The substitution principle in sustainability is the maxim of how processes, services, and products should be replaced with alternatives that result in a lower environmental impact.
An example of a strong, hazard-based interpretation of the substitution principle in application regarding chemicals considers the overall impact: "hazardous chemicals should be substituted by less hazardous alternatives or preferably alternatives that mitigates identifiable hazards impacts".
[1] The principle has historically been promoted by environmental groups.
[2] Critics of the principle claim it is very difficult to implement in reality, especially in terms of legislation.
Nonetheless, the concept is an important one and a key driver behind identifying "Substances of Very High Concern" in REACH and the development of hazardous substance lists such as the SIN List and the 'ETUC Trade Union Priority List'.