All chemicals, even those essential to life, can be toxic if they are in the environment at levels that are too high. Scientists use information from experiments on animals and from studying workers exposed to chemicals at their jobs to set levels of chemicals that are not likely to cause harm. Acceptable levels often depend on who is exposed, or whether the chemical in question is in the soil, in drinking water, or in the air. For example, levels that are not likely to harm children are often lower than those for adults, and acceptable levels in air are often much lower than those in soil.
Acceptable levels also depend on what happens on the property where the chemicals are. For example, acceptable levels at a playground are usually lower than acceptable levels at a factory. IDEM uses the best available information about levels of chemicals found at a property, how toxic those chemicals are, and who might be exposed to those chemicals to decide whether something needs to be done to protect people and the environment. IDEM posts copies of current and past tables of acceptable levels on the IDEM website. Appendix A of IDEM's Remediation Closure Guide describes the process that IDEM uses to derive its screening levels.