Ecological risk assessments are generally considered either prospective or retrospective in nature. Prospective risk assessments are performed for products prior to introduction to the environment; ecological risk assessment for antibiotics has largely focused on these regulatory assessment paradigms. Currently, ecological risk of pharmaceuticals to aquatic organisms is characterized deterministically with a hazard quotient (HQ), which describes the relationship between a predicted environmental concentration (PEC) and a predicted no effect concentration (PNEC), and traditional endpoints (e.g., survival, growth). If an HQ (PEC/PNEC) is >1, then a risk to ecosystems is considered possible.
While tiered approaches are employed during prospective assessments of antibiotics, several criticisms of current regulatory risk assessment paradigms have recently surfaced in the peer-reviewed literature. Specifically, environmental introduction concentration (EIC) modeling, trigger values for tiered decisions, and selection of single-species toxicity tests and response variables for human and veterinary medicines have all been questioned. For example, several government agencies employ EICaquatic and EICsoil trigger values for assessment of human and select veterinary medicines of 1 mgl-1 and 100 mgkg-1, respectively, below which ecological effects are believed to be minimal. In addition, a tenfold dilution factor is applied to an EICaquatic to estimate PECs of antibiotics released from municipal effluent discharges. This dilution factor is not appropriate for effluent-dominated ecosystems.
In many cases, standardized single-species acute toxi-city tests are used to establish PNEC values for antibiotics if a trigger value is exceeded. After data from these studies are collected, default assessment factors (AFs) of 100 or 1000 are applied to EC50 values to account for potential chronic effects. However, it is not transparent how default AFs were derived from empirical data. While an AF approach is appropriate for some classes of compounds, pharmaceuticals may have acute to chronic effects ratios (ACRs) higher than the default AFs currently used in regulatory risk assessments. If chronic testing is required, standardized single-species toxicity tests with response variables such as algal or juvenile fish growth or D. magna reproduction are assessed. However, alternative endpoints or response variables such as those described in the effects section above may be more appropriate to assess chronic toxicity for antibiotics and other pharmaceuticals.
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