Human Health Risk Assessment for Priority Substances

1.0 Introduction

The Canadian Environmental Protection Act (CEPA), proclaimed on June 30, 1988, and amended on June 30, 1989, authorizes the Ministers of the Environment and of Health to investigate a wide variety of substances that may be present in the environment and cause adverse effects on the environment or on human health.

Under CEPA, "substance" means "any distinguishable kind of organic or inorganic matter, whether animate or inanimate, and includes:

any mixture that is a combination of substances ...,

any complex mixtures of different molecules that are contained in effluents,
emissions or wastes that result from any work, undertaking or activity."

This definition of a "substance" encompasses discrete chemical compounds, classes of chemicals, emissions and effluents, and products of biotechnology, including microorganisms. Therefore, all of the above are candidates for assessment under the legislation.

According to a definition in section 11 of the Act:

"... a substance is toxic if it is entering or may enter the environment in a quantity or concentration or under conditions"

1. having or that may have an immediate or long-term harmful effect on the environment;
2. constituting or that may constitute a danger to the environment on which human life depends; or
3. constituting or that may constitute a danger in Canada to human life or health."

Under Part II of CEPA ("Toxic Substances"), there is a legal framework for assessing and controlling substances that may already be present in the Canadian environment (existing substances). Both Ministers (Environment and Health) must publish and may amend a Priority Substances List, respond to public nominations for additions to the List within 90 days, and conduct an assessment to ascertain whether or not each listed substance is "toxic". If the report of the assessment is not published within 5 years of a substance being added to the List, this could lead to the establishment of a Board of Review. The Ministers must also publish a summary of the Assessment Report in the Canada Gazette along with an indication of whether they intend to recommend the development of regulations to control the substance.

The first Priority Substances List, published in the Canada Gazette in February 1989, comprised 44 substances including discrete chemicals, classes of substances and complex mixtures. Priority must be given to these substances in assessing whether they are "toxic or capable of becoming toxic" under section 11 of CEPA.

The determination of "toxic", under paragraph 11(c) of CEPA (i.e., with respect to direct effects on human health), for Priority Substances is the responsibility of the Bureau of Chemical Hazards of Health Canada, and is based on careful consideration of the principles outlined in this report. Though developed specifically to ensure consistency of approach for Priority Substances, some of the aspects addressed herein also apply to other substances being assessed under CEPA.

In scientific parlance, toxicity is the inherent capability of a substance to cause harm, which does not take into account exposure. However, the definition of "toxic" under section 11 of the Act is a legal one which may be equated with risk since it embodies the concept that harm to the environment or to human health is a function of both the intrinsic toxicity (i.e., toxicity in the traditional sense) and the extent of exposure. In addition, the inclusion of the word "may" in the definition with respect to both entry into the environment ("a compound may enter the environment in a quantity or concentration or under conditions") and effects ("that may constitute a danger in Canada to human life or health") allows the approach to designation of "toxic" with respect to human health 11(c) to be developed in a manner which is consistent with current principles of halth risk assessment. Assessment under paragraph 11(c), for which there are three endpoints (i.e., designation of "toxic", "not considered to be toxic", or "insufficient information to conclude whether or not the compound is toxic") does not address any aspects of risk management, which are not considered at this stage. Designation of a substance as "toxic" under the Act sets the stage for adding the substance to Schedule I of CEPA (the "List of Toxic Substances") and for reviewing options for controlling risks to human health and/or to the environment. Provisions concerning the need for controls on a substance that is deemed to be "toxic" are included under other sections and paragraphs of the Act (for example, paragraph 13(l)(c) and sections 33 and 34). It should be noted that the designation of a substance as "toxic" under CEPA does not necessarily mean that controls will be imposed. Such decisions can only be made in a subsequent risk management phase that includes a judicious balancing of the risks and benefits associated with continued use of the substance (that is, based on subsequent analysis of social and economic as well as scientific factors).

In this report, initially, a brief description of aspects considered in the evaluation of data relevant to assessment of "toxic" under paragraph 11(c) of CEPA is presented. This is followed by a description of the principles used in the assessment of exposure and the approach to evaluation of effects for different types of substances ("non-threshold toxicants", "threshold toxicants", "possible threshold toxicants" and mixtures). More detailed information is included in the Appendices.

Although there may be uncertainty in the available scientific data used as the basis for determination of whether or not a substance is "toxic" under paragraph 11(c) of CEPA, every effort is made to take these uncertainties into account in the approaches described here. It should also be emphasized that fundamental to these assessments is the application of sound scientific judgement on a case-by-case basis. Moreover, these approaches are subject to change, to incorporate new developments in risk assessment methodology.