Atmosphere

The Environmental Protection Agency has expressed fears that even short lived halocarbons may have a significant detrimental effect on the global atmosphere, as well as concerns about our relative lack of understanding of the environmental effects of these compounds1. Several chlorinated solvents are listed by the U. S. Environmental Protection Agency (U.S. EPA) as a hazardous air pollutant (HAP) under the U.S. Clean Air Act. However, various environmental NGOs and organisations maintain that current regulation of chlorinated solvent disposal is inadequate - for example, chlorinated solvents are not regulated under the Montreal Protocol despite evidence that they may contribute to ozone depletion.

Photochemical Smog

Global Warming

Dichloromethane has a Global Warming Potential (GWP) ten times greater than that of carbon dioxide, whilst trichloromethane has a GWP 30 times greater. At the time of writing the GWPs of tetrachloroethene and trichloroethene are not known, but are expected to be comparable to those for DCM and TCM.

Acid Rain

In the lower atmosphere degradation of chlorinated solvents is initiated by a reaction with the hydroxyl radical, and forms a variety of products including hydrochloric acid, formic acid, and phosgene (the colourless gas infamous for its use as a chemical weapon during World War One). These compounds dissolve in cloud and rain water and are ultimately deposited from the atmosphere in acid rain and snow.

Further, trichloroacetic acid (TCA) can be formed as a minor product in the atmospheric deg-radation of some chlorinated solvents. Studies have shown that TCA is broadly distributed in precipitation, surface water and soil on a global scale. Since the observed levels in soil in some areas have been found to exceed the accepted 'safe' levels (2.4 μg/kg for terrestrial organisms) the European Commission instructed producers of the relevant solvents to carry out extensive studies of the origin and fate of environmental TCA. Although the results of these studies suggest that TCA levels in soils could not be explained by precipitation alone, the European Union Risk Assessment on nevertheless concluded that “it is considered unlikely that depo-sition of TCA from the atmosphere will by itself lead to levels of TCA in soil that pose a risk for ter-restrial organisms”.