Because the chemicals that are used in personal care products usually end up in our creeks, rivers and oceans, it is important to consider whether they are toxic to aquatic plants and animals.
Aquatic plants and animals are usually quite susceptible to any toxic effects of these chemicals, because they spend their lives living in it. Plants and animals on land might receive doses of a chemical, but they often do not have the same level of continuous exposure.
Because aquatic plants and animals are so susceptible, considering aquatic toxicity is also a good screening tool for worst-case environmental harm. There are always exceptions to the rule, but the consensus is that if a chemical is not harmful to aquatic plants and animals it is probably low risk. One important exception to this rule is if a chemical is bioaccumulative.
To do aquatic toxicity testing, scientists usually test on species from three groups of organisms – fish, aquatic invertebrates (like water fleas, which you might know as Sea Monkeys) and algae. This helps to make sure that the results are representative across a wide range of aquatic plants and animals.
The United Nations publishes the Globally Harmonised System of Classification and Labelling of Chemicals, which are international rules on what test results are required for a chemical to be considered very toxic, toxic or harmful. The descriptions on this site are made according to this criteria.