Occupational Exposure Limits (OELs)
Exposure Limits for Mixtures of Chemical Substances
When applying Occupational Exposure Limits (OELs) to assess health hazards arising from simultaneous exposure to two or more chemical substances, special considerations must be taken into account. Appendix (E) of this guideline briefly presents these considerations, together with the relevant calculation methods and practical examples.
Changes in Working Conditions and Work Schedules
Application of OELs under Unusual Environmental Conditions
When employees are exposed to airborne contaminants under temperature and pressure conditions that differ significantly from Normal Temperature and Pressure (NTP) conditions (24°C and 760 mmHg), caution must be exercised when comparing sampling results with Occupational Exposure Limits.
For aerosols, the Time-Weighted Average (TWA) exposure concentration, calculated from the sampled air volume without temperature and pressure correction, should be compared directly with the established OEL.
For gases and vapors, several methods may be used for comparison. A simple approach is as follows:
1. Determine the exposure concentration in mass-per-volume units (mg/m³) without correcting for temperature and pressure.
2. If the exposure limit is expressed in units other than mg/m³, convert it to mg/m³. For unit conversions, the molar volume of a gas should be assumed to be 24.04 liters per mole.
3. Compare the measured concentration with the exposure limit using identical units.
When comparing sampling results obtained under non-standard atmospheric conditions with established exposure limits, several assumptions are made. One assumption is that the volume of air inhaled by a worker during a workday under varying temperature and pressure conditions does not differ significantly from that under standard conditions. Another assumption for gases and vapors is that the absorbed dose is related to the partial pressure of the inhaled substance.
For exposures occurring under extremely high or low pressure conditions, particular care should be exercised when comparing measured concentrations with established exposure limits.
Unusual Work Schedules
The application of Occupational Exposure Limits to work schedules that differ substantially from the conventional 8-hour workday and 40-hour workweek requires special analysis to ensure an equivalent level of worker protection.
Shorter workweeks may permit workers to hold additional jobs involving similar exposures. Consequently, although exposure in each individual job may remain below the OEL, cumulative exposure across multiple jobs may exceed acceptable limits.
Several mathematical models have been developed to evaluate non-standard work schedules. Based on toxicological principles, the general objective of these models is to identify exposure levels that ensure daily or weekly peak body burdens do not exceed those associated with a standard 8-hour/day, 40-hour/week schedule.
Brief and Scala Model
One of the most widely used approaches for evaluating non-standard work schedules is the Brief and Scala Model.
This model reduces the Occupational Exposure Limit proportionally to:
- Increased exposure duration; and
- Reduced recovery (non-exposure) time.
It is commonly applied to work schedules exceeding 8 hours per day or 40 hours per week.
The model should not be used for situations involving very high exposure concentrations over extremely short periods (for example, exposure to five times the TWA-OEL for three hours followed by no exposure for the remainder of the shift). In such cases, Short-Term Exposure Limits (STELs) and excursion limits should be applied.
The Brief and Scala Model recognizes that during a 12-hour work shift:
- Exposure duration is 50% greater than during an 8-hour shift under equivalent conditions.
- Recovery and detoxification time is reduced by approximately 25% (from 16 hours to 12 hours).
The model also recognizes that repeated exposures over extended work schedules may place additional burdens on the body's detoxification mechanisms, increasing the likelihood of toxic substance accumulation in target organs.
As a result, even when exposures remain within conventional OELs, worker protection during extended work schedules may be reduced.
Calculation of the Reduction Factor (RF)
The Brief and Scala Model first calculates a Reduction Factor (RF), which is then multiplied by the published TWA-OEL to obtain an adjusted exposure limit.
Adjusted OEL
Adjusted OEL = TWA-OEL × Reduction Factor
Daily Work Schedules Exceeding 8 Hours
For daily work periods exceeding 8 hours:
RF = (8/h) × ((24 − h)/16)
Where:
- h = number of working hours per day
Weekly Work Schedules Exceeding 40 Hours
For weekly work periods exceeding 40 hours:
RF = (40/h) × ((168 − h)/128)
Where:
- h = number of working hours per week
Example
If the TWA-OEL for a chemical substance is 50 ppm:
- Under a 10-hour work shift, the adjusted OEL becomes approximately 35 ppm.
- Under a 12-hour work shift, the adjusted OEL becomes approximately 25 ppm.
Important Note
When both daily and weekly working hours exceed standard limits (for example, 10 hours per day and 60 hours per week), the reduction factor should be calculated using both equations, and the lower adjusted value should be adopted.
Recommendations for Using the Brief and Scala Model
a) Systemic Health Effects
Where the OEL is based on systemic (acute or chronic) health effects, the reduction factor should be applied and the resulting adjusted value used as the revised TWA-OEL.
b) Excursion Limits
For non-standard work schedules, excursion limits should also be adjusted. The adjusted excursion factor is calculated as:
EF = (EF₈ − 1) × RF + 1
Where:
- EF = Adjusted Excursion Factor
- EF₈ = Excursion Factor for the standard 8-hour workday
- RF = OEL Reduction Factor
c) Extremely Long Work Periods
The model is not applicable to work periods exceeding 16 hours, such as those encountered in:
- Submarines
- Spacecraft
- Similar environments where living and working occur within the same confined space
In such situations, conventional OELs are generally not appropriate.
d) Very Short Work Periods
The model should not be applied to work periods shorter than 4 hours per day or less than 20 hours per week.
e) Applicability to TWA-Based OELs
The reduction factor is intended for OELs expressed as Time-Weighted Averages (TWAs).
f) Ceiling Limits
The model may also be applied to substances with Ceiling Limits (identified by the code "C"), except where the ceiling designation is based solely on sensory irritation, since irritation responses do not necessarily increase linearly with work duration.
Alternative Models
Although the Brief and Scala Model is simpler to use than complex pharmacokinetic models, alternative methods have also been developed.
One such approach, developed by the Robert-Sauvé Research Institute for Occupational Health and Safety (IRSST) and the University of Montreal, is based on Haber's Law and produces adjusted exposure limits that closely approximate those obtained through physiological pharmacokinetic modeling.
This method classifies chemical substances according to their toxicological effects and applies different adjustment procedures to each category.
The detailed methodology and classification tables are provided in separate guidance documents issued by occupational health authorities.
Units of Occupational Exposure Limits (OELs)
Occupational Exposure Limits are typically expressed as:
- ppm (parts per million), or
- mg/m³ (milligrams per cubic meter)
Airborne contaminants may occur as:
Gas
A gas is a substance whose molecules move freely within a container under normal temperature and pressure conditions. Gases have neither a fixed shape nor a fixed volume.
Vapor
A vapor is the gaseous phase of a substance that exists as a liquid or solid under normal conditions. Vapor concentration is generally expressed in terms of vapor pressure and varies with temperature and pressure.
Aerosol
An aerosol is a suspension of solid particles or liquid droplets in a gas.
Common aerosol forms include:
- Dust
- Mist
- Fume
- Fiber
- Smoke
- Smog
Aerosols are often categorized according to their aerodynamic behavior and deposition location within the human respiratory system.
Conversion Between ppm and mg/m³
For gases and vapors at NTP conditions, where the molar volume equals 24.04 liters/mol, the following conversion formulas apply:
From mg/m³ to ppm
OEL (ppm) = OEL (mg/m³) × 24.04 / Molecular Weight
From ppm to mg/m³
OEL (mg/m³) = OEL (ppm) × Molecular Weight / 24.04
For exposure limits expressed in elemental form, the atomic weight of the element should be used rather than the molecular weight of the compound.
Biological Exposure Indices (BEIs)
The symbol BEI indicates that a Biological Exposure Index has been established for a chemical substance.
The following specialized BEI designations are also used:
BEIA
Biological Exposure Index for acetylcholinesterase-inhibiting pesticides.
BEIM
Biological Exposure Index for methemoglobin-forming agents.
BEIP
Biological Exposure Index for polycyclic aromatic hydrocarbons (PAHs).
Biological monitoring is recommended for evaluating total exposure from all sources, including:
- Dermal exposure
- Ingestion
- Non-occupational sources
Carcinogenicity
A carcinogen is any agent capable of causing benign or malignant tumors.
Evidence of carcinogenicity may be derived from:
- Toxicological studies
- Epidemiological studies
- Mechanistic investigations
This standard adopts the carcinogenicity classification system of the American Conference of Governmental Industrial Hygienists (ACGIH), using categories designated by the letter A followed by a numerical classification indicating the level of carcinogenic evidence.
Detailed definitions are provided in Appendix A.
Inhalable Fraction and Vapor (IFV)
The symbol IFV is assigned to substances with sufficient vapor pressure to exist in both particulate and vapor phases at concentrations relevant to the TWA-OEL.
When evaluating IFV substances, occupational hygienists should assess both particulate and vapor phases during exposure monitoring.
Particular attention should be given to:
- Spray application operations
- Temperature-dependent physical state changes
- Situations where vapors dissolve into or adsorb onto airborne particles
Sensitization
The sensitization notation identifies substances that have been demonstrated, through human or animal studies, to cause allergic sensitization.
Two specific symbols are used:
DSEN
Indicates the potential to cause skin sensitization.
RSEN
Indicates the potential to cause respiratory sensitization.
These symbols do not necessarily imply that sensitization was the primary basis for establishing the OEL. However, where sensitization data are available, they are carefully considered during OEL development.
For substances whose OELs are based on sensitization effects, compliance with the OEL is expected to protect workers from becoming sensitized. These limits are not intended to protect workers who have already developed sensitization.
Sensitization may occur through:
- Inhalation exposure
- Skin contact
- Conjunctival exposure
The absence of a sensitization notation does not necessarily indicate that a substance lacks sensitizing properties; it may simply reflect insufficient scientific evidence.
Sensitization typically occurs through immunological mechanisms and should not be confused with irritation, hypersensitivity, or individual susceptibility. Once sensitization develops, subsequent exposures—even at very low concentrations—may trigger significant adverse reactions.