Statistical mathematical models SOPs and exposure scenario types

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

SOPs for residential exposure assessments have been developed by the EPA 1997a and EU EC, 2002. In both cases, the objective of the SOPs is to provide standard default methods for developing residential exposure assessments for both application and post- application exposures, when applicable monitoring data are limited or not available. The SOPs cover calculation algorithms for estimating dermal, inhalation or incidental inges- tion doses for a total of 13 major residential exposure scenarios: a lawns; b garden plants; c trees; d swimming pools; e painting with preservatives; f fogging; g Pesticides: risks and hazards 514 The two model types described above have different attributes and are strictly applica- ble only within their defined realms of use. In general, mathematical mechanistic models are concerned with specific circumstances or processes such as paint spraying and drum filling and usually cannot be used for more general applications. They are likely to give a lot of detailed information on the exposure to be expected. Empirical or knowledge- based models, on the other hand, tend to be applicable to a wider range of circumstances and are based on many years of accumulated experience. While they can give a broad idea of likely exposure, they are imprecise. They are, however, able to predict exposure to new and existing substances and to assess likely exposure across a range of uses of a substance. Model estimates are more widely used for exposure assessment of new sub- stances, because the data available are often poor or non-existent EC, 1996.

14.7.2.3. Statistical mathematical models

Statistical models have not been used yet for exposure estimations in the EU. Such models use empirical relationships to predict exposures from statistical distributions. In princi- ple, they reflect a combination of empirical and mechanistic models, together with consi- deration of the distribution of the input parameters. One of the most important steps in the procedure is represented by the implementation of the probabilistic approach, which allows the use of distributions in the calculation. While these models are in use in other related fields, such as the dietary exposure from pesticide residues, their application to exposure to biocidal products is still experimental. One of the reasons for developing statistical models and replacing deterministic models with probabilistic models is the recognition that deterministic models, by nature, tend to be overly conservative. In fact, they tend to introduce several conservative assumptions in a serial way, and the resulting so-called point estimate reflects worst-case scenarios so extreme as to become clearly unrealistic. Unlike deterministic models, probabilistic models integrate the distribution of discrete input variables and generate a final risk dis- tribution as a continuous variable, allowing the assessor to select the extent of protection or uncertainty desired. A recent paper by Lunchick 2001 has an interesting discussion of the pros and cons of deterministic exposure assessments versus probabilistic exposure assessments. He also provides an example of operator exposure where the deterministic exposure estimate corresponds to the 96th percentile of the probabilistic exposure distri- bution. How regulators will decide to interpret the results of probabilistic assessments is still open. These models demand the regulator to set the level of desired conservatism and in some cases determine the extent of protection of the population at risk. Current exposure models are limited to considering a single source and a single agent at a time. However, aggregate and cumulative exposure analyses are now a requirement of registration or regulation in the EU and North America EPA, 2001b, 2002a–c; PMRA, 2003. Definitions of aggregate and cumulative risk are as follows. • An aggregate risk involves the likelihood of the occurrence of an adverse health effect from all routes of exposure to a single substance. Public Health Significance of Urban Pests 517 large biomonitoring surveys of general populations, such as the survey conducted by the National Center for Environmental Health CDC, 2005 in the United States, that revea- led a widespread presence of small amounts of measurable pesticide residues in the urine of subjects who were not employed in agriculture nor were known to be professional users of pesticides. It is difficult to say how much the home use of pesticides and biocidal products contri- butes to the pesticide residues detected in general population surveys. In principle, the possibility that home use may be responsible for the urinary occurrence of pesticide resi- dues in some subjects and in others may add considerably to dietary exposure, cannot be excluded. However, the frequency and continuity basically daily of dietary exposure, when compared with the infrequent and discontinuous and relatively low level of resi- dential exposure for home use of biocides, make dietary exposure more likely blame- worthy than home exposure for the widespread pesticide residue contamination at the general population level.

14.8. Examples of residential risk assessments

This section covers understanding and comparing risks from routes of residential expo- sure, chlorpyrifos and pyrethrin exposures and risks, and a summary of examples of resi- dential risk assessments.

14.8.1. Understanding and comparing risks from routes of residential exposure

Housing residents are likely to be exposed to pesticides used in the home. The most com- mon routes of exposure are the dermal and inhalation routes, with unintentional inciden- tal oral exposure being attributable primarily to toddlers putting their fingers in their mouth after crawling over treated surfaces or touching pets. To better understand the Pesticides: risks and hazards 516 crack-and-crevice treatments; h pet treatments; i detergents; j impregnated mate- rials; k termiticides; l inhalation of residues from indoor treatments; and m rodenti- cides. Default values for the underlying exposure factors, such as amount used or der- mal transfer factors, are specified. These default values represent reasonable worst-case values. While the SOPs provide methods and default assumptions for conducting scree- ning-level residential exposure assessments for indoor and outdoor settings, they do not preclude the use of more sophisticated methods including stochastic analyses and the replacement of default values for exposure parameters with new data. WHOPES has published an easy to understand quantitative exposure and risk assessment for insecti- cide-treated mosquito nets, which provides a stepwise approach to assessing toxicity, exposure and risk WHOPES, 2004.

14.7.3.1. Comparing pesticide risks from residential and dietary exposures

When discussing exposure of the general population to pesticides, as a result of home use and application, it has to be kept in mind that dietary exposure to pesticide residues is known to occur in most parts of the world. This dietary exposure is regulated by law in most countries and is legally permitted in so far as it is considered to be safe or acceptable when the use of pesticides on specific agricultural products is authorized. Since such expo- sure involves millions of people, the regulating authorities in the various countries of the world have set up complex monitoring systems aimed at checking regularly the residues present on a large variety of food items sampled from the market. The results of this monitoring are collected and analysed periodically to ensure that the foods on the mar- ket comply with the legally established levels of residues and that the health risk for consumers remains within the limits set up by legislation. A general overview of the results produced by the residue monitoring systems indicates that the proportion of samples of food items found to be irregular is generally rather low and decreasing over time; for example, in the EU, from a frequency of detection of irre- gular samples of 5–6 typically observed in the mid-1990s, the actual frequency of detec- tion has decreased to 2–3 and in some countries to 1. Two major reasons account for a sample being defined as irregular. • A sample contains a pesticide residue permitted, but in a concentration slightly excee- ding the maximum residue limit MRL. • A food item contains the residue of a pesticide the use of which is not legally permitted for that crop. A toxicological assessment of such irregularities in pesticide residues suggests that the overall risk for the population is minimum, if any, given the margin of safety adopted in setting MRLs and the modestly excessive levels found in the samples. These population-level data, though reassuring in terms of toxicological risk, do indicate that consumers normally ingest pesticide residues with their diet, that only about 50 of the food items on the market do not have measurable residues in or on them and that a fair percentage of food items 10–20 bear more than one residue up to six at the same time. These analytical observations are in agreement with the results obtained by Table 14.2. Regulatory end-points and toxic effects of chlorpyrifos and pyrethrins Acute Sub-chronic Pesticide Regulatory end-point Toxic effect Regulatory end-point Toxic effect NOAEL NOAEL Chlorpyrifos Incidental oral route = Plasma and RBC a — Cholinesterase 0.5 mgkg cholinesterase inhibition inhibition Dermal route = Dermal route = 0.15 mgkgday 0.3 mgkgday Inhalation route b = Inhalation route b = 0.1 mgkgday 0.1 mgkgday Pyrethrins Inhalation route = Neurotoxicity Inhalation route Neurotoxicity 7.67 mgkgday LOAEL = 2.57 mgkgday Incidental oral route = Incidental oral route = 20 mgkgday 6.4 mgkgday a RBC = red blood cell. b As a conservative assumption, 100 lung absorption was assumed. Source: EPA 2000d, 2006a.