Chronic toxicity

Chronic toxicity, the development of adverse effects as a result of long term exposure to a contaminant or other stressor, is an important aspect of aquatic toxicology. Adverse effects associated with chronic toxicity can be directly lethal but are more commonly sublethal, including changes in growth, reproduction, or behavior. Chronic toxicity is in contrast to acute toxicity, which occurs over a shorter period of time to higher concentrations. Various toxicity tests can be performed to assess the chronic toxicity of different contaminants, and usually last at least 10% of an organism’s lifespan. Results of aquatic chronic toxicity tests can be used to determine water quality guidelines and regulations for protection of aquatic organisms.

Chronic toxicity is the development of adverse effects as the result of long term exposure to a toxicant or other stressor. It can manifest as direct lethality but more commonly refers to sublethal endpoints such as decreased growth, reduced reproduction, or behavioral changes such as impacted swimming performance.

Chronic toxicity tests are performed to determine the long term toxicity potential of toxicants or other stressors, commonly to aquatic organisms. Examples of common aquatic chronic toxicity test organisms, durations, and endpoints include:

Results from chronic toxicity tests can be used to calculate values that can be used for determining water quality standards. These include:

The no observed effects concentration (NOEC) is determined as the highest tested concentration that shows no statistically significant difference from the control. The lowest observed effects concentration (LOEC) is the lowest concentration of those tested that produced a statistically significant difference from the control. NOECs and LOECs can be derived from both acute and chronic tests and are used by agencies to set water quality standards.

The maximum acceptable toxicant concentration (MATC) is calculated as the geometric mean of the NOEC and LOEC. MATC is sometimes called the chronic value (CV) and defined as “the concentration (threshold) at which chronic effects are first observed”.

The predicted no effects concentration (PNEC) is calculated from toxicity tests to determine the concentration that is not thought to cause adverse effects to aquatic organisms. Determination of aquatic PNEC values requires toxicity test results from freshwater fish (e.g. ‘‘Pimephales promelas’’, freshwater invertebrates (e.g. ‘‘Daphnia magna’’), and freshwater algae (e.g. ‘‘Raphidocelis subcapitata’’) The probable effects concentration (PEC), the concentration predicted to be in the environment, is compared with the PNEC in risk assessment. The PEC takes into account both acute and chronic exposures to toxicants.

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