Comment pubs.acs.org/est
The Carlin Effect
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impacted by effluent from an upstream wastewater treatment plant that received unexpectedly high concentrations of unmonitored chemicals is to rethink our policy of using sewers as a repository for industrial waste. In the United States and many other countries, industrial source control programs are stuck in an era when the primary goal was to avoid shock loadings that could compromise the biological treatment process. Now that we recognize municipal wastewater effluent as a drinking water source and, in some cases, an important component of flows in rivers, commercial and industrial dischargers need to be held to a higher standard. When it is impractical to remove chemicals of concern prior to discharge to the sewer, we need to consider product substitution or closed-loop manufacturing processes. The Carlin Effect can also affect the air we breathe. For example, efforts to control smog-forming gases demonstrated that a relatively small number of vehicles with malfunctioning catalytic converters were responsible for much of the pollution. After recognizing the importance of the outliers, regulators employed on-road emissions sampling and “smoking vehicle” programs to improve air quality. In those cases, thinking only in terms of typical operating conditions failed to detect and address the small number of car owners who tampered with their emissions control devices. After we control the most obvious sources of pollution, we need to turn our attention to the challenge posed by the outliers. Sometimes we can anticipate these pollution sources by considering industrial practices or by analyzing the ways that people might abuse a chemical or a pollution control device. Other times, this will require more comprehensive monitoring programs and use of the latest analytical tools for detection of nontarget compounds. Either way, I am convinced that if we were to dedicate more effort to this phenomenon, we would find other situations in which the Carlin Effect threatens public health and the environment. In the absence of this information, researchers and policymakers should refrain from drawing conclusions about the overall safety of a particular practice on the basis of a small set of samples. A modest effort can provide a good estimate of the median concentration, but if we hope to detect the Carlin Effect, we need to put in the extra work that is required to capture the outliers.
he comedian and social critic George Carlin once said, “Somewhere in the world is the world’s worst doctor. And what’s truly terrifying is that someone has an appointment with him tomorrow morning.” Over the past few years, I have observed a phenomenon relevant to our community that adheres to the spirit of Carlin’s observation: a pollution source that does not pose a significant risk when evaluated on the basis of typical conditions can morph into a serious threat in a small number of cases in which abnormally high concentrations occur. Our scientific training leads us to think of contaminant releases in terms of normal or log-normal distributions. As a result, we tend to ignore or downplay outliers to the distribution. The truly terrifying consequence of this habit is that it inevitably results in some people or ecosystems being exposed to dangerous conditions tomorrow morning (and every morning thereafter). We can observe the Carlin Effect in efforts related to organic contaminants in municipal wastewater. For example, in 2010 USGS researchers reported that the effluent from treatment plants in cities where pharmaceuticals were manufactured contained concentrations of opioids and muscle relaxants that were almost 3 orders of magnitude higher than those observed at treatment plants in cities where consumers and hospitals were the only sources of the chemicals (10.1021/es100356f). Such outliers are not restricted to medicines. When researchers apply a sensitive analytical technique to monitor a large number of samples they always seem to find a few anomalies. The Carlin Effect is particularly worrying when it comes to compounds that pose human health risks at low concentrations. Since researchers started studying the carcinogen NDMA in wastewater about 10 years ago, a handful of cases have been reported in which extremely high concentrations of the compound or its precursors were traced back to an industrial source that accounted for a small fraction of the flow of wastewater coming into the treatment plant. Current plans for greatly expanding the number of potable water reuse systems in California, Texas, and other places struggling with water shortages are being made on the basis of the experiences of a relatively small number of facilities. Because these advanced treatment plants were early adopters of a new technology, their operators employed state-of-the-art analytical techniques to monitor unregulated chemicals. When elevated concentrations of organic contaminants were detected, they identified the responsible industries and controlled the contamination at its source. As the next generation of potable water reuse projects starts operating, they will monitor regulated contaminants and a handful of chemicals that proved to be problematic to the early adopters. But this approach will not detect anomalously high concentrations of a chemical that was not present in the community where the early adopters were located. The Carlin Effect is not restricted to potable water reuse. Plenty of cities deliver drinking water that was sourced from wastewater-impacted rivers (10.1021/es402792s). Therefore, the best way to ensure that no one will drink water that is © 2015 American Chemical Society
David Sedlak, Editor-in-Chief
Published: August 28, 2015 10757
DOI: 10.1021/acs.est.5b04133 Environ. Sci. Technol. 2015, 49, 10757−10758
Environmental Science & Technology
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Comment
AUTHOR INFORMATION
Notes
Views expressed in this editorial are those of the author and not necessarily the views of the ACS. The authors declare no competing financial interest.
10758
DOI: 10.1021/acs.est.5b04133 Environ. Sci. Technol. 2015, 49, 10757−10758
