Evaluation of the Webler-Brown model for estimating tetrachloroethylene exposure from vinyl-lined asbestos-cement pipes

Our plain-English summary of Evaluation of the Webler-Brown model for estimating tetrachloroethylene exposure from vinyl-lined asbestos-cement pipes, by L.A. Spence, A. Aschengrau, L.E. Gallagher, et al (Environmental Health, 2008).

The study

• Authors: L.A. Spence, A. Aschengrau, L.E. Gallagher, T.F. Webster, T.C. Heeren, D.M. Ozonoff
• Journal: Environmental Health
• Published: 2008;7:24
• Cited by: 14 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Background: From May 1968 through March 1980, vinyl-lined asbestos-cement (VL/AC) water distribution pipes were installed in New England to avoid taste and odor problems associated with asbestos-cement pipes. The vinyl resin was applied to the inner pipe surface in a solution of tetrachloroethylene (perchloroethylene, PCE). Substantial amounts of PCE remained in the liner and subsequently leached into public drinking water supplies.

Methods: Once aware of the leaching problem and prior to remediation (April-November 1980), Massachusetts regulators collected drinking water samples from VL/AC pipes to determine the extent and severity of the PCE contamination. This study compares newly obtained historical records of PCE concentrations in water samples (n = 88) with concentrations estimated using an exposure model employed in epidemiologic studies on the cancer risk associated with PCE-contaminated drinking water. The exposure model was developed by Webler and Brown to estimate the mass of PCE delivered to subjects' residences.

Results: The mean and median measured PCE concentrations in the water samples were 66 and 0.5 microg/L, respectively, and the range extended from non-detectable to 2432 microg/L. The model-generated concentration estimates and water sample concentrations were moderately correlated (Spearman rank correlation coefficient = 0.48, p < 0.0001). Correlations were higher in samples taken at taps and spigots vs. hydrants (rho = 0.84 vs. 0.34), in areas with simple vs. complex geometry (rho = 0.51 vs. 0.38), and near pipes installed in 1973-1976 vs. other years (rho = 0.56 vs. 0.42 for 1968-1972 and 0.37 for 1977-1980). Overall, 24% of the variance in measured PCE concentrations was explained by the model-generated concentration estimates (p < 0.0001). Almost half of the water samples had undetectable concentrations of PCE. Undetectable levels were more common in areas with the earliest installed VL/AC pipes, at the beginning and middle of VL/AC pipes, at hydrants, and in complex pipe configurations.

Conclusion: PCE concentration estimates generated using the Webler-Brown model were moderately correlated with measured water concentrations. The present analysis suggests that the exposure assessment process used in prior epidemiological studies could be improved with more accurate characterization of water flow. This study illustrates one method of validating an exposure model in an epidemiological study when historical measurements are not available.

In plain English

This study tested how well an exposure model developed by Webler and Brown estimated levels of the solvent tetrachloroethylene (PCE) in drinking water. The authors had used the model in their earlier Cape Cod cancer case-control studies. PCE leached into water from the vinyl lining of vinyl-lined asbestos-cement pipes. The exposure studied was PCE, not asbestos. The authors compared model estimates with 88 drinking water samples that Massachusetts regulators collected in 1980 in nine towns. Measured PCE ranged from undetectable to 2432 μg/L, with a mean of 66 and a median of 0.5, and 49% of samples had no detectable PCE. Measured and estimated levels were moderately correlated (Spearman correlation 0.48), and the model explained 24% of the variance. The correlation was higher for samples from taps and spigots than from hydrants (0.84 vs. 0.34), and higher in simple than in complex pipe layouts (0.51 vs. 0.38). The authors concluded that the estimates were moderately concordant with the measurements. They noted limits: single spot samples, a lab method that could underestimate PCE by as much as 80%, and simplified assumptions about water flow.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Exposure-model validation study comparing historical measured water concentrations with model-generated estimates
• Who was studied: 88 eligible drinking water samples collected by the Massachusetts Department of Environmental Protection in 1980, before remediation, at locations with vinyl-lined asbestos-cement pipe in nine Massachusetts towns (Barnstable, Bourne, Brewster, Chatham, Falmouth, Provincetown, Sandwich, Plymouth, Wareham). 112 samples were initially identified and 24 were excluded.
• Exposure: Tetrachloroethylene (PCE) leaching into drinking water from the vinyl liner (Piccotex resin applied in a PCE solution) of vinyl-lined asbestos-cement water pipes installed May 1968 to March 1980
• Main results:
  • Across all 88 samples, the mean measured PCE concentration was 66 μg/L and the median was 0.5 μg/L
  • Individual measured concentrations ranged from undetectable to 2432 μg/L
  • 49% of samples had undetectable PCE (detection limit 0.5 μg/L)
  • Measured concentrations and model point-concentration estimates were moderately correlated across all samples (Spearman 0.48, p less than 0.0001)
  • Spearman correlation for all samples: 0.48, p less than 0.0001
  • Model estimates explained 24% of the variance in measured PCE concentrations
  • Correlation was higher for samples taken at taps and spigots (n = 10) than at hydrants (n = 18): 0.84 vs. 0.34
  • Correlation was higher in areas with simple than complex pipe geometry: 0.51 vs. 0.38
  • In a sensitivity analysis adjusting for measurement error, the mean Spearman correlation was 0.44 (range 0.29 to 0.53)
  • Among samples with detectable PCE only, the correlation fell to 0.41 (p = 0.005)
  • In multiple logistic regression, undetectable PCE was more common near the earliest installed pipes (adjusted OR 8.4), where flow was high (adjusted OR 6.3), and at the beginning and middle of the pipes (adjusted OR 2.1)
  • Model and measured concentrations fell into the same percentile category in 54.5% of samples
  • For classifying the upper 50th and upper 75th percentiles of measured concentrations, the model's sensitivities were 63% and 59%, and its specificities were 62% and 86%
• The authors' conclusion: The authors concluded that the Webler-Brown model generated exposure estimates moderately concordant with historically measured PCE data, and that the analysis supports the model used in their previous epidemiologic studies
• Limitations the authors state:
  • The historical measurements were themselves error-prone, an 'alloyed gold standard'
  • Each location had a single spot sample, and short-term fluctuations in PCE were not captured
  • According to a DEP analysis, the head space laboratory method underestimated PCE concentrations by as much as 80%
  • The type of water fixture sampled was missing for many samples (60 of 88)
  • Simplifying assumptions about the rate and direction of water flow were needed, especially affecting areas with complex pipe configurations
  • Parcel maps dated from 1988 or later, 8 to 20 years after pipe installation, which could misstate housing density and water flow
  • The model assumes a uniform amount of PCE in the liner, even liner thickness and constant water temperature, which did not hold in reality

Topics

Medical terms in this study, explained in our Lexicon: Asbestos-Cement Pipe, Asbestos-Cement.

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