Historical Use of Chlorinated Solvents and Their Stabilizing Compounds - Environmental Investigation and Remediation - page 26

a vapor degreaser. Subjecting chlorinated solvent vapors to intense ultraviolet light from arc welding
will also form phosgene gas. Health and safety guidance for arc welding calls for no welding within
200 ft of a degreaser using chlorinated solvents (AFSCME, 2006).
Several studies have been performed for measuring the by-products that form when TCE is
photocatalytically decomposed because of arc welding in proximity to a vapor degreaser. The major
products are phosgene and carbon dioxide; small quantities of dichloroacetic acid and trichloro-
acetaldehyde remain at the end of the reaction (Blake et al., 1993; Rice and Raftery, 1999).
Perchloroethylene is known to break down to form phosgene, chloroacetic acid, and trichloroacetic
acid (Skeeters, 1960a). Of the chlorinated solvents, perchloroethylene gives off the most phosgene
when heated by l ame or when in contact with a very hot surface or when subjected to intense ultra-
violet light (Smallwood, 1993).
Dichloromethane and carbon tetrachloride can produce phosgene; methyl chloroform also forms
phosgene as a photocatalytic breakdown product, but to a lesser degree than TCE (Dreisbach, 1987;
European Chemicals Bureau, 2000a). Phosgene is a particularly dangerous toxin. It was used as a
chemical warfare agent in World War I. There are few symptoms immediately following exposure.
Symptoms may appear within 24 h of exposure, but can show up as long as 72 h afterward. Phosgene
is a gas at room temperature and combines with water inside the respiratory tract to form carbon
dioxide and hydrochloric acid, which dissolves the membranes in the lungs; l uid then i lls the lungs,
and death results from a combination of blood loss, shock, and respiratory failure. Even a very small
amount of phosgene may be deadly, although early symptoms of exposure such as dizziness, chills,
and cough usually take 5 or 6 h to manifest (AFSCME, 2006). Phosgene can react with DNA and
with enzymes (polymerases) that are responsible for the replication of DNA in cells. Phosgene is
considered carcinogenic, even at low exposure levels. The lethal concentration of phosgene gas for
which half the exposed people would survive (human LC 50 ) is 500 ppm (Lim et al., 1996).
Illnesses and fatalities have been attributed to degreasing operations in which destabilized sol-
vents have evolved phosgene gas. In one incident, TCE was overheated when it was mistakenly used
in a perchloroethylene degreaser. The thermal breakdown of TCE caused the death of an operator
(Spolyar et al., 1951). The literature holds many other examples of similar fatalities, such as the
chain-smoking proprietor of a dry-cleaning establishment who smoked while operating dry-cleaning
machinery in an atmosphere laden with perchloroethylene vapors. They formed phosgene gas as he
drew the vapors through his burning cigarette (Derrick and Johnson, 1943).
Several of the metal inhibitors used for the normally nonl ammable methyl chloroform are highly
l ammable and can change the l ash point associated with this solvent. Under severe conditions,
stabilizers in methyl chloroform solvent can render it l ammable, a fact that proved fatal to a welder
working on the solid rocket boosters for the space shuttle. On August 10, 1983, an accident killed a
welder who was working in a vapor degreaser pit i lled with methyl chloroform vapors at a Utah
facility that cleaned and inspected the used cases for the solid rocket boosters used on the space
shuttle. The welder had been told that methyl chloroform would not burn. While in a basket sus-
pended from an overhead crane inside the pit, he was attempting to weld a bracket into place when
a ball of l ame developed by his chest and then expanded; he jumped or fell into the pit. A i reball
erupted from the pit, and the welder was burned to death or asphyxiated by the dense methyl
chloroform vapors. Vapor combustibility tests showed that the liquid solvent would not burn, but the
vapors would burn after the liquid was gone. Testing showed that degreasing grades of methyl
chloroform were easier to ignite than vapors of pure or technical grade methyl chloroform. A high
proportion of solvent stabilizers remaining in the solvent after the accident made it easier to ignite
a sample (de Nevers, 1986).
Several stabilizers of methyl chloroform, such as nitromethane and 1,4-dioxane, may impart
l ammability to methyl chloroform if concentrated; these two compounds may have caused the Utah
welding accident. The properties of stabilizers most commonly added to the major solvents and the
industrial operations that lead to their partitioning are discussed in Chapter 3. Operator safety for
industrial processes using chlorinated solvents is the subject of a large body of regulation.