the limit of the time-weighted average of the concentration of a chemical agent in the air within the breathing zone of a worker in relation to a specified reference period (unless otherwise specified), occupational exposure limit - a concentration in the workplace air to which most people can be exposed without experiencing harmful effects. There are three different types of exposure limits in common use: TWA (Time-weighted average), STEL (Short-term exposure limit) and C (ceiling) exposure limit., Occupational Exposure Limits (OEL's) can be described as limits for airborne concentrations of substances, which are believed to not cause health effects to nearly all workers exposed to those substances during their working life, piirnorm - ohtliku aine keskmine kontsentratsioon töökeskkonna sissehingatavas õhus tööpäeva (8h) ja töönädala (40h) jooksul, mis ei kahjusta tervist kogu tööstaaži kestel, kuid ei välista terviseriski, sest keemiliste ainete toime oleneb organismi individuaalsest eripärast, piirnorm - millegi suhtes lubatud kõrgeim määr