OHS Code Explanation Guide

Published Date: July 01, 2009
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Part 4 Chemical Hazards, Biological Hazards and Harmful Substances

Section 16 Worker exposure to harmful substances

This section requires an employer to ensure that worker exposure to a harmful substance is kept as low as reasonably practicable/reasonably achievable and does not exceed the substance’s OELs. This is based on the principle that for each substance there is a safe or tolerable level of exposure below which no significant adverse health effects are likely to occur. Many factors affect total exposure, including

(a) the potential for absorption into the body by inhalation, ingestion or skin absorption,
(b) the duration of exposure, and
(c) the effect of simultaneous exposure to multiple substances.

The OHS Act requires that employers ensure, as far as it is reasonably practicable to do so, the health and safety of workers at their work site. In this section, the term “reasonably achievable” is used. Understanding of the term reasonably achievable comes from the Canadian Nuclear Safety Commission Regulatory Guide (2004, for “Keeping Radiation Exposures and Doses as Low as Reasonably Achievable (ALARA)”. Though the term reasonably achievable has not been given definite meaning by the Canadian Court system, it is generally accepted in industry and by regulators to encompass the same considerations as the concept of “reasonably practicable”.

Reasonably practicable is a concept used by the courts and is assessed using the “reasonable person test”. This test asks what a dozen of your peers e.g. twelve workers with equal qualifications and experience, would consider reasonable in a similar set of circumstances. The persons would likely review what happened and compare it against what they do in their own operations. Some of them might do more, others less. The result would be a balanced and wise judgment that could be defended to others. Reasonably practicable is a term that has been tested in the courts and supports a high standard of effective workplace protection for workers.

Factors that might be considered when evaluating exposure to a harmful substance include

  • What is common practice in other workplaces that use the substance or process? Are exposure levels at the workplace similar to those at other workplaces that use the substance or process?
  • Has the employer assessed whether exposure can be eliminated by substitution with a less toxic substance or other control measures? If these measures have not been implemented, what is the rationale for not doing so?
  • Are workers exposed to multiple substances at the workplace that may have synergistic, potentiating or additive effects?
  • Are workers experiencing adverse health effects even though exposure may be at or below the OELs?
For more information

Due Diligence
Bulletin LI015

The OHS Code requires that exposure be kept as low as reasonably practicable/reasonably achievable where there are harmful substances used in the workplace for which there are currently no OELs. Employers determining safe levels of exposure in such circumstances should consult other jurisdictions and organizations as well as the product manufacturer to obtain guidance on safe exposure limits. For example, the American Industrial Hygiene Association publishes Workplace Environmental Exposure Levels (WEELs) for a wide variety of substances. Section 9 of the OHS Act allows an occupational health and safety officer to enforce an exposure limit from another jurisdiction or organization if there is no OEL.

The three main routes of entry of a substance into the body are

(1) inhalation – by being inhaled,
(2) dermal – by being absorbed through the skin, and
(3) oral – by being swallowed.

Inhalation is the most common route of entry. Most exposure standards, including the OELs, are based on exposure resulting from the inhalation of substances suspended in air, either as a gas, vapour or aerosol such as dust, mist or fume.

Another way substances enter the body is absorption through the skin. The amount of chemical absorbed through the skin depends on the chemical and it is important to take this into consideration when determining exposure. Substances for which exposure via skin absorption is a potentially significant route of exposure have the designation “2” in the substance interaction column in Schedule 1, Table 2.

Oral exposure or ingestion usually occurs by accident through the contamination and subsequent ingestion of food or materials that come into contact with the mouth. Contaminants can also be ingested through hand-to-mouth contact such as nail biting or hand contamination of food or smoking materials.

Individual susceptibility to adverse effects from exposure to substances varies widely. A small percentage of workers may feel discomfort at or below the OEL. The OEL should not be used as a fine line between safe and unsafe conditions or as an index of relative toxicity.  Some workers may be affected more seriously due to aggravation of a pre-existing condition or by development of an occupational illness. In addition, some individuals are extremely sensitive to certain industrial chemicals due to genetic factors, personal habits such as smoking or alcohol use, the use of drugs or medications, pre-existing health conditions or previous exposure. These workers may not be adequately protected from adverse health effects resulting from exposure to substances that are at concentrations at or below their OEL. The extent to which these workers need more protection should be evaluated by an occupational physician.

Compliance in cases of short-term excursions

Short term exposure limits are concentrations of a substance to which it is believed that most workers may be exposed for a short period of time without suffering from adverse health effects such as irritation and chronic or irreversible tissue damage. The worker also should not be physically impaired to a degree that could increase the likelihood of accidental injury, impair self rescue or reduce work efficiency. It is not a separate exposure limit. It supplements the 8-hour OEL.

Employers must comply with the following rules on short-term excursion limits:

(a) worker exposure measured over any 15-minute period must not exceed the 15-minute OEL. Worker exposure to a substance measured over successive 15 minute periods, at a concentration above its 8-hour OEL, but at or below its 15-minute OEL, must not happen more than four times per day. There must be at least 60 minutes between successive exposure periods in this concentration range and the 8-hour OEL may not be exceeded for the work shift;

(b) worker exposure must never exceed ceiling levels which are denoted in Table 2 of Schedule 1 by a “c”. Ceiling limits can be measured using a direct reading instrument such as a colorimetric detector tube or an instrument with a diffusion controlled sensor which effectively averages measurements over approximately 1 minute; and

(c) if there is no 15-minute or ceiling OEL listed for a substance, the 8-hour OEL may not be exceeded and the “3X and 5X” rule applies. A worker must not be exposed to 3 times the 8-hour OEL for more than a total of 30 minutes during a continuous 24-hour period and 5 times the 8-hour OEL at any time. However, in no case can the immediately dangerous to life and health (IDLH) concentration be exceeded.

As defined in Part 1 of the OHS Code, IDLH means “circumstances in which the atmosphere is deficient in oxygen, or the concentration of a harmful substance in the atmosphere

(i) is an immediate threat to life,
(ii) may affect health irreversibly,
(iii) may have future adverse effects on health, or
(iv) may interfere with a worker’s ability to escape from a dangerous atmosphere”.

Some materials, i.e. hydrogen fluoride gas and cadmium vapour, may produce immediate transient effects that, if severe, may pass without medical attention, but are followed by sudden, possibly fatal collapse 12-72 hours after exposure. The victim “feels normal” from recovery from transient effects until collapse. Such materials in hazardous quantities are considered to be “immediately dangerous to life and health”.

IDLH concentrations are described in NIOSH publication NTIS Publication No. PB-94-195o47: Documentation for Immediately Dangerous to Life or Health Concentrations (IDLH), May 1994. This publication documents the criteria and information sources that have been used by NIOSH to determine immediately dangerous to life or health concentrations.

For more information

American Conference of Governmental Industrial Hygienists
The Effects of Unusual Work Schedules and Concurrent Exposures on Occupational Exposure Limits (OELs)
Bulletin CH055
Documentation for Immediately Dangerous to Life or Health Concentrations