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Aldehydes constitute a group of relatively reactive organic compounds. They occur as natural (flavoring) constituents in a wide variety of foods and food components, often in relatively small, but occasionally in very large concentrations, and are also widely used as food additives. Evidence of carcinogenic potential in experimental animals is convincing for formaldehyde and acetaldehyde, limited for crotonaldehyde, furfural and glycidaldehyde, doubtful for malondialdehyde, very weak for acrolein and absent for vanillin. Formaldehyde carcinogenesis is a high-dose phenomenon in which the cytotoxicity plays a crucial role. Cytotoxicity may also be of major importance in acetaldehyde carcinogenesis but further studies are needed to prove or disprove this assumption. For a large number of aldehydes (relevant) data on neither carcinogenicity nor genotoxicity are available. From epidemiological studies there is no convincing evidence of aldehyde exposure being related to cancer in humans. Overall assessment of the cancer risk of aldehydes in the diet leads to the conclusion that formaldehyde, acrolein, citral and vanillin are no dietary risk factors, and that the opposite may be true for acetaldehyde, crotonaldehyde and furfural. Malondialdehyde, glycidaldehyde, benzaldehyde, cinnamaldehyde and anisaldehyde cannot be evaluated on the basis of the available data. A series of aldehydes should be subjected to at least mutagenicity, cytogenicity and cytotoxicity tests. Priority setting for testing should be based on expected mechanism of action and degree of human exposure.

Procedures for the selection of compounds with high health hazard potential are reviewed, and major aspects of the assessment of health risks associated with exposure to mixtures of chemicals are discussed. Examples are given of additivity and synergism of effects following exposure to mixtures. Using these data from the literature a two-step procedure for the safety evaluation of the mixture of chemicals occurring at a specific workplace is suggested. The first step consists of estimating the relative health risk associated with each chemical; the estimation is based on the ratio between exposure level and degree of toxicity. Those chemicals representing a high risk are then selected for further consideration. The second step comprises the risk assessment, focusing on prediction of the risk associated with exposure to the mixture of selected chemicals. To allow such prediction the (presumable) mode of action of the selected compounds should be considered. A practical way to find out whether exposure to chemicals at a specific workplace is of serious health concern, could be the conduct of both a 4-week toxicity study in rats and 2 different types of genotoxicity studies with the mixture of selected chemicals, using exposure concentrations related to those occurring in practice.

Biparental sexual reproduction is usual in planthoppers. In this they resemble their other relatives among the Auchenorrhyncha. Den Bieman and de Vrijer (1987) discovered the only known example of a truly parthenogenetic planthopper, but pseudogamy has been reported in a few species. The evolution of these forms of reproduction will be discussed further below. Biparental reproduction involves a complicated sequence of behavior that results in contact between receptive males and females of the same species. These behavior patterns, from long-range mate location to copulation and transfer of gametes, may best be included under the general title of mating behavior.