Good Modeling Practice Guidelines for Applying Multimedia Models in Chemical Assessments
Multimedia mass balance models of chemical fate in the environment have been used for over 3 decades in a regulatory context to assist decision making. As these models become more comprehensive, reliable, and accepted, there is a need to recognize and adopt principles of Good Modeling Practice (GMP) to ensure that multimedia models are applied with transparency and adherence to accepted scientiﬁc principles. We propose and discuss 6 principles of GMP for applying existing multimedia models in a decision-making context, namely 1) speciﬁcation of the goals of the model assessment, 2) speciﬁcation of the model used, 3) speciﬁcation of the input data, 4) speciﬁcation of the output data, 5) conduct of a sensitivity and possibly also uncertainty analysis, and ﬁnally 6) speciﬁcation of the limitations and limits of applicability of the analysis. These principles are justiﬁed and discussed with a view to enhancing the transparency and quality of model-based assessments.
Bioaccumulation of Organic Contaminants in Humans: A Multimedia Perspective and the Importance of Biotransformation
Bioaccumulation is an important component of the exposure hazard assessment and risk assessment of organic chemicals. Screening criteria for chemical hazard used in national and international regulations are based on the paradigm that partitioning properties are the primary chemical determinants of bioaccumulation. We use a holistic multimedia perspective to evaluate the partitioning property paradigm with respect to assessing human bioaccumulation. Multimedia bioaccumulation factors (mmBAFs) for humans were modeled for hypothetical chemicals with a wide range of physical-chemical properties. Varying partitioning properties over 12 orders of magnitude (a plausible range for nonionizing organics) resulted in only modest changes in mmBAFs (a factor of similar to 10) for all but very volatile or hydrophilic chemicals. In contrast, varying biotransformation rate constants over 6 orders of magnitude resulted in substantial differences in mmBAFs (greater than a factor of 10(9)). Our model results are supported by empirical observations of well characterized pollutants, which demonstrate that chemicals with similar partitioning properties can have very different bioaccumulation behavior. Susceptibility to biotransformation clearly determines bioaccumulation in humans for many chemicals. We conclude that a holistic multimedia perspective for bioaccumulation assessment is necessary to develop regulations, criteria, and policies that are protective of human health and the environment.
Intrinsic Human Elimination Half-Lives of Polychlorinated Biphenyls Derived from the Temporal Evolution of Cross-Sectional Biomonitoring Data from the United Kingdom
BACKGROUND: Most empirical estimates of human elimination kinetics for persistent chemicals reflect apparent elimination half-lives that represent the aggregated effect of intrinsic elimination, ongoing exposure, and changes in body weight. However, estimates of intrinsic elimination at background levels are required for risk assessments for the general population. OBJECTIVE: To estimate intrinsic human elimination half-lives at background levels for nine polychlorinated biphenyl (PCB) congeners, we used a novel approach based on population data. METHODS: We used a population pharmacokinetic model to interpret two sets of congener-specific cross-sectional age-concentration biomonitoring data of PCB concentrations measured in lipid and blood samples that were collected from 229 individuals in 1990 and 2003. Our method is novel because it exploits information about changes in concentration in the human population along two dimensions: age and calendar time. RESULTS: Our approach extracted information about both elimination kinetics and exposure trends from biomonitoring data. The longest intrinsic human elimination half-lives estimated in this study are 15.5 years for PCB-170, 14.4 years for PCB-153, and 11.5 years for PCB-180. CONCLUSIONS: Our results are further evidence that a maximum intrinsic elimination half-life for persistent chemicals such as PCBs exists and is approximately 10-15 years. A clear conceptual distinction between apparent and intrinsic half-lives is required to reduce the uncertainty in elimination half-lives of persistent chemicals. The method presented here estimates intrinsic elimination half-lives and the exposure trends of persistent pollutants using cross-sectional data available from a large and growing number of biomonitoring programs.
Using COSMOtherm to predict physicochemical properties of poly- and perfluorinated alkyl substances (PFASs).
Recently, there has been concern about the presence of poly- and perfluorinated alkyl substances (PFASs) in the environment, biota and humans. However, lack of physicochemical data has limited the application of environmental fate models to understand the environmental distribution and ultimate fate of PFASs. We employ the COSMOtherm model to estimate physicochemical properties for 130 individual PFASs, namely perfluoroalkyl acids (including branched isomers for C4–C8 perfluorocarboxylic acids), their precursors and some important intermediates. The estimated physicochemical properties are interpreted using structure-property relationships and rationalised with insight into molecular interactions. Within a homologous series of linear PFASs with the same functional group, both air–water and octanol–water partition coefficient increase with increasing perfluorinated chain length, likely due to increasing molecular volume. For PFASs with the same perfluorinated chain length but different functional groups, the ability of the functional group to form hydrogen bonds strongly influences the chemicals’ partitioning behaviour. The partitioning behaviour of all theoretically possible branched isomers can vary considerably; however, the predominant isopropyl and monomethyl branched isomers in technical mixtures have similar properties as their linear counterparts (differences below 0.5 log units). Our property estimates provide a basis for further environmental modelling, but with some caveats and limitations.
Estimating Physicochemical Properties of Poly- and Perfluorinated Alkyl Substances (PFAS) with a Quantum Chemistry-Based Model.
BETR Global – A geographically-explicit global-scale multimedia contaminant fate model
We present two new software implementations of the BETR Global multimedia contaminant fate model. The model uses steady-state or non-steady-state mass-balance calculations to describe the fate and transport of persistent organic pollutants using a desktop computer. The global environment is described using a database of long-term average monthly conditions on a 15 degrees x 15 degrees grid. We demonstrate BETR Global by modeling the global sources, transport, and removal of decamethylcyclopentasiloxane (D5). (C) 2011 Elsevier Ltd. All rights reserved.
Assessment of Nonoccupational Exposure to DDT in the Tropics and the North: Relevance of Uptake via Inhalation from Indoor Residual Spraying
BACKGROUND: People who live in dwellings treated with indoor residual spraying (IRS) of DDT [1,1,1-trichloro-2,2-bis(p-chlorophenyl) ethane] for disease-vector control in the tropics and indigenous populations in the Arctic who comsume marine mammals experience high nonoccupational exposure to DDT. Although the use of DDT in IRS is rising, the resulting nonoccupational exposure is poorly characterized. OBJECTIVES: We have provided a comparative assessment of exposure to DDT and its metabolites in the general population of the tropical and northern regions and in highly exposed populations in these regions. METHODS: We compiled > 600 average or median DDT concentrations from the peer-reviewed literature, representing > 23,000 individual measurements in humans, food, air, soil, and dust. We use Monte Carlo sampling of distributions based on these data to estimate distributions of population-and route-specific uptake. We evaluate our exposure estimates by comparing them with biomonitoring data. RESULTS: DDT concentrations are highest in people living in IRS-treated houses and lowest in the northern general population, differing by a factor of about 60. Inuits and the general population in the tropics have similar concentrations. Inhalation exposure explains most of the difference in concentration between the highly exposed and the general population in the Tropics. Calculated exposure levels are consistent with human biomonitoring data. CONCLUSIONS: Nonoccupational inhalation exposure is a relevant exposure pathway for people living in homes treated by IRS of DDT. Continued monitoring of time trends and DDE to DDT ratios in the Tropics and in the North is needed to identify a possible slowdown in concentration decline and the influence of ongoing DDT use.
Global Distribution of Linear and Cyclic Volatile Methyl Siloxanes in Air
The global distribution of linear and cyclic volatile methyl silxoanes (VMS) was investigated at 20 sites worldwide, including 5 locations in the Arctic, using sorbent-impregnated polyurethane foam (SIP) disk passive air samplers. Cyclic VMS are currently being considered for regulation because they are high production volume chemicals that are potentially persistent, bioaccumulative, and toxic. Linear and cyclic VMS (including L3, L4, L5, D3, D4, DS, and D6) were analyzed for in air at all urban, background, and Arctic sites. Concentrations of D3 and D4 are significantly correlated, as are D5 and D6, which suggests different sources for these two pairs of compounds. Elevated concentrations of D3 and D4 on the West coast of North America and at high elevation sites suggest these sites are influenced by trans-Pacific transport, while D5 and D6 have elevated concentrations in urban areas, which is most likely due to personal care product use. Measured concentrations of 1)5 were compared to modeled concentrations generated using both the Danish Eulerian Hemispheric Model (DEHM) and the Berkeley-Trent Global Contaminant Fate Model (BETR Global). The correlation coefficients (r) between the measured and modeled results were 0.73 and 0.58 for the DEHM and BETR models, respectively. Agreement between measurements and models indicate that the sources, transport pathways, and sinks of D5 in the global atmosphere are fairly well understood.
The State of Multimedia Mass-Balance Modeling in Environmental Science and Decision-Making
Estimation of the Source Strength of Polybrominated Diphenyl Ethers Based on Their Diel Variability in Air in Zurich, Switzerland
Diel (24-h) concentration variations of polybrominated diphenyl ethers (PBDEs) in air were measured in the center of Zurich, Switzerland, and on Uetliberg, a hill about 5 km from the city center. Air samples were collected simultaneously at both sites over 4 h time periods for 3 consecutive days during a stable high pressure system in August 2007. Higher PBDE concentrations in the city compared to the Uetliberg site indicate that Zurich is a likely source of PBDEs to the atmosphere. A multimedia mass balance model was used to (i) explain the diel cycling pattern of PBDE concentrations observed at both sites in terms of dominant processes and (ii) estimate emission rates of PBDEs from the city to the atmosphere. We estimate that Zurich emits 0.4, 6.2, 1.6, and 0.4 kg year(-1) of the PBDE congeners 28, 47, 99, and 100, respectively. On a per-capita basis, these estimates are within the range or somewhat above those obtained in other studies using approaches based on emission factors (EF) and PBDE production, usage, and disposal data, or concentration measurements. The present approach complements emission estimates based on the EF approach and can also be applied to source areas where EFs and PBDE material flows are poorly characterized or unknown, such as electronic waste processing plants.
Assessing the impact of weather events at mid-latitudes on the atmospheric transport of chemical pollutants using a 2-dimensional multimedia meteorological model
We investigate the long-range transport potential (LRTP) of five different classes of hypothetical chemical pollutants (volatile, multimedia, semivolatile, particle-associated and hydrophilic) during a low pressure weather event using a novel 2 (x- and z-axis)-Dimensional Multi-Media Meteorological Model (2D4M). The atmosphere (z-axis) is described by three atmospheric layers, where two layers constitute the boundary layer and the third layer the free troposphere. The 2D4M can describe distinct weather events on a regional scale and calculate the LRTP of chemicals as a function of time during these events. Four weather factors are used to model weather events and their influence on the atmospheric transport of chemicals: (1) temperature, (2) wind speed and mixing dynamics of the troposphere, (3) hydroxyl radical concentrations and (4) precipitation. We have modeled the impact of variability in each of these factors on LRTP of pollutants during a front event associated with a low pressure period that interrupts a dominant high pressure system. The physico-chemical properties of the pollutant determine which specific weather factors contribute most to variability in transport potential during the event. Volatile and multimedia chemicals are mainly affected by changing atmospheric mixing conditions, wind speeds and OH radical concentrations, while semivolatile substances are also affected by temperature. Low-vapor-pressure pollutants that are particle-associated, and water-soluble pollutants are most strongly affected by precipitation. Some chemical pollutants are efficiently transported from the boundary layer into the upper troposphere during the modeled low pressure event and are transported by much higher wind speeds than in the boundary layer. Our model experiments show that the transport potential of volatile, multimedia and semivolatile compounds is significantly increased during a front event as a result of efficient tropospheric mixing and fast wind speeds in the upper troposphere, whereas low-volatility and hydrophilic chemicals are largely scavenged from the atmosphere. In future LRTP assessment of chemical contaminants as required by the Stockholm Convention and the convention on long-range transboundary air pollution, it is therefore advised to prioritize volatile, multimedia and semivolatile chemicals that are identified in initial screening. (C) 2010 Elsevier Ltd. All rights reserved.