In most cases, there is more than one chemical exposure standard for a chemical. The Occupational Safety and Health Administration (OSHA) develops Permissible Exposure Limits (PEL), the American Conference of Governmental Industrial Hygienists (ACGIH) has Threshold Limit Values (TLV), and the National Institute of Occupational Safety and Health (NIOSH) has Recommended Exposure Limits (REL). Compliance with OSHA PELs are required by law but are made by a consensus of scientists, business, and labor. The ACGIH is an organization of industrial hygienists that only relies on experimental and epidemiological information. NIOSH is a governmental organization that also makes standards that are based on science.
It is always best to follow standards based on the latest scientific information. In most cases, those are ACGIH and/or NIOSH limits. Industrial hygienists will recommend based on the OSHA PEL for compliance only. You wouldn’t want your doctor to recommend treatment based upon 30 year old information.