Indoor air escaping from homes and buildings is a significant source of the polychlorinated biphenyls (PCBs) found in outside air, a new study reports. Its findings support a growing body of evidence indicating that indoor air contributes more to outdoor PCB pollution than other known sources, such as soil.

Since inside air concentrations of PCBs are about 30 times higher than outside air levels, this overlooked source creates a special problem in urban areas where older, PCB-emitting structures are clustered. Removing the PCBs from existing buildings is the best way to lower the pollutants’ levels, both inside and out, and reduce human exposure, say the authors.

Indoor cleanup of PCB contamination at University of Minnesota

Context

Polychlorinated biphenyls-- PCBs -- are a family of chlorinated compounds -- 209 in total. All 209 varieties have chlorine molecules, from one to ten, attached to biphenyl, a molecule that consists of two benzene rings that resembe eyeglasses (below).

PCBs persist in the environment, bioaccumulate in fat, cause disease and have numerous adverse environmental effects. While most production of PCBs ended in the 1970s (Russia is an exception) they are still found everywhere in the environment because of their persistence.

In the US alone, more than 1.5 billion pounds of the chemicals were produced for use in electrical equipment, hydraulics, paints, plastics and many other industrial and commercial uses. PCBs entered the environment during manufacture, use and disposal, with the majority coming from accidental spills and leaks during transport; illegal or improper disposal of industrial wastes and consumer products; burning of some wastes in incinerators; and leaks or fires in products containing PCBs, including old electrical transformers and inadequate hazardous waste sites.

Caulking and sealing materials in buildings built from the1950s to the 1970s are the primary sources of PCBs in older structures. These materials are deteriorating, posing higher risks for those exposed to the air born pollutants on a regular basis. Health risks include cancer, brain and nervous system problems stemming from prenatal and childhood exposures and reproductive and fertility effects. Newer buildings, which do not have PCBs, have lower associated risks.

Several studies suggest buildings are a continual source of these pollutants. Although total exposure to PCBs has declined in the United Kingdom, indoor air concentrations remained significantly higher than outdoor air levels and were almost constant during the last decade. Another extensive study done in Switzerland reported similar results, finding indoor air contains much higher concentrations of PCBs than outdoor air.

Other studies comparing levels in urban and rural areas of Canada found that concentrations of PCBs were considerably higher closest to the urban center and faded away on either sides of the central city area. This phenomenon, called ‘urban pulse,’ has been observed with other dioxin-like contaminants, such as the fire retardants polybrominated diphenylethers (PDBEs). Widespread indoor generation of these contaminants in, and their subsequent ventilation from, older buildings may explain such a pulse.

What did they do? Air and soil were sampled in 10 locations during 1 year in a 79 kilometer urban-rural transect line across West Midlands (near and around Birmingham, United Kingdom). This long stretch was chosen to study the urban pulse phenomenon. Air and soil were collected at each location (map below) and analyzed for nine congeners and for total PCBs.

Transect in UK along which samples were taken. Site 10 was not followed for the whole study.

Their analysis depended upon the fact that some PCBs (e.g. congeners 95, 136, and 149) exist in two distinct chemical forms known as enantiomers, left-handed and right-handed versions of the same molecule. Chemists shine a special kind of polarized light through mixtures to find out how much of each form is present. If a solution is a mixture with roughly even amounts of both form, the light will scatter, no longer polarized. If it is dominated by either the left- or right-handed version, it will remain polarized. Chemists describe the first condition, when polarization is preserved, as displaying chirality. When instead it is scattered, it's called a racemic mixture.

The research team took advantage of the fact that some soil microbes specifically degrade one enantiomer of PCBs leaving the other unaffected (a process called enantioselective degradation), thus producing a chiral signature. On the other hand, the PCBs remain as a racemic mixture in air samples, where no such microbes are present to perform enantioselective degradation. Using the chiral signature, the authors can identify whether PCBs detected in an outdoor air sample actually originated from volatilization from soil or from ventilation of indoor air.

What did they find? Concentrations of PCBs, both in outdoor air and in soil, were highest in the Birmingham city center (site 7; 588 picograms per cubic meter (pg/m3) in air and 13,300 picogram/gram (pg/g) dry weight of soil), decreased with distance from the city and were lowest in the rural areas (site 1; 66 pg/m3 in air and 574 pg/g dry weight of soil). The scientists describe this pattern of high PCB levels in the city center as an "urban pulse."

Total PCBs in air measured at different sampling stations along the UK transect. Numbers correspond to sites identified in map, above. Highest levels were observed within urban Birmingham. Site 10 was dropped from the study.

Specific chiral PCBs (congeners #95, 136 and 149) were racemic or near-racemic in both indoor and outdoor air samples. In contrast, the same congeners of PCBs in soil displayed a chiral signature. From these data, the authors concluded that the "similarity between chiral signatures in indoor and outdoor air, coupled with the disparity between those in outdoor air and soil for many cases, suggests strongly that ventilation of indoor air is a far more significant source of PCBs than volatilization from soil."

This result is summarized in the diagram below: racemic mixtures of PCBs emerge from indoor air in buildings, whereas chiral PCBs evaporate from soils. Measurements of PCBs in outdoor air show that the racemic mixtures dominate.

What does it mean? The findings strongly indicate that older buildings are the major source of PCBs in outdoor air, contributing more than PCBs released from soil, at least in urban areas that resemble Birmingham in patterns of deployment of PCBs in the built environment.

The authors conclude that "future reductions in PCB concentrations in outdoor air and ultimately human exposure appear best achieved by action to remove remaining sources of PCBs from existing structures."