Indoor air is a major source of PCBs
New research contradicts the prevailing theory that soil volatilization is the primary source of PCBs in the atmosphere.
Research published today on ES&T’s Research ASAP website (DOI: 10.1021/es062218c) uses an innovative approach to show that indoor air releases far more PCBs to the atmosphere around England’s second-largest city than does the area’s soil. The findings go against the widely held hypothesis that soil volatilization is the main source of PCBs in the environment, according to Stuart Harrad and his colleagues at the University of Birmingham (U.K.), where the measurements were made.
PCBs are one of the 12 “dirty dozen” pollutants banned by the United Nations’ Stockholm Convention on Persistent Organic Pollutants. The paper adds important new data to the growing body of research showing that older consumer products still in use are a significant source of PCBs on a global basis, says Terry Bidleman of Environment Canada’s Air Quality Processes Research Division.
The ventilation of contaminated indoor air “to outside is what is driving outdoor air concentrations, which in turn is what drives food concentrations,” Harrad explains. At present, food is the main route through which most people are exposed to PCBs, explains Linda Birnbaum, director of the experimental toxicology division of the U.S. EPA’s National Health and Environmental Effects Research Laboratory.
Although the levels of dioxins and dioxin-like PCBs have been falling, the concentrations of nondioxin-like PCBs in U.K. food have not declined since 1992, according to a draft report (594KB PDF) by the country’s Department for Environment Food and Rural Affairs (Defra). It documents that concentrations of these nondioxin-like PCBs—which are primarily what Harrad’s group was monitoring—have increased in 9 out of 11 food groups monitored between 1992 and 2001. “This is not inconsistent with there being a continuing and nondiminishing source of PCBs to the environment that is helping to maintain concentrations in food,” Harrad says.
Harrad’s new data corroborate recent measurements of the sources of PCBs in and around Toronto, says Tom Harner, a research scientist with Environment Canada. “We now have two comprehensive studies that have investigated the plume of PCBs in urban areas, both implicating indoor air as the major source and both showing strong gradients as you move away from the most heavily populated areas,” Harner says.
Harrad and his colleagues took the concept one step further than the Toronto study by distinguishing between sources of PCBs from indoor air and soils. To do so, they exploited the fact that some of the individual PCB compounds, or congeners, are chiral molecules that have two nonsuperimposable mirror-image forms called enantiomers. Because soil microbes preferentially consume one enantiomer of each pair, the PCBs have characteristic chiral signatures when they volatilize from the soil in warm weather. PCBs emitted into air from other sources bear unaltered racemic chiral signatures that contain equal amounts of each enantiomer. Harrad’s study showed signatures in outdoor air matched the racemic signatures in indoor air, and not the altered soil signatures.
Harrad’s use of these chiral signatures is an innovative way to distinguish between PCB sources, says Bidleman, who has done similar work on chiral signatures of chlorinated pesticides. The paper makes the strongest case for PCB–95, he says, adding that he would like to see more data collected. “The policy implications are very, very large,” he stresses, a comment echoed by others. The findings could “lead to a reevaluation of remediation and cleanup priorities,” adds Martin Scheringer of the Swiss Federal Institute of Technology’s Institute for Chemical and Bioengineering.
Since 2000, researchers in Germany, Sweden, and Switzerland have documented that indoor air can contain significantly higher levels of PCBs than the concentrations recorded inside Birmingham buildings, Harrad points out. “Before the ban of PCB in open systems in 1972, PCBs were used as plasticizers for joint sealants, [which can contain up] to 30% of PCB,” explains Martin Kohler, an analytical chemist for Empa, the Swiss government’s Materials Science and Technology research institution. Kohler was involved in a nationwide study published in 2005 that revealed “clearly elevated PCB indoor air levels” in many Swiss buildings.
Josef Tremp, an environmental scientist with the Swiss Federal Office of the Environment who collaborated with Kohler, adds that the PCBs in such joint sealants are often emitted directly to outdoor air. He nevertheless says: “I fully agree with the authors’ conclusion that further reductions in PCB concentrations in outdoor air—as well as in soil, sediments, and biota and ultimately human exposure—are best achieved by action to remove remaining sources of PCBs in use in the built environment.”
