The following is a general review describing the current state of knowledge on the toxic responses observed for a wide variety of species exposed to polycyclic aromatic hydrocarbons (PAHs). The term 'ecotoxicology' mainly refers to the effects that toxicants have on ecosystems; however, this is a very broad concept. Even though humans are generally considered a component of ecosystems, such evaluations are usually not a part of ecotoxicological investigations. This article focuses on the adverse effects from exposure to PAHs that occur in most species except Homo sapiens. Ecotoxicology as a field is broad and covers the direct and indirect effects on ecosystems, in addition to laboratory studies that focus on specific contaminants and their effects on whole organisms. As ecotoxicologists, we are generally interested in any adverse responses that affect population integrity; therefore, we generally focus on effects relating to survival, growth, and reproduction. Over the last several years, however, other relevant responses have been elevated in importance because of their potential effect on population dynamics. For example, alterations to the immune system or organismal behavior can certainly lead to an increase in mortality for individuals. For most of these 'endpoints', ecotoxicologists study not only the higher-level responses, such as reduced biomass, immobilization, or fewer offspring, but many of the physiological and biochemical mechanisms that lead to the higher-level responses. In most cases, the decisions regarding toxic concentrations are made on statistically significant changes for the higherlevel responses and some physiological alterations. Unlike toxicity profiling for humans, evidence of biochemical change is usually considered not sufficient for regulatory action to protect an ecosystem.

Most of the data we have for ecotoxicological responses come from controlled laboratory studies, which will be the focus of this article. Field studies demonstrating alterations to species or ecosystems are inherently difficult to perform and interpret. Such studies usually suffer from confounding factors that preclude definitive conclusions. In most cases, multiple contaminants occur at a site and these are often correlated with each other, which is an important consideration when characterizing specific cause and effect relationships. Other confounding factors involve myriad uncontrolled environmental parameters that vary between reference and impacted sites.

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