Combined Exposure to Mixture of Chemicals. An Impossible Challenge?

Abstract

The harmful effects of the human activities on health and the environment are known for a very long time but the public awareness is recent and dates from second half of the 20th century. Living organisms are almost constantly exposed to many stressors. Among them, chemical pollutants play a major role. A wide range of chemical substances act as pollutants, ranging from simple inorganic ions to complex organic molecules. Some metals such as cadmium, mercury, lead provoke adverse effects of human health when they are present at high level of exposure. Radioactive isotopes may be harmful to organisms, depending on the dose and type of radiation. Numerous organic compounds are also known to be noxious: hydrocarbons (i.e. benzene, polycyclic aromatic hydrocarbons, PAHs), polychlorinated phenols (PCPs) polychlorinated biphenyls (PCBs), polychlorinated dibenzodioxins (PCDDs), polychlorinated dibenzofurans (PCDFs), polybrominated biphenyls (PBBs), organochlorine pesticides (OCs), organophosphorus pesticides (OPs), carbamates, pyrethroid insecticides, phenoxy herbicides, rodenticides, organometallic compounds and so on (Walker et al., 2001). Some of these chemicals are of concern because of their human toxicity. Other chemicals cause damages to non-human biota but are not believed to be harmful to humans. Finally, some other pollutants are not directly toxic to humans or other biota at current environmental concentrations, but are capable to modify environmental features causing major environmental damage (i.e. chlorinated fluorinated carbons, CFCs, known to drastically disturb the chemistry of the stratosphere). In this chapter, only the pollutants harmful to living organisms are considered, keeping in mind that non chemical stressors may act at the same time on biota. Any substance can have adverse effects on cell biology and/or on whole organism, but this depend on dose and chemical speciation. Toxicity and ecotoxicity are defined as the capacity to cause injury to a living organism (human or not) defined with reference to the quantity of substance absorbed, the way in which the substance is taken up and distributed in time (single or repeated doses), the type and severity of injury, the time needed to produce the injury, the nature of the organism(s) affected, and other relevant conditions (Duffus et al., 2009). Any chemical of concern has to be taken up by an organism before it can produce an effect. Once absorbed, the potentially toxic substance will be distributed throughout the organism and the absorption of the toxicant will result in a toxic effect and a response defined as the percentage of the exposed population showing the defined toxic effect. To quantitatively