| Food-borne hazard remains a real and formidable problem on the global scale, causing great human suffering and significant economic loss. The food-borne risk to human health can arise from hazards that are biological, chemical or physical in nature. A key discipline for future reducing food-borne illness and strengthening food safety systems is risk assessment.Risk assessments are performed in a four-step process:hazard identification, hazard characterization, exposure assessment, and risk characterization. Information of hazard identification and hazard characterization can usually be extracted from the work of FAO/WHO, and applied directly. Exposure assessment, an essential element for quantifying risk, is defined as "the qualitative and/or quantitative evaluation of the likely intake of biological, chemical, and physical agents via food as well as exposures from other sources if relevant". It is recommended that national authorities that will to perform their own dietary exposure assessments use national food consumption and concentration data, but international toxicological reference values. And then, the latter step (risk characterization) integrates the information collected in the preceding three steps.Dietary exposure assessment is increasingly being performed internationally to evaluate safety and quality of agricultural products. Although model construction is by no means a new subject in the statistical literature, a good model for dietary exposure assessment must be derived from realistic scenarios and high-quality data. In order to feasibly evaluate safety and quality of agricultural products, this paper improved the simple-distribution method to bring forward a feasible and realistic model specific for the dietary exposure assessment of Chinese agricultural products. It also included an intensive simulation study on the relationship between sample size and high percentile estimates referring to contamination data of total mercury exposure in milled rice, which would be helpful to the sample survey of dietary exposure assessment. And then a case study (survey and dietary exposure assessment of total mercury in milled rice farmed in6provinces of southern China) was conducted to examine the reliability and practicality of the proposed model. Finally, software called "dietary exposure assessment model" was developed based on model construction theories. The main contents include:(1) Proposed a feasible and realistic model specific for the dietary exposure assessment of Chinese agricultural productsDeterministic model and probabilistic model are two main approaches to combine or integrate the data to provide an estimate of exposure. Deterministic model is easy to carry out, but its single risk estimate is conservative and rough. In comparison, probabilistic model use all available data and knowledge, and that variability and uncertainty can be quantified. And also there is a high demand for empirical data through the transition from non-parametric to parametric techniques. But the insufficiency of consumption data of Chinese agricultural products resulting from the lack of data communication can’t meet the data bulk requirement of probabilistic model. In order to make full use of the existed information to evaluate safety and quality of Chinese agricultural products pertinently, this paper focusing on the residue/concentration data, improved the simple-distribution method to bring forward a feasible and realistic model specific for the dietary exposure assessment of Chinese agricultural products. The improved simple-distribution was a method that employed a fixed value for food consumption variables and an empirical distribution of chemical concentrations in that food. The new model that assumed individuals with high exposure to the substance in question consume the relevant foods predominantly at average level would generally overcome this particular problem of "worst-case" assumptions. And to make the assessment more refined, age-gender groups quoting from the Chinese national health and nutrition survey were introduced into the model to take account of variation in consumption among people of interest preliminarily. The measure method of variability and uncertainty was also described in detail.(2) Launched a simulation study on appropriate sample sizes of contaminated data for dietary exposure assessmentA percentile is the value of a variable below which a certain percent of observations fall. As exposure assessment mostly involves positively skewed data, its result should be presented in the form of percentiles. However, how to determine an appropriate sample size to obtain the most economic and effective percentile estimation has seldom been studied. With the benefit of computer simulation, this paper set out to study the relationship using contamination data of total mercury in milled rice as materials. Weibull distribution and Log-normal distribution were used to fit the data. And Log-normal distribution has been further selected to study the impact of distribution pattern and variation on percentile estimates. The simulation results showed as follows:(1) Accurately estimating a higher percentile of the positively skewed distribution would require a larger sample size. And a larger sample size always resulted in a more accurate and more stable estimated percentile. With a sample size of500, we accurately estimated all the target percentiles of the2positively skewed distributions except the result of P99.9.(2) Estimating the same percentile using Log-normal distribution required far larger sample size than it needed when using a normal distribution. Moreover, the sample size needed for Log-normal distribution estimation was positively correlated to the magnitude of the variation of the skewed distribution. Considering the positively skewed data, an economic and effective assessment must be based on an appropriate sample size.C3) Performed a case study to verify the reliability and practicality of the proposed model and sampling guidelineThe case study investigated the occurrence of total mercury in milled rice from6provinces of southern China during harvest in2009, and determined the dietary exposure of target population using an improved non-parameter simple-distribution method with information on rice consumption and body weight. Altogether1321milled rice samples were collected from Jiangxi, Hubei, Hunan, Guangdong, Guangxi and Sichuan Provinces. Total mercury was measured by inductively coupled plasma-mass spectrometry (ICP-MS), and the limit of detection (LOD) for it was0.0008mg-kg-1. The analytical results showed that76.2%of the samples contained detectable concentrations of total mercury, which ranged from0.0008to0.0634mg-kg-1, but levels were generally low, with only2.3%of the samples having concentrations above0.02mg-kg-1, the maximum level (ML). On the other hand, there was an apparent regional difference found for the concentrations of total mercury in milled rice. The provisional tolerable weekly intake (PTWI) of total mercury, as recommended by Joint FAO/WHO Expert Committee on Food Additives (JECFA) is5μg·(kg bw)-1. The estimated exposure values for populations of interest were compared to the PTWI. For the relevant population, this study confirmed the low probability of health risks from total mercury via the milled rice from the6provinces of southern China. But exposure to total mercury for the population below14years old at P99.9represented41.5%-62.9%of the PTWI. While for children aged2-4years and boy aged4-7years, their estimated exposure at P99.9were all over60%of the PTWI suggesting a potential risk. Compared with the previous research result gotten from the total dietary study, the proposed model and sampling guideline were reliable and practicable.(4) Developed and released software "dietary exposure assessment model"In order to enhance Chinese assessment level of quality and safety for agricultural products,"dietary exposure assessment model" was developed based on model construction theories. And through appropriately setting, the improved simple-distribution method can be implemented in the software. A case study was conducted to compare "dietary exposure assessment model" with@Risk and SAS. The results showed that:the dietary exposure assessment model not only got accurate results, but also behaved effect, feasible and friendly. |
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