According to a new study, exposure to toxins from hazardous waste sites may be a significant risk factor for developing primary biliary cirrhosis (PBC). Published in the March 2006 issue of Hepatology, researchers found significant clusters of the disease near Superfund toxic waste sites (SFS) and that the majority of patients in New York City who need liver transplants because of PBC, reside near SFS.
PBC is an uncommon liver disease of unknown cause, though it reportedly appears in geographic clusters. Researchers, led by Aftab Ala, M.D. of Mount Sinai School of Medicine in New York City, examined the prevalence and potential clustering of PBC near New York City's most toxic waste sites, which are state-designated SFS.
The researchers collected zip code data on 172 patients in New York City in need of orthotopic liver transplant (OLT) between 1995 and 2003 due to either PBC or primary sclerosing cholangitis (PSC). They compared the patients' zip codes to zip codes of known SFS in the city. Of the city's 174 zip codes, 89 included or bordered SFS and 85 did not.
The researchers then calculated the expected prevalence of PBC-OLT and PSC-OLT, adjusting for demographic characteristics. They compared the mean standardized prevalence ratios in zip codes containing or adjacent to a SFS to the remaining zip codes. Lastly, they utilized SaTScan, a statistical software package, to detect specific clusters.
"The prevalence ratio of PBC-OLT, not PSC-OLT, was significantly higher in zip codes containing or adjacent to SFS," the authors report. The standardized prevalence ratio of PBC-OLT cases near SFS was statistically significantly higher than in zip codes that did not have, nor border with, a toxic site. Meanwhile, the standardized prevalence ratio of PSC-OLT cases near SFS was not statistically different from zip codes without a toxic site nearby. The density of SFS and the prevalence ratio of PBC-OLT were both highest in the borough of Staten Island, while Manhattan had the lowest values for both.
The cluster-detection software uncovered five clusters of PBC-OLT. One was on Staten Island, two were in Brooklyn, and two in Queens. SFS near these clusters were contaminated by volatile organic compounds, like benzene and toluene, and chlorinated hydrocarbons, like tri- and tetra-chloroethylene. The cluster on Staten Island was statistically significant, and located near a SFS contaminated with volatile aromatic hydrocarbons and trichloroethylene. "This is the first epidemiological study to show a statistically significant clustering of PBC patients near known sources of environmental toxins and one of relatively few studies linking proximity to SFS to a specific disease," the authors write. Though the exact mechanism by which proximity to toxins may increase PBC patient exposure is not clear, the authors suggest inhalation is the likely method. Still, confounding factors may play a role, such as socioeconomics that influence where people live.
"Our observations further support the hypothesis that environmental toxin exposure is a risk factor for PBC," the authors conclude. They urge researchers in other locations to further investigate the association.
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