What does CMO mean in UNCLASSIFIED

A Calculated Mean Organism (CMO) is a hypothetical organism used in water quality assessment to represent the average sensitivity of aquatic organisms to toxic substances. It is calculated using data from standardized toxicity tests conducted on a range of aquatic species, including fish, invertebrates, and algae.

CMO provides a simplified and standardized way to assess the potential toxicity of water samples. It allows for comparison of toxicity data across different species and testing methods, enabling scientists and regulators to make informed decisions about water quality management.

CMO is calculated by taking the geometric mean of the toxicity values (e.g., EC50 or LC50) obtained from toxicity tests conducted on different aquatic species. The geometric mean is used to account for the log-normal distribution of toxicity values, which is common in environmental data.

CMO provides an average representation of aquatic organism sensitivity, but it does not account for species-specific differences or the potential for synergistic or antagonistic effects between multiple toxicants. Additionally, CMO may not accurately represent the sensitivity of organisms under field conditions, where environmental factors can influence toxicity.

CMO is often used as a screening tool to identify water samples that may require further toxicity testing. It can also be used to set water quality criteria and develop effluent limits for industrial discharges. By comparing the toxicity of a water sample to the CMO, regulators can assess the potential risk to aquatic organisms.