Air contaminants are listed in the three tables in 29 CFR 1910 Subpart Z. Both the concentration of the substance and the length of time an employee may be exposed to it are restricted, i.e., there are permissible exposure limits (PELs), ceiling limits, and 8-hour time weighted averages (TWAs) in the "Z Tables" that must be complied with. Employers must not permit their employees to be exposed to any of the regulated air contaminants for a longer period of time or in a greater concentration than is stated in these tables.