developed to suit this purpose. Toxicological effects Toxicity can be broken down into two broad categories of direct and indirect toxicity. Direct toxicity results from a toxicant acting at the site of action in or on the organism. Indirect toxicity occurs with a change in the physical, chemical, or biological environment. Lethality is most common effect used in toxicology and used as an endpoint for acute toxicity tests. While conducting chronic toxicity tests sublethal effects are endpoints that are looked at. These endpoints include behavioral, physiological, biochemical, histological changes. There are a number of effects that occur when an organism is simultaneously exposed to two or more toxicants. These effects include additive effects, synergistic effects, potentiation effects, and antagonistic effects. An additive effect occurs when combined effect is equal to a combination or sum of the individual effects. A synergistic effect occurs when the combination of effects is much greater than the two individual effects added together. Potentiation is an effect that occurs when an individual chemical has no effect is added to a toxicant and the combination has a greater effect than just the toxicant alone. Finally, an antagonistic effect occurs when a combination of chemicals has less of an effect than the sum of their individual effects. Important aquatic toxicology resources ASTM International (formerly American Society for Testing and Materials). A consensus-based organization, representing 135 countries, that develops and delivers international voluntary standard methods for aquatic toxicity testing. Standard Methods for the Examination of Water and Wastewater. A compilation of techniques for water analysis, jointly published by the American Public Health Association (APHA), the American Water Works Association (AWWA), and the Water Environment Federation. "Ecotox." A database maintained by EPA that offers single chemical toxicity information for both aquatic and terrestrial purposes. Society of Environmental Toxicology and Chemistry (SETAC). A nonprofit, worldwide society working to promote scientific research to further our understanding of environmental stressors, environmental education, and the use of science in environmental policy. EPA publishes guidance manuals outlining aquatic toxicity test procedures. Organisation for Economic Co-operation and Development (OECD). A forum for governments to work together to promote policies for the betterment of people’s social and economic well-being around the world. One way in which they accomplish this is through the development of aquatic toxicity test guidelines. Environment and Climate Change Canada. Canada's lead federal agency for environmental protection. Terminology Median Lethal Concentration (LC50) – The