Exposure assessments temporality

The first step in a wildlife exposure assessment is to document the occurrence and persistence of a pesticide in the study area throughout the study duration. Several articles in this book describe the experimental designs and best practices to conduct field crop and environmental dissipation (air, soil and water) studies. This article presents methods to quantify spatial and temporal distributions of pesticide presence in ecosystems following normal application and resultant exposure of nontarget wildlife. 

Perhaps most easy to overlook are spatial and temporal dependencies. For example, the hydrologic component of the pesticide root zone model-exposnre analysis modeling system (PRZM-EX AMS) treats mnltiple field plots over whole watersheds as independent, nnconpled, simple, 1-dimensional flow systems. In reality, the field plots are coupled systems that exhibit complex 3-dimensional water flow and pesticide transport (US SAP 1999). These higher order processes introduce spatial dependencies that may need to be considered in the assessment. Temporal autocorrelations are also likely when assessing exposure. 

The scope and purpose of the exposure assessment inform the formulation of one or more scenarios for which exposures are to be estimated. The exposure estimation approach should be capable of faithfully representing the key structural assumptions of the scenario, such as exposed population, agents to be considered, spatial and temporal scope, microenvironments and so on (see section 3.2.1 for a complete list). If the modelling approach omits any of the relevant elements, then the estimates could be biased. If the modelling approach includes irrelevant, unnecessary or superfluous elements, then, at best, the model is likely to be more cumbersome to deal with than it needs to be, or, at worst, biases may be introduced. 

The data needed for this exposure assessment include surface water concentrations based on limited monitoring data from fresh and ocean water, laboratory-scale BCF experiments and activity patterns (variation in long-term average fish consumption). We also have biomonitoring data derived from limited geographical, temporal and population subgroup coverage. 

An adequate assessment of exposure to mixtures may require development of improved tools for measurement or detection of chemicals, but also for assessing temporal aspects of exposure. 

Hope BK. 2005. Performing spatially and temporally explicit ecological exposure assessments involving multiple stressors. Human Ecol Risk Assess 11 539-565. 

Mukeijee, S., W.D. Ellenson, R.G. Lewis, R.K. Stevens, M.C. Somerville, D.S. Shadwick and R.D. Willis (1997). Soil characterizations conducted in the lower Rio Grande Valley of Texas, m. Residential microenvironmental measurements with applications for regional and temporal-based exposure assessments. Environ. Int., 23, 657-673. 

Residential risk assessment to pesticides typically involves more than one sonrce and mnitiple pathways and rontes, e.g. a given active ingredient may be nsed for multiple indoor apphcations, and in some cases, for outdoor applications. In some cases, the applications may overlap with respect to timing (calendar days). The potential co-occurrence of applications and potential exposures requires temporal product use information. Such information is rarely available. 

Longitudinal exposure assessment methods and measurements have emerged to address temporal and spatial aspects of aggregate exposures. Longitudinal... 

Important to the concept of exposure assessment is the profile of temporal exposure that a person experiences. Key elements of a temporal exposure profile include ... 

Exposure has been defined as the concentration of toxic materials in space and time at the interface with target populations (Travis et aL, 1983). The respective parameters required for environmental hazard assessments relate to the spatial and temporal abundance of the contaminants in the various compartments of the ecosystem and hence their bioavailability. Reference environmental descriptions must include the biota exposed to the released chemicals and the hydrological, topographical, geological and meteorological characteristics of the environment that affect the transport and transformation of the contaminants. The key step in exposure assessment is the use of transport and transformation models to quantify the movement of contaminants from the source through the environment to the target populations. 

An adequate characterization of the dose/exposure portion of reported dose—responses for lead requires discussion of the merits and limits of those exposure biomarkers which have traditionally been used and others which are relatively new and gaining favor. These two broad groups of exposure assessment instruments are further stratified in terms of temporal and toxicokinetic relationships for lead s associated toxic responses and in terms of accessibility and acceptability for routine use in the research and medical communities. 

Antignac J-P, Cariou R, Maume D et al (2008) Exposure assessment of fetus and newborn to brominated flame retardants in France preliminary data. Mol Nutr Food Res 52 258-265 Asante KA, Adu-Kumi S, Nakahiro K et al (2011) Human exposure to PCBs, PBDEs and HBCDs in Ghana Temporal variation, sources of exposure and estimation of daily intakes by infants. Environ Int 37(5) 921-928... 

Temporal Relationships of Adverse Events. The temporal relationship between duration of product exposure and development of an adverse event is important in assessing causality. But how can data on temporal relationships be systematically summarized in a database containing thousands or even hundreds of thousands of subjects Temporal relationships cannot be clearly elicited if only frequencies of adverse events between treatment and control groups are compared. There can be many disparities in the subjects time of exposure or time at risk. Toxic manifestations of drugs may not occur until several months or even years after the initial exposure to the drug. How do we systematically assess delayed toxicity of a previously prescribed drug from the effect of a newly prescribed drug Such a scenario occurred with reported cases of pancreatitis associated with valproic acid therapy, in which some cases appeared several years after therapy [2]. 

Discern spatial differences and temporal changes in mercuty exposures (i.e., an assessment of concentrations in wildlife, as well as, potentially, effects) in wide-ranging individual species. 

As probabilistic exposure and risk assessment methods are developed and become more frequently used for environmental fate and effects assessment, OPP increasingly needs distributions of environmental fate values rather than single point estimates, and quantitation of error and uncertainty in measurements. Probabilistic models currently being developed by the OPP require distributions of environmental fate and effects parameters either by measurement, extrapolation or a combination of the two. The models predictions will allow regulators to base decisions on the likelihood and magnitude of exposure and effects for a range of conditions which vary both spatially and temporally, rather than in a specific environment under static conditions. This increased need for basic data on environmental fate may increase data collection and drive development of less costly and more precise analytical methods. 

This form of risk assessment is based on the concept of defining an exposure level, the derived standard, expressed usually on a temporal basis (e.g., daily, weekly), which is considered to offer sufficient reassurance of protection of human health, and then comparing this with an estimated level of exposure. If the estimated exposure is higher than the standard, then further regulatory intervention may be needed. Please see Section 5.12 for a discussion of the health implications of exceeding the tolerable intake. 

It is essential to define the assessment scenario within which the assessment endpoint will be assessed. The assessment scenario should specify the spatial, temporal, and ecological boundaries within which the endpoint is assessed, since these have substantial implications for the structure of the assessment model and the scope of the input data. The assessment scenario should also describe those aspects of the ecosystem that are relevant to the assessment, that is, those aspects that have an influence on the mechanisms of exposure and effects that will be assessed. This step is important in all ecological risk assessments it places the assessment activity into the real context of an ecosystem, helps to prevent construction of inappropriate models, and helps with interpretation and communication of results. 

The units of analysis should be determined by the needs of the assessment, not by the data that happen to be available. Careful consideration is required to identify which biological, spatial, and temporal units are appropriate for each assessment. This will depend on the nature and degree of spatial and temporal variation in the many factors that affect exposure and effects, including the following ... 

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