Solid wastes generated from urban and industrial sources contain a large number of ingredients, some of which are toxic. The substances are considered toxic when the concentration exceeds a particular value below which it may not endanger public health.

Various tests and criteria have been devised by different agencies to determine as to whether a given substance is toxic or hazardous. It is necessary to assess the intrinsic properties of the waste to judge whether its uncontrolled release in the environment would lead to toxic effects on human or other living organisms. The possible toxic effect also depends upon the quantity of the waste.

A preliminary decision model for screening and selecting hazardous compounds and ranking of hazardous wastes has been developed by EPA. The criteria used in the screening model relate to only the intrinsic hazard of the wastes on uncontrolled release to the environment in respect of its quantity or the pathway to humans or other critical organisms.

The criteria such as toxicity, phytotoxicity, genetic activity and bioconcentration are used for this purpose. Substances or materials can be classified as hazardous or otherwise depending on the dose administration, exposure mode and time of exposure.

On this basis, EPA has given the following scale of activity:

$LD_{50}$ = Lethal Dose to kill 50% population