Philip H. Howard

Handbook of environmental degradation rates

Some of the processes this book includes are aerobic and anaerobic biodegradation, direct photolysis, hydrolysis, and reaction with various oxidants or free radicals (e.g., hydroxyl radical and ozone in the atmosphere). Experimental data are used and cited when available, and validated estimation methods are used when no experimental data are available.

Computer estimation of the Atmospheric gas-phase reaction rate of organic compounds with hydroxyl radicals and ozone

Abstract The Atmospheric Oxidation Program (AOP) is a computer program that estimates the rate constant for the atmospheric, gas-phase reaction between photochemically produced hydroxyl radicals (OH) and organic chemicals. It also estimates the rate constant for the gas-phase reaction between ozone and olefinic/acetylenic compounds. AOP, which uses estimation methods developed by Atkinson and co-workers, estimates more accurate rate constants than the PCFAP (Fate of Atmospheric Pollutants) program that was part of the U.S. EPAs Graphical Exposure Modeling System (GEMS). Due to its superior predictive ability, the EPA is currently using AOP to evaluate the atmospheric fate of compounds defined under Sections 4, 5 and 6 of the Toxic Substances Control Act (TSCA).

Identifying New Persistent and Bioaccumulative Organics Among Chemicals in Commerce II: Pharmaceuticals

The goal of this study was to identify commercial pharmaceuticals that might be persistent and bioaccumulative (P&B) and that were not being considered in current wastewater and aquatic environmental measurement programs. We developed a database of 3193 pharmaceuticals from two U.S. Food and Drug Administration (FDA) databases and some lists of top ranked or selling drugs. Of the 3193 pharmaceuticals, 275 pharmaceuticals have been found in the environment and 399 pharmaceuticals were, based upon production volumes, designated as high production volume (HPV) pharmaceuticals. All pharmaceuticals that had reported chemical structures were evaluated for potential bioaccumulation (B) or persistence (P) using quantitative structure property relationships (QSPR) or scientific judgment. Of the 275 drugs detected in the environment, 92 were rated as potentially bioaccumulative, 121 were rated as potentially persistent, and 99 were HPV pharmaceuticals. After removing the 275 pharmaceuticals previously detected in the environment, 58 HPV compounds were identified that were both P&B and 48 were identified as P only. Of the non-HPV compounds, 364 pharmaceuticals were identified that were P&B. This study has yielded some interesting and probable P&B pharmaceuticals that should be considered for further study.

Estimating log P with atom/fragments and water solubility with log P

An atom/fragment contribution method that predicts log P is described. Coefficient values for 150 atom/fragments and 250 correction factors have been derived from a training set of 2473 compounds. When applied to an independent validation set of 10589 compounds, the method estimates log P with excellent accuracy (correlation coefficient r 2 of 0.943, standard deviation of 0.473, and absolute mean error of 0.354). A method that predicts water solubility from log P is also described.

Development of a general kinetic model for biodegradation and its application to chlorophenols and related compounds.

Mesure des cinetiques de biodegradation de divers composes: phenol, resorcinol, p-cresol, acide benzoique et derives chlore du phenol, anisole et du resorcinol

Workgroup report: review of fish bioaccumulation databases used to identify persistent, bioaccumulative, toxic substances.

Chemical management programs strive to protect human health and the environment by accurately identifying persistent, bioaccumulative, toxic substances and restricting their use in commerce. The advance of these programs is challenged by the reality that few empirical data are available for the tens of thousands of commercial substances that require evaluation. Therefore, most preliminary assessments rely on model predictions and data extrapolation. In November 2005, a workshop was held for experts from governments, industry, and academia to examine the availability and quality of in vivo fish bioconcentration and bioaccumulation data, and to propose steps to improve its prediction. The workshop focused on fish data because regulatory assessments predominantly focus on the bioconcentration of substances from water into fish, as measured using in vivo tests or predicted using computer models. In this article we review of the quantity, features, and public availability of bioconcentration, bioaccumulation, and biota–sediment accumulation data. The workshop revealed that there is significant overlap in the data contained within the various fish bioaccumulation data sources reviewed, and further, that no database contained all of the available fish bioaccumulation data. We believe that a majority of the available bioaccumulation data have been used in the development and testing of quantitative structure–activity relationships and computer models currently in use. Workshop recommendations included the publication of guidance on bioconcentration study quality, the combination of data from various sources to permit better access for modelers and assessors, and the review of chemical domains of existing models to identify areas for expansion.

Recent developments in broadly applicable structure-biodegradability relationships.

Biodegradation is one of the most important processes influencing concentration of a chemical substance after its release to the environment. It is the main process for removal of many chemicals from the environment and therefore is an important factor in risk assessments. This article reviews available methods and models for predicting biodegradability of organic chemicals from structure. The first section of the article briefly discusses current needs for biodegradability estimation methods related to new and existing chemicals and in the context of multimedia exposure models. Following sections include biodegradation test methods and endpoints used in modeling, with special attention given to the Japanese Ministry of International Trade and Industry test; a primer on modeling, describing the various approaches that have been used in the structure/biodegradability relationship work, and contrasting statistical and mechanistic approaches; and recent developments in structure/biodegradability relationships, divided into group contribution, chemometric, and artificial intelligence approaches.

Improved estimation of solubility and partitioning through correction of UNIFAC-derived activity coefficients

Octanol-water partition coefficients (K/sub ow/) of 75 compounds ranging over 9 orders of magnitude are correlated by log K/sub ow/ = -0.40 + 0.73 log (..gamma../sub W/)/sub U/ -0.39 log (..gamma../sub 0/)/sub U/ (r = 0.98), where (..gamma..//sub W/)/sub U/ and (..gamma../sub 0/)/sub U/ are UNIFAC-derived activity coefficients in water and octanol, respectively. The constants 0.73 and -0.39 are obtained empirically and are intended to compensate for group nonadditivity. Correction factors of similar magnitude are obtained in independent correlations of water solubility with (..gamma../sub W/)/sub U/ and of octanol solubility with (..gamma../sub 0/)/sub U/, thereby confirming the validity of the approach.

A new biodegradation prediction model specific to petroleum hydrocarbons

A new predictive model for determining quantitative primary biodegradation half-lives of individual petroleum hydrocarbons has been developed. This model uses a fragment-based approach similar to that of several other biodegradation models, such as those within the Biodegradation Probability Program (BIOWIN) estimation program. In the present study, a half-life in days is estimated using multiple linear regression against counts of 31 distinct molecular fragments. The model was developed using a data set consisting of 175 compounds with environmentally relevant experimental data that was divided into training and validation sets. The original fragments from the Ministry of International Trade and Industry BIOWIN model were used initially as structural descriptors and additional fragments were then added to better describe the ring systems found in petroleum hydrocarbons and to adjust for nonlinearity within the experimental data. The training and validation sets had r2 values of 0.91 and 0.81, respectively.

Identifying New Persistent and Bioaccumulative Organics Among Chemicals in Commerce. III: Byproducts, Impurities, and Transformation Products

The goal of this series of studies was to identify commercial chemicals that might be persistent and bioaccumulative (PB) and that were not being considered in current wastewater and aquatic environmental measurement programs. In this study, we focus on chemicals that are not on commercial chemical lists such as U.S. EPAs Inventory Update Rule but may be found as byproducts or impurities in commercial chemicals or are likely transformation products from commercial chemical use. We evaluated the 610 chemicals from our earlier publication as well as high production volume chemicals and identified 320 chemicals (39 byproducts and impurities, and 281 transformation products) that could be potential PB chemicals. Four examples are discussed in detail; these chemicals had a fair amount of information on the commercial synthesis and byproducts and impurities that might be found in the commercial product. Unfortunately for many of the 610 chemicals, as well as the transformation products, little or no information was available. Use of computer-aided software to predict the transformation pathways in combination with the biodegradation rules of thumb and some basic organic chemistry has allowed 281 potential PB transformation products to be suggested for some of the 610 commercial chemicals; more PB transformation products were not selected since microbial degradation often results in less persistent and less bioaccumulative metabolites.