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Water: Water Quality Standards Academy

Deriving the CMC: Step 3—Calculate the Final Acute Value

Aquatic Life Criteria

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Basic Course: Supplemental Topics

Steps in the
FAV Calculation
  • Calculate the GMAVs.
  • Rank the GMAVs.
  • Calculate the FAV.

The lowest four values are then used in regression to estimate the concentration that would cause the threshold effect (i.e., LC50) for the fifth percentile most sensitive species. For acute toxicity tests, this fifth percentile of the effect concentrations is considered the FAV.


Key Point.In most cases, there are not enough data to interpolate the fifth percentile concentration. Thus, typically the four lowest GMAVs are selected for extrapolating.

If a SMAV for a commercially or recreationally important species (e.g., rainbow trout) falls below the FAV, the 1985 Guidelines say that this SMAV can be substituted for the fifth percentile of the GMAVs to protect that important species.


Key Point. If acceptable data are available for a large number of appropriate taxa from an appropriate variety of taxonomical and functional groups, a reasonable level of protection will likely be provided if all except a small fraction of the taxa are covered. To be practical, EPA selected the fifth percentile as this small fraction.

The ALC derivation graph below highlights the data and approach used in calculating the FAV.


For informational purposes only–Not official statements of EPA policy.

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