Some statistical and conceptual issues in the detection of low-level environmental pollutants

With increasing concern over chemicals that are potential health hazards at low levels, determination of limits of detection have undergone considerable scrutiny. Most traditional detection limit estimators suffer from extensive statistical and/or conceptual limitations. In this paper, traditional detection limit estimators are described and critically evaluated. Using the terminology of Currie (1968), methods are categorized into decision limits versus detection limits. The methods are further categorized into single concentration design versus calibration design methodologies. While the single concentration design methods are useful for fixing ideas and clarifying definitions, they are shown to be extremely limited in practice since dependence of variability on concentration can neither be estimated or incorporated. Calibration-based detection limit estimators are described, compared and contrasted. Generalizations to non-constant variance, multiple future detection decisions and simultaneous control of Type I and II errors are provided. The various calibration-based methods are illustrated using real data and experimental design issues for detection limit studies discussed.     

Do plants’ emissions respond to ambient environmental quality? Evidence from the clean water act

In this paper we show that plants respond to downstream ambient water quality after controlling for permitted levels of pollution. We find if past water quality declines by one percent, plants reduce current pollution by 0.35 percent. The magnitude of this coefficient is comparable to the coefficient on permitted discharge levels i.e. regulatory stringency itself. Results are consistent with two mechanisms. First, a decline in water quality may lead to more stringent permits that would raise the cost of abatement of a plant significantly. Second, the plant is likely to be subject to increased public pressure in response to poor water quality. Indeed, as expected, the impact of water quality becomes stronger in locations with higher median household income, higher percent carpooling to work, or lower percent of manufacturing employment but surprisingly with lower median age of residents, lower percent with bachelor׳s degree or higher percent of families with children.