presence of one or more chlorine atoms in their molecular structure. This chemical addition makes the resulting substance highly effective at dissolving fats, oils, and greases. For nearly a century, these compounds were widely used across industrialized nations due to their low flammability and exceptional solvency. This widespread adoption, however, has led to a lasting environmental and public health legacy that is still being addressed today.
Chemical Identity and Common Applications
The defining feature of a chlorinated solvent is the substitution of one or more hydrogen atoms with chlorine atoms on a hydrocarbon molecule, such as methane or ethane. This modification yields various compounds, including trichloroethylene (TCE), perchloroethylene (PCE), and methylene chloride (DCM). These compounds are often grouped chemically as chlorinated methanes, ethanes, or ethenes, based on their foundational structure.
Their ability to dissolve organic materials made them useful in numerous manufacturing and commercial processes. Historically, a common application was in cold and vapor degreasing, where they cleaned metal parts for the automotive, aerospace, and electronics industries. For example, TCE was widely used as a degreasing agent, while PCE became the standard solvent for dry cleaning operations.
Beyond cleaning, these solvents serve as intermediates in the production of other chemicals, including refrigerants and plastics. Methylene chloride (DCM) continues to be used in paint stripping formulations and as a processing aid in the manufacture of certain pharmaceuticals.
Environmental Persistence and Contamination
Many chlorinated solvents are classified as Dense Non-Aqueous Phase Liquids (DNAPLs). They are denser than water and only sparingly soluble, causing them to sink vertically through the soil and aquifer material instead of floating on the groundwater surface. This downward migration can lead to the formation of “source zones” of concentrated contamination deep underground.
Once a DNAPL source zone is established, the solvent slowly dissolves into the flowing groundwater, often for decades or even centuries. This creates a widespread area of dissolved contamination known as a plume, which can travel substantial distances from the release point. The compounds are also resistant to natural breakdown processes (biodegradation) in many aquifer conditions. This combination of high density, low solubility, and stability makes remediation of contaminated groundwater sources difficult and costly.
Mechanisms of Human Exposure
Human exposure to chlorinated solvents occurs through several pathways. In the workplace, inhalation represents the most frequent route of exposure, particularly for workers involved in metal degreasing or dry cleaning operations where the compounds readily evaporate. High concentrations of solvent vapors in occupational settings can quickly enter the bloodstream via the lungs.
A separate, non-occupational inhalation risk comes from vapor intrusion, where solvents volatize from contaminated groundwater plumes beneath a building. These vapors can seep through cracks in basement foundations or utility conduits and accumulate inside the indoor air of homes and businesses. Ingestion is another pathway, primarily occurring when drinking water wells draw from a groundwater source contaminated by a solvent plume.
Dermal exposure involves direct contact, such as handling products that contain the solvents or contact with contaminated soil or water. While less common than inhalation or ingestion for long-term systemic effects, skin absorption can be a significant route of exposure, especially in occupational settings where protective measures are inadequate.
Acute and Chronic Health Effects
Exposure to chlorinated solvents can result in both immediate and delayed health consequences, depending on the dose and duration. Acute exposure, involving a high concentration over a short period, often targets the central nervous system (CNS). Symptoms can mimic alcohol intoxication, beginning with dizziness, headaches, and fatigue, and potentially progressing to confusion or unconsciousness at very high levels. Direct contact with the liquid or concentrated vapor can also cause irritation to the eyes, skin, and respiratory tract.
Chronic exposure is associated with damage to specific organs and systems. The liver and kidneys are vulnerable as they are the body’s main detoxification organs. Long-term exposure to certain solvents, like trichloroethylene (TCE), is correlated with toxicity to both the liver and kidneys, and is suspected to increase the risk of developing neurological conditions like Parkinson’s disease.
Furthermore, several common chlorinated solvents are classified as probable or known human carcinogens by various regulatory bodies. For instance, trichloroethylene (TCE) is linked to kidney cancer, while perchloroethylene (PCE) has been associated with an increased risk of laryngeal cancer at the highest cumulative exposure levels.