• Short-Term Exposure Limit (STEL): This is a 15-minute time-weighted average exposure that should not be exceeded at any time during a workday, even if the 8-hour TWA is within limits. STELs are set to protect against acute effects such as irritation, narcosis, or other adverse health effects that may occur from short-term exposure.
• Ceiling Limit (C): This is a concentration that should not be exceeded at any point in time during the workday. Ceiling limits are typically set for substances that can cause immediate and severe health effects upon brief exposure.

2. Based on Establishing Authority and Legal Enforceability:

• Permissible Exposure Limits (PELs): These are legally enforceable limits established by governmental regulatory agencies, such as the Occupational Safety and Health Administration (OSHA) in the United States. Employers are required to comply with PELs, and failure to do so can result in penalties.
• Threshold Limit Values (TLVs): These are recommendations established by the American Conference of Governmental Industrial Hygienists (ACGIH). TLVs are health-based guidelines and are not legally enforceable standards themselves, but they are widely recognized and often adopted or referenced by regulatory bodies and industrial hygiene professionals worldwide.
  • TLVs also include TWA, STEL, and Ceiling designations.
• Recommended Exposure Limits (RELs): These are recommendations established by the National Institute for Occupational Safety and Health (NIOSH) in the United States. Similar to TLVs, RELs are based on scientific data and are provided to OSHA for consideration in developing legally enforceable standards.
• Workplace Exposure Limits (WELs): Used in the UK, these are GB occupational exposure limits approved by the Health and Safety Executive (HSE) and are legally binding under the Control of Substances Hazardous to Health Regulations (COSHH).
  • Within WELs, there are sometimes Occupational Exposure Standards (OES) for substances where a safe threshold can be identified, and Maximum Exposure Limits (MEL) for substances with serious health implications where a safe threshold cannot be set (e.g., carcinogens). For MELs, exposure must be reduced to as low as reasonably practicable.
• Indicative Occupational Exposure Limit Values (IOELVs) and Binding Occupational Exposure Limit Values (BOELVs): These are established at the European Union level through directives. IOELVs are non-binding values that Member States should consider when setting their national OELs, while BOELVs are legally binding and must be implemented by Member States.
• Other Country-Specific OELs: Many other countries have their own specific terms and lists of OELs established by their respective regulatory agencies (e.g., MAK values in Germany, VLE in France).

3. Based on the Nature of the Substance and Risk Assessment:

• Health-Based OELs: For substances where a “safe threshold” can be determined, meaning there’s a concentration below which no adverse health effects are expected.
• Risk-Based OELs: For substances where a safe threshold cannot be determined (e.g., genotoxic carcinogens or respiratory sensitizers), every level of exposure carries some risk. In these cases, OELs are set to an “acceptable” or “tolerable” risk level, which is often a policy decision.

Factors influencing OELs:

• Toxicity of the substance
• Exposure routes (inhalation, skin contact, etc.)
• Duration and frequency of exposure
• Available scientific data (animal studies, human data, epidemiological studies)

It’s important to remember that OELs are guidelines to protect worker health and should be used in conjunction with a comprehensive risk assessment and control measures (e.g., engineering controls, administrative controls, personal protective equipment). The absence of an OEL for a substance does not necessarily mean it is safe.