When determining acceptable exposure limits for potentially hazardous substances, there are several measures to draw from.
The standards set by the Occupational Safety and Health Administration (OSHA) are the only legally-binding limits in place nationwide. Two other major systems of evaluation, in addition to state standards, are used by most corporations and universities to establish safe exposure limits for employees.
Here’s a quick rundown of the major measurement systems for workplace chemical exposure, and what they represent.
Permissible Exposure Limit (PEL)
The Permissible Exposure Limit is a standard set by OSHA establishing safe levels of exposure to a hazardous substance. These legally-enforceable limits currently apply to fewer than 500 chemicals used in commercial products, industry, and research. Many stakeholder organizations (including OSHA itself) consider the current PELs to be outdated, as most were established shortly after OSHA’s creation in 1970.
Most PEL standards are based on a time-weighted average (TWA), which sets limits for exposure over an assumed 8- to 10-hour day, 40-hour-week working life. Some also include standards for short-term exposure limits (STEL).
Recommended Exposure Limit (REL)
The Recommended Exposure Limit is an occupational standard set by the National Institute for Occupational Safety and Health (NIOSH). These limits establish a permissible level of maximum exposure designed to protect workers over the course of their working life, in combination with proper worker training and PPE.
While not a federal legal standard, RELs are treated as guidelines by many organizations, and are used by OSHA and the Mine Safety and Health Administration (MSHA) in developing standards.
Threshold Limit Value (TLV)
Similar to the REL, the Threshold Limit Value is a guideline for safe exposure levels to specific chemicals over the working life of an employee. It is designed to be used by research and industry professionals as supplements to existing safety and health programs, not as a freestanding exposure limit.
TLVs are established for over 700 chemicals by the American Conference of Governmental Industrial Hygienists. ACGIH committees research and make recommendations on the level of exposure a typical worker can face without adverse effects.
In addition to the three measurement systems outlined above, OSHA also recommends California Division of Occupational Safety and Health (Cal/OSHA) Permissible Exposure Limits as an alternative source. CAL-OSHA standards are no less protective and in many cases more protective than existing OSHA standards, and constitute the most extensive list of PELs of any state.
Hazardous Substance Example: Acetone
Acetone is a solvent with many industrial uses. It is also found in common household products such as nail polish remover, identified by its pungent, fruity odor. Acetone is a nose and throat irritant. Chronic effects include respiratory tract irritation, dizziness, and loss of strength. Acetone is highly flammable.
Permissable Exposure Limit:
1000 ppm (2400 mg/m3)
500 ppm (590 mg/m3) 8-hr TWA
Recommended Exposure Limit:
250 ppm (590 mg/m3) 10-hr TWA
Threshold Limit Value:
250 ppm (594 mg/m3) 8-hr TWA
500 ppm (1,187 mg/m3) STEL
As seen above, the time-weighted average (TWA) exposure limit for Acetone is lower under REL and TLV standards than under OSHA standards.
Commercially available nail polish remover normally contains between 40% and 70% acetone, though percentages can be higher.
PPM= Parts per million
mg/m3= Milligrams per cubic meter
TWA= Time-Weighted Average. TWA concentrations of a contaminant are for up to an 8-hr or 10-hr work day during a 40-hr work week.
STEL= Short-term exposure limit. The STEL is the 15-minute TWA exposure that should not be exceeded at any time during a work day.