Elsevier

Chemosphere

Volume 43, Issues 4–7, May 2001, Pages 787-799
Chemosphere

The use of Monte-Carlo simulation techniques for risk assessment: study of a municipal waste incinerator

https://doi.org/10.1016/S0045-6535(00)00435-5Get rights and content

Abstract

The incremental lifetime risks due to polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) for the residents living in the surroundings of a municipal solid waste incinerator (MSWI) have been assessed. Two different pathways of exposure to PCDD/Fs, ingestion through the diet and exposure from MSWI emissions, were compared. Monte-Carlo simulations were carried out to obtain variability and uncertainty propagation The joint analysis of uncertainty and variability included a sensitivity analysis that identified the contribution to variance by different inputs. In general terms, PCDD/F ingestion through the diet contributed with more than 99% of the total risk, whereas direct exposition to PCDD/F emissions from the MSWI was less than 1% The results show that the median (50% percentile) of non-carcinogenic risk due to PCDD/Fs in the population living in the surroundings of the MSWI was 0.72 and the ratio of the 95th percentile and fifth percentile was about 2. With respect to the total carcinogenic risk, the median increment in individual lifetime was 7.90×10−5, while the ratio between the 95th percentile and the fifth percentile was about 1.5. In this analysis, a sequential structural decomposition of the relationships between the input variables has been used to partition the variance in the output (risk) in order to identify the most influential contributors to overall variance among them.

Introduction

Currently, most municipal solid waste (MSW) is disposed in sanitary landfills. However, because of increasing regulations and cost of using landfills, in recent years there has been more interest in alternatives to solid waste disposal. Incineration has become a reasonable mean by which communities can dispose of their solid trash (Valberg et al., 1996). However, although incineration is an effective way of treating solid wastes, the potential public health effects associated with stack emissions have caused a serious concern. Some of the chemicals emitted by MSW incinerators are constituents of the waste, which travel through the combustion chamber and are not captured by the pollution control equipment. These contaminants include toxic metals such as mercury, arsenic, chromium and cadmium among others, as well as organic compounds that escape of the combustion or are only partially oxidized. Other pollutants, such as polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) are by-products formed in the combustion train (Zemba et al., 1996, Lisk, 1988).

PCDDs and PCDFs, especially the 2,3,7,8-containing congeners, have generated the most public concern because they are perceived as the most hazardous. Although many of the dioxin and furan congeners are highly toxic, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) has generated the most interest due to its extreme toxicity in some species of experimental animals, persistence in the environment, and bioaccumulation in the food chain. In addition, TCDD has resulted to be a carcinogen and reproductive toxic in experimental animals, whereas it is also considered human carcinogen by the IARC.

Quantitative risk assessment tools have been used for almost a decade to evaluate stack emissions from municipal solid waste incinerators (MSWI). Frequently, human health risk assessment is performed as a part of the siting process for newly proposed facilities, while in many cases is a regulatory requirement of environmental impact assessment. Risk assessment is also sometimes conducted to evaluate environmental implications of design changes (such as control technology retrofits) in operating facilities (Zemba et al., 1996).

Recent emphasis in the risk analysis has focused on uncertainty in the risk assessment process. To perform risk assessment, model uncertainty and data uncertainty should be taken into account. `Uncertainty' can be described as a lack of knowledge regarding the true value of a parameter. However, since a lack of knowledge does not imply total ignorance, it is possible to characterise the relative likelihood of different values for a parameter. On the other hand, `variability' can be described as the actual differences in the value of a parameter that affects risk between the members of a population (heterogeneity) (Cohen et al., 1996). A key difference between uncertainty and variability is that uncertainty can be reduced and, at least in theory, eliminated by gathering additional information. However, gathering additional information, cannot eliminate the variability (Cohen et al., 1996). The variability may be due to a spectrum of human physiological behaviours such as breathing and drinking rates. In turn, the uncertainty may be due to a lack of precision regarding biological, chemical and physical data and processes.

Recently, the concentrations of PCDD/Fs were determined in soil and vegetation samples collected near a new MSWI (Tarragona, Catalonia, Spain) (Schuhmacher et al., 1998a, Schuhmacher et al., 1998b). Food samples, which were randomly obtained from local markets and supermarkets, were also analysed for PCDD/Fs (Domingo et al., 1999). In the current study, the development of a methodology for estimating the distribution of daily PCDD/F intake for the population living near to a MSWI is presented. This method compares the intakes due to direct exposure pathways (MSWI) with those from indirect pathways. The objectives of the study were the following: (1) To estimate the total exposure to PCDD/Fs of the population living at the surroundings of the MSWI and to evaluate the health risks by means of a Monte-Carlo simulation. (2) To compare human health risks caused by direct exposure to the incinerator with those originated by other exposure pathways. (3) To determine which inputs contribute significantly to the output uncertainty.

Section snippets

Experimental

The NAS report classifies the risk assessment process into four broad components: hazard identification, exposure assessment, dose–response assessment and risk characterisation (NRC, 1993). The first component involves an evaluation of whether a particular chemical can cause an adverse health effect in humans. The hazard identification process in itself is a qualitative risk assessment that examines both the potential for exposure and the nature of the adverse effect expected. For exposure

Risk characterisation

Risks for adverse human health effects are estimated assuming to be carcinogens or non-carcinogens.

  • Non-carcinogenic risk: To determine if the contaminant poses a risk to human health, daily intake is compared with the reference dose (RfD) for chronic exposure. The tolerable daily intake (TDI) established by the WHO was here used. Recently, a TDI range of 1–4 pg I-TEQ/kg body weight was established for PCDD/Fs (van Leeuwen et al., 2000).

  • Carcinogenic risk: It is calculated by multiplying the

Results and discussion

In the present study, ranges of exposure rather than single point estimates were developed in order to account for both the variability among members of a population and for uncertainties in the input variables. Even if it were possible to eliminate the uncertainty associated with the input variables, because of the variability, a probability density function would still be required.

The determination of which form of distribution function should be assigned to each parameter depends on both

Acknowledgements

The authors acknowledge the financial support of SIRUSA (Tarragona) and the Regional Government of Catalonia (Generalitat de Catalunya), Spain.

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