[1] The REL is a level that NIOSH believes would be protective of worker safety and health over a working lifetime if used in combination with engineering and work practice controls, exposure and medical monitoring, posting and labeling of hazards, worker training and personal protective equipment.
The Pocket Guide is a source of general industrial hygiene information for workers, employers, and occupational health professionals.
To really use these guidelines well, safety professionals need to understand the recommended exposure levels, how to measure them, and ways to make sure workers aren't exposed to harmful stuff.
These guidelines aim to strike a balance; it is to keep workers safe from harm without going overboard and making things inefficient in the workplace.
The RELs, unlike permissible exposure limits or PELs set by OSHA, are merely guidelines -- they are not legally enforceable.