Mathematical mechanistic models Empirical models Some existing models

Public Health Significance of Urban Pests 513 rical models, they are likely to account for the many variables that influence exposure. Currently, no empirical models exist for predicting consumer exposures, since the avai- lable databases on exposure measurements are not sufficiently large. Pesticides: risks and hazards 512 in use, including propel- lant, held within the can. T he use of exposure models requires the selec- tion of various input para- meters. Insufficiently detailed information on exposure scenarios or lack of sufficient data may require the use of default values. Input data or default values used for the calculations must be clearly documented. Computer programs have been developed to imple- ment mathematical pre- dictive models and empiri- cal models. Statistical models have been develo- ped using available data and appropriate statistical methods. Model choice should be jus- tified by showing that the model uses the appropriate exposure scenario – for example, as judged from the underlying assumptions of the model. Expert judgement may be required to check the realism of the exposure value derived from a model, particularly if default or so-called reasonable worst-case values have been used. Modelling exposure can be performed either by taking discrete values point estimates or distributions for the model variables probabilistic modelling. Generally, exposure models fall into one of three types: mathematical mechanistic models, empirical or knowledge-based models, and statistical mathematical models. T hese models predict exposure levels from a mechanistic description of a process, an empirical database or statistical relationships.

14.7.2.1. Mathematical mechanistic models

Normally, mathematical models are based on mass balance equations. These models can incorporate the physical and chemical properties of the substance, together with patterns of use. They are used to characterize the rate of release of the product into a space and its subsequent behaviour. Mathematical models should cover all relevant processes or tasks that contribute to exposure in a scenario. For many tasks, a number of different models may be appropriate.

14.7.2.2. Empirical models

Empirical models are probably best described as models based on exposure measurements obtained from real situations. This type of model can be used to predict the likely expo- sures in other comparable situations. If sufficient and high-quality data are used in empi- Fig. 14.7. Tiered approach to estimation of oral exposure Source: Modified from EC 2002. Public Health Significance of Urban Pests 515 • A cumulative risk involves the likelihood of the occurrence of an adverse health effect from all routes of exposure to a group of substances sharing a common mechanism of toxicity.

14.7.2.4. Some existing models

Newly emerging exposure models are set up to accommodate aggregated residential exposure scenarios that contain multiple sources of a chemical. These models are mostly initiated in response to the demands of the Food Quality Protection Act in the United States. They aggregate exposure from multiple sources, at the cost of requiring good input data for each source. Some examples of these newly emerging exposure models are listed below. A modelling effort by the Hampshire Research Institute, funded by the EPA, developed a dietary and non-dietary residential exposure model that helps estimate aggregate expo- sure over a person’s lifetime Price, Young Chaisson, 2001. Another model can estimate daily annual non-dietary residential exposure and works in a probabilistic environment. The outputs from this model can be linked with the dietary exposure estimation model to produce an aggregate model. Exposure to pest control products applied by consumers can be assessed with the ConsExpo Consumer Exposure model Delmaar, Park van Engelen, 2005; Bremmer et al., 2006; RIVM, 2006. It is relevant to the Biocides Directive EC 1998, 2002. In the SHEDS stochastic human exposure and dose simulation model, sequential der- mal and non-dietary ingestion exposure and dose–time profiles are simulated by combi- ning measured surface residues and residue transfer efficiency with actual micro-level activity data quantified from videotapes Zartarian et al., 2000. This model has been used for some specialized residential exposure assessments EPA, 2002d. The REx residential exposure model is another model for aggregated exposure assess- ment. It is structured according to EPA SOPs for pesticidal residential exposure assess- ment EPA, 2000c. All these models can be used and refined by regulatory agencies as needed.

14.7.3. SOPs and exposure scenario types