When talking about exposure to hazardous chemicals, the safety industry has developed some acronyms that make it difficult to understand what the actual exposure limitations are.
ppm: parts per million
OSHA: Occupational Safety and Health Administration http://www.osha.gov/
NIOSH: National Institute for Occupational Safety and Health http://www.cdc.gov/niosh/
ACGIH: American Conference of Governmental Industrial Hygenists http://www.acgih.org/home.htm
TWA: Time Weighted Average. The averaged exposure to a chemical over certain period of time (usually 8 hours for OSHA).
PEL: Permissible Exposure Limit. Usually provided in either ppm or in a concentration such as mg/M³ (micrograms per cubic meter) over an 8 hour TWA.
STEL: Short Term Exposure Limit. Provided in either ppm or in a concentration such as mg/M³ (micrograms per cubic meter) over a short term time period such as 15 minutes. This is also a TWA (time weighted average).
REL: Recommended Exposure Limit. This is purely a NIOSH term that takes into account OSHA data as well as “real life”, and sets a limit at which health and life may be affected for exposures beyond the REL.