The Effects of Unusual Work Schedules and Concurrent Exposures on Occupational Exposure Limits (OELs)

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Alberta Human Resources and Employment

Summary Statement

The article describes the impact that factors such as duration of exposure and existence of multiple, simultaneous exposures have on OELS (Occupational exposure limits).
November, 2000

Alberta Government Technical Bulletin


Alberta’s Occupational Exposure Limits (OELs) for airborne substances (vapors, gases, fumes, dusts and fibers) are contained in the Chemical Hazards Regulation (Alberta Regulation 393/88). Employers are required to ensure that a worker’s exposure to any substance is kept as low as reasonably practicable and does not exceed the substance’s OEL. Since many factors affect total exposure, it is important to be aware of and consider the impact of these factors to prevent overexposure.

Three of the most significant factors to consider are:

    (1) the potential for absorption into the body by all routes of exposure;
    (2) the duration of exposure; and
    (3) the effect of simultaneous exposure to multiple agents.

These factors are important as they determine the toxic outcome of exposure. This Safety Bulletin deals with the adjustment of airborne exposure limits but employers should be aware that unusual work schedules may have an impact on many other aspects of health and safety on the job. A change in the length of the workday will also affect allowable exposure levels for physical hazards such as noise. This Safety Bulletin explores the impact of several key factors and how these factors should be considered in the evaluation of workplace exposure.

Routes of exposure

The three main routes of exposure and absorption in the occupational setting are:

    (1) dermal (through the skin);
    (2) oral (through the gastrointestinal tract); and
    (3) inhalation (through the respiratory system).

Routes of absorption for specific substances are identified in the Material Safety Data Sheets (MSDSs) for those substances.

Dermal exposure

Work practices involving the handling of chemicals or close contact with chemicals during maintenance, degreasing or cleaning activities can result in significant dermal uptake for some chemicals. Even if inhalation exposure is controlled, a dose equivalent to or greater than that from inhalation exposure alone can be achieved as a result of absorption through the skin. Without adequate assessment of the properties of the chemical and potential for dermal exposure, the worker may not be adequately protected.

Materials with the potential for significant absorption through the skin are identified with a “skin” notation in the OELs. Dermal exposure can be controlled by:

    (a) substitution of a chemical with one that is not absorbed through the skin;
    (b) a process change to eliminate skin contact; or
    (c) the use of appropriate personal protective equipment (PPE).

The MSDS, chemical supplier or PPE manufacturer must be consulted to ensure that material from which the PPE is made provides an adequate barrier to the chemical. Gloves are made from a variety of materials (polyvinyl chloride, natural rubber, neoprene, etc.) and the degree of protection provided varies with the properties of the chemical. The protection offered by different materials is rated as "fair", "good", "excellent" or "not recommended" as determined by manufacturer testing. For example, a glove made of polyvinyl chloride is not recommended for use as a protective barrier against acetone. The use of inappropriate PPE gives workers a false sense of security.

Oral exposure – ingestion of chemicals

Ingestion of chemicals in the workplace is largely accidental through the contamination and subsequent ingestion of food or materials that are brought into contact with the mouth e.g. tobacco products, chewing gum. Contaminants can also be ingested through hand to mouth contact such as nail biting or hand contamination of food. Exposure to metals and their oxides e.g. lead and lead oxide, has caused occupational poisoning. To prevent accidental ingestion, the Chemical Hazards Regulation prohibits eating, drinking and smoking in areas likely to be contaminated by harmful substances.

Inhalation exposure

Most airborne exposure standards, including Alberta’s OELs, make reference to an 8 hour, 15 minute, or ceiling exposure limit. The value represents the time-weighted average concentration of the airborne substance over the specified exposure period. When accounting for unusual work schedules, adjustments are generally made to 8 hour exposure limits. When an 8 hour exposure limit is set, the basic premise is that nearly all workers can be exposed day after day (8 hrs/day, 40 hrs/week) to these concentrations without suffering adverse health effects.

Established on the basis that they protect nearly all workers, susceptible groups or those with preexisting medical conditions may not be protected by the exposure limit. Factors such as age, sex, reproductive status (pregnancy), genetic factors and lifestyle factors (smoking, alcohol use, etc.) may also play a role in the biological outcome of exposure to chemicals, particularly if exposure is close to the OEL. It is also thought that patterns of exposure and the impact of shift work, which may be combined with extended work hours, can also affect the biological outcome. Although it is not possible to adjust the OEL for each of these parameters, they should be considered in the overall strategy to protect workers.

The impact of unusual work schedules on exposure limits

Nontraditional work schedules are becoming more common in the workplace. There is an increasing trend towards extended work hours with more days off between shifts. Many continuous process operations such as chemical manufacturing, oil refineries, steel mills, drilling rigs and paper mills require two or three shifts in a 24-hour period to accommodate continuous production. Workers may routinely work overtime during periods of heavy demand. A second job may also result in workers being exposed to chemicals for extended periods. Whether called a compressed work week, novel work schedule or extended work day, this prolonged exposure time can have a health impact where workers are exposed to physical and chemical hazards.

Exposure limits are based on the assumption that exposure occurs for an 8 hour period after which the body is no longer exposed but allowed to recover for the next 16 hours. Where the worker is exposed for more than 8 hours in a day, these assumptions do not hold true and the worker could be at increased risk. Numerous biological factors come into play when adjusting the OEL. The booklet produced each year by the American Conference of Governmental Industrial Hygienists (ACGIH) — Threshold Limit Values (TLVs) and Biological Exposure Indices (BEIs) — should be consulted to ensure it is appropriate to adjust the limit. For example, it is unnecessary to adjust limits where they are based on odor. Although limits can be adjusted downwards to accommodate longer periods of exposure, standards can never be adjusted upwards to accommodate shorter periods of exposure.

Models to adjust exposure limits of airborne substances for unusual work schedules

The risk of an increased exposure to certain chemicals (body burden) has been recognized and several models proposed to modify the 8 hrs/day, 40 hrs/week standard to a “nonstandard” work day. The intent of the models is to maintain the same overall body burden yet preserve the same margin of safety as the original standard.

Two main models are used to adjust occupational exposure standards. Each model has its strengths and weaknesses, requiring specific types of information to be applied properly.

Pharmacokinetic models

Pharmacokinetic models use information such as the biological half life of the substance and exposure time to predict peak body burden. Pharmacokinetic models most accurately predict body burden and therefore result in the least conservative recommendations when adjusting to unusual work schedules. The difficulty with adjusting exposure standards based on this model is that biological half lives are not available for many chemicals. These models are suitable only for chemicals with standards based on accumulated body burden. They are not suitable for chemicals with standards based on odor, irritancy, or other non-systemic health effects.

A number of pharmacokinetic models are available for use. The one most widely used is the Hickey and Reist model (Hickey J, Reist P; 1977).

Brief and scala model

The simplest and most conservative model is that developed by Brief and Scala. It compensates for unusual work schedules by reducing the permissible concentration in proportion to both the increase in exposure time and the reduction in recovery time. Daily and weekly exposures are addressed by the following formulae:

Daily Adjustments of Occupational Exposure Limits:

Daily Re duction Factor = {8/h x (24-h/16)

    where h = hours worked per day

Adjusted Exposure Limit = 8hr OEL x Daily Re duction Factor

Weekly Adjustments of Occupational Exposure Limits

Weekly Re duction Factor = {40/h x (168-h)/128}

    where h = hours worked per day

Adjusted Exposure Limit = 8hr OEL x Daily Re duction Factor

Note: The adjusted exposure limit should be calculated using each equation and the most restrictive value adopted.

In summary, there are differences in the complexity of information required to apply each of the models. When adjustment values are compared, the Brief and Scala Model is the most conservative and results in the greatest reduction of the exposure limit. When adjustments to exposure limits are necessary, it is recommended that a competent person be consulted to ensure that the adjustment is appropriate and applicable as the models are theoretical and involve assumptions that may not apply to every chemical. An understanding of the chemical is required and caution must be taken where limited toxicity data is available, the toxic effect being avoided is serious, or the chemical accumulates following repeated exposure. However, the benefits of adjusting exposure limits outweigh the uncertainty of the models. Where unusual work schedules are common, the need to adjust exposure limits should be explored and the most appropriate model selected.

Concurrent multiple chemical exposures

Another consideration in the evaluation of workplace exposure is the effect of concurrent chemical exposures. In fact, exposure to a single chemical in the workplace occurs rarely. Exposure to several chemicals can result from complex work processes, breakdown products, or from work performed by others in the area. Nevertheless, standards are generally established based on information, testing or experience resulting from exposure to a single chemical. The resulting biological effect of exposure to several chemicals is rarely known but available data indicate that interactions between chemicals is more likely to occur under conditions of high exposure.

The combined effects of chemicals are described as independent, additive, antagonistic, synergistic or potentiating; these effects are described in Table 1. If known, information on potential health effects, both individual and interactive, are described in the MSDS. In evaluating the impact of concurrent chemical exposures, materials acting independently can be evaluated individually. Where the potential for synergistic or potentiating effects are suspected, this enhancement of toxic effect must be reflected in the allowable exposure. However, there is no model for adjustment of the exposure limit to account for synergistic or potentiating effects. The easiest solutions are to either find a substitute for one of the chemicals to avoid the potential effect or ensure exposure is maintained as low as reasonably practicable. In the occupational setting, antagonistic effects are not used as a basis for increasing exposure limits.

Where chemicals are known to have additive effects, the Chemical Hazards Regulation contains a formula which is intended to prevent overexposure:

C1/T1 ± C2/T2 ± C3/T3 ±...Cn/Tn < 1

where C1 C2 C3...C n = actual airborne concentrations of each containment and

T1 T2 T3...Tn = respective 8 hr OEL

To prevent overexposure, the sum of the standardized exposures must not exceed 1.

The assessment of worker exposure must be comprehensive to ensure that total exposure is not underestimated. The potential for exposure from all forms of contaminants (gases, vapors, dusts) and all routes of exposure (dermal, oral, inhalation) must be considered. In addition, the interaction of these materials and the duration of exposure must be accounted for. Only when all factors are considered and adjusted are workers protected.

Table 1. Effects Caused by Concurrent Exposures

Independent The toxicity of each compound is produced by independent mechanisms and/or act upon separate organs or systems. Independent substances exert their own toxicity without influence or interference from one another. 2 + 3 = 2 + 3 Silica Dust and Carbon Monoxide
Additive Compounds with similar toxicity produce a response that is equal to the sum of the effects produced by each of the individual compounds acting alone. 2 + 3 = 5 Xylene and Toluene
Antagonistic Toxicity of one chemical is reduced by exposure to another. 2 + 3 <= 5 BAL and Lead
Potentiating Where one substance does not have a toxic effect on a certain organ but when combined with exposure to another chemical, it makes the latter much more toxic. 0 + 3 >= 3 Isopropanol and Carbon Tetrachloride
Synergistic Two materials act together to produce toxicity greater than that produced by either material if administered separately. 2 + 3 >= 5 Carbon Tetrachloride and Ethanol
Adapted from Whylie and Elias (1992)


American Conference of Governmental Industrial Hygienists. Documentation of the Threshold Limit Values and Biological Exposure Indices. 6 th edition; 1991.

Brief RS, Scala, RA. Occupational Exposure Limits for Novel Work Schedules. Am. Ind. Hyg. Assoc. J. 36: pp 467, 1975.

Harris RL, Cralley LJ, Cralley LV (editors). Patty's Industrial Hygiene and Toxicology, 3 rd ed. Vol. 3, Part A, PP 191-348, 1994.

Hickey J, Reist P. Application of Occupational Exposure Limits to Unusual Work Schedules. Am.Ind. Hyg. Assoc.J. 38(11); PP 613-621, 1977.

Wylie DN, Elias JD. Adjustments of TLV's to Accommodate Specific Conditions in the Workplace. Presented to the AIHA - Alberta Local Section, 1992.

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