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Evaluation of two methods of interpolating quarterly trihalomethane levels between sampling dates

Journal of Exposure Science & Environmental Epidemiology volume 19, pages 405–413 (2009)Cite this article

Abstract

Epidemiological studies of the relationship between disinfection byproducts (DBPs) and adverse birth outcomes often use a single quarterly sample result to characterize an exposure period during a pregnancy. Concentrations of trihalomethanes (THMs), a frequently studied class of DBPs, can fluctuate considerably between sampling periods so that a single point measurement in time may not adequately characterize levels over an exposure period. In addition to obtaining compliance samples that are required quarterly at a limited number of sampling sites, the New York City water supply system takes monthly THM samples at a large number of sites. We used these monthly data to investigate two methods for interpolating between quarterly measurements: cubic splines and linear interpolation. We based interpolations on 1 month in each calendar quarter and checked the interpolated values against the actual data values from the other 2 months of the quarter. Both methods produced generally acceptable fits: 90% of the observed discrepancies between fitted and actual values were less than 14 μg/l and over half were less than 5 μg/l for total trihalomethanes (THM4). These numbers are about half the change in THM4 level from one quarter to the next and compare favorably with the range of 0–80 μg/l or more used in many studies. Comparable results were obtained for chloroform and bromodichloromethane, the two THMs with levels high enough to analyze. Linear interpolation generally performed better than the spline fit, producing lower average discrepancies and fewer large discrepancies. Linear interpolation can be a useful and easily applied method for developing improved exposure estimates using quarterly sampling data.

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Acknowledgements

We thank Allison F. Hinckley and Gwen Babcock for SAS programming and Roger Sokol for assisting with data interchange. Richter was supported by Grant U50/CCU223284-01 from the United States Centers for Disease Control and Prevention under their Environmental and Health Effect Tracking initiative. We also thank two anonymous reviewers for their helpful comments.

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Authors and Affiliations

  1. New York State Department of Health, Troy, New York, USA

    Wayne Richter & Thomas F Hart Jr

  2. National Center for Environmental Assessment, Office of Research and Development, U.S. Environmental Protection Agency, Research Triangle Park, North Carolina, USA

    Thomas Luben

  3. New York City Department of Environmental Protection, 59-17 Junction Boulevard, Flushing, New York, USA,

    Salome Freud

  4. Department of Environmental and Radiological Health Sciences, Colorado State University, Fort Collins, Colorado, USA

    John R Nuckols

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  1. Wayne Richter
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  2. Thomas F Hart Jr
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  3. Thomas Luben
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  4. Salome Freud
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  5. John R Nuckols
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Correspondence to Wayne Richter.

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Although the research described in this article has been reviewed by the U.S. EPA, it does not necessarily reflect the views of the agency, and no official endorsement should be inferred. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.

Supplementary Information accompanies the paper on the Journal of Exposure Science and Environmental Epidemiology website (http://www.nature.com/jes)

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Richter, W., Hart, T., Luben, T. et al. Evaluation of two methods of interpolating quarterly trihalomethane levels between sampling dates. J Expo Sci Environ Epidemiol 19, 405–413 (2009). https://doi.org/10.1038/jes.2008.30

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