Risk assessment of mixtures of pesticides. Current approaches and future strategies.

The risk assessment of pesticide residues in food is based on toxicological evaluation of the single compounds and no internationally accepted procedure exists for evaluation of cumulative exposure to multiple residues of pesticides in crops, except for a few groups of pesticides sharing a group ADI. However, several attempts have been suggested during the last decade. This paper gives an overview of the various approaches. It is of paramount importance to consider whether there will be either no interaction or interaction between the compounds in the mixture. When there are no interactions several approaches are available for the risk assessment of mixtures of pesticides. However, no single simple approach is available to judge upon potential interactions at the low doses that humans are exposed to from pesticide residues in food. In these cases, PBTK models could be useful as tools to assess combined tissue doses and to help predict potential interactions including thresholds for such effects. This would improve the quality of the risk assessment.

The influence of thresholds on the risk assessment of carcinogens in food.

The risks from exposure to chemical contaminants in food must be scientifically assessed, in order to safeguard the health of consumers. Risk assessment of chemical contaminants that are both genotoxic and carcinogenic presents particular difficulties, since the effects of such substances are normally regarded as being without a threshold. No safe level can therefore be defined, and this has implications for both risk management and risk communication. Risk management of these substances in food has traditionally involved application of the ALARA (As Low as Reasonably Achievable) principle, however ALARA does not enable risk managers to assess the urgency and extent of the risk reduction measures needed. A more refined approach is needed, and several such approaches have been developed. Low-dose linear extrapolation from animal carcinogenicity studies or epidemiological studies to estimate risks for humans at low exposure levels has been applied by a number of regulatory bodies, while more recently the Margin of Exposure (MOE) approach has been applied by both the European Food Safety Authority and the Joint FAO/WHO Expert Committee on Food Additives. A further approach is the Threshold of Toxicological Concern (TTC), which establishes exposure thresholds for chemicals present in food, dependent on structure. Recent experimental evidence that genotoxic responses may be thresholded has significant implications for the risk assessment of chemicals that are both genotoxic and carcinogenic. In relation to existing approaches such as linear extrapolation, MOE and TTC, the existence of a threshold reduces the uncertainties inherent in such methodology and improves confidence in the risk assessment. However, for the foreseeable future, regulatory decisions based on the concept of thresholds for genotoxic carcinogens are likely to be taken case-by-case, based on convincing data on the Mode of Action indicating that the rate limiting variable for the development of cancer lies on a critical pathway that is thresholded.

Risk assessments of polychlorinated dibenzo- p-dioxins, polychlorinated dibenzofurans, and dioxin-like polychlorinated biphenyls in food.

The polychlorinated dibenzo-p-dioxins (PCDD), polychlorinated dibenzofurans (PCDF), and dioxin-like polychlorinated biphenyls (dioxin-like PCB) are ubiquitous in food of animal origin and accumulate in fatty tissues of animals and humans. The most toxic congener is 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). The toxic responses include dermal toxicity, immunotoxicity, carcinogenicity, and reproductive and developmental toxicity. Toxic equivalency factors have been established for the other PCDD, PCDF and dioxin-like PCB relative to TCDD, and the combined toxicity of a sample can be expressed as toxic equivalent (WHO-TEQ). The EC Scientific Committee for Food evaluated these compounds in 2001. The assessment used the most sensitive adverse toxicological end-points of TCDD in experimental animals. These were developmental and reproductive effects in the male offspring of rats administered TCDD during pregnancy. Because of the large difference between rats and humans in the biological half-life of TCDD, the assessment used a body burden approach to compare across species and derived a tolerable weekly intake of 14 pg TCDD/kg of body weight (bw), which was extended to include all the 2,3,7,8-substituted PCDD and PCDF, and the dioxin-like PCB, and expressed as a group tolerable weekly intake of 14 pg WHO-TEQ/kg bw. The FAO/WHO Joint Expert Committee on Food Additives (JECFA) performed a similar assessment whereas the US Environmental Protection Agency (US EPA) has paid more attention to human data on carcinogenicity.

Workshop on trichothecenes with a focus on DON: summary report.

A number of mycotoxins of the class of trichothecenes are produced by a variety of Fusarium fungi commonly found on cereals. Unfavourable weather conditions may lead to a high level of Fusarium infections in crops such as wheat and correspondingly high trichothecene contents. The ILSI Europe Natural Toxin Task Force therefore organised a workshop on trichothecenes with a special focus on deoxynivalenol (DON). A number of experts reviewed the current knowledge on trichothecenes with respect to occurrence, including aspects of mould growth, toxin formation, storage and effects of processing; prevention; analytical methodologies, including sampling; surveillance and exposure assessments; and toxicology and risk assessment. A number of recommendations were given under the headings: prevention, sampling and analytical methods, exposure assessment, and toxicology. Gaps in knowledge were also identified.

Ochratoxin A: previous risk assessments and issues arising.

Ochratoxin A (OTA) causes nephropathy in all species tested with large sex and species differences in potency, pigs being most sensitive. It has been linked to Balkan endemic nephropathy (BEN) in humans. Embryotoxicity, teratogenicity, and immunotoxicity occur only at doses higher than those causing nephrotoxicity. OTA has long serum half-lives in various species including humans. OTA produced renal tumours in mice and rats. The male rat was most sensitive, renal carcinomas occurring after 70 microg/kg bw per day but not 21 microg/kg bw per day. OTA was not mutagenic in most studies in bacteria and mammalian cells, but produced DNA damage and chromosomal aberrations in mammalian cells in vitro, and in mice in vivo. DNA adducts found in the kidneys of mice and rats dosed with OTA, did not contain fragments of OTA. OTA in food has been evaluated by the Joint FAO/WHO Expert Committee on Food Additives (JECFA), and by the EC Scientific Committee on Food (SCF). JECFA established a provisional tolerable weekly intake (PTWI) of 100 ng/kg bw based on the LOEL for renal effects in pigs. Conversely, SCF recommended reducing exposure to OTA as much as possible, e.g. below 5 ng/kg bw per day. Both committees recommended further studies to clarify the mechanism by which OTA induces nephrotoxicity and carcinogenicity.