ceiling limit 

OCCUPATIONAL MEDICINE, TOXICOLOGY

The concentration of a substance, defined by OSHA¹ and NIOSH², that should not be exceeded for any (emphasis added) period of time, as it is regarded as immediately dangerous to life or health.

¹Occupational Safety and Health Administration ²National Institute for Occupational Safety and Health

Synonym Short-term exposure limit 

Reference http://en.wikipedia.org/wiki/Permissible_exposure_limit