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The Science Behind Environmental Assessments of Priority Substances under the Canadian Environmental Protection Act
The Canadian Environmental Protection Act (CEPA), proclaimed in 2000, focuses on the cradle-to-grave management of toxic substances. CEPA requires the Ministers of Environment and Health to compile and publish a list of substances to be assessed on a priority basis to determine if they are toxic as defined under the Act. This list is known as the Priority Substances List, or PSL.
Environment Canada has published a Guidance Manual for conducting environmental risk assessments under the Priority Substances Assessment Program. The manual summarizes the scientific risk assessment process from an environmental perspective. Health Canada assesses PSL substances to determine the human health risks they pose.
PSL substances must undergo assessments to determine whether they are "toxic" as defined under CEPA. The definition, contained in Section 64 of the Act, takes into account the likelihood and magnitude of releases into the environment and the harm a substance may cause to human health or ecosystems at levels occurring in the Canadian environment:
« A substance is toxic if it is entering or may enter the environment in a quantity or a concentration or under conditions that:
(a) have or may have an immediate or long-term harmful effect on the environment or its biological diversity, or
(b) constitute or may constitute a danger to the environment on which life depends, or
(c) constitute or may constitute a danger in Canada to human life or health. »
If a substance is found to be "toxic," the federal government works with the provinces, territories, industry, nongovernment organizations and other interested parties to develop a management plan to reduce or eliminate the harmful effects the substance has on the environment and the health of Canadians.
Environmental assessments can be complex and time-consuming, since the effects of substances on numerous species must be considered. Sometimes the effects are direct, such as death or reproductive abnormalities; at other times the effects are more subtle, such as disruptions to the structure or function of an ecosystem.
Environmental Resource Group (ERG)
Assessors at Environment Canada are assisted by an Environmental Resource Group which is formed for each substance. The ERG can include experts from industry, academia and members of other government departments or other levels of government who have particular expertise in dealing with the substance and who are invited by Environment Canada to participate. The ERG participates actively in the assessment process by performing tasks ranging from the review of draft documents and assistance in data collection to the development or writing of sections of supporting documents or assessment reports.
Three-tiered Assessment Approach
Environmental risk assessments of priority substances involve three major steps: Problem Formulation, analysis, and risk characterization. The risk characterization process is further structured in three tiers, to ensure that substances undergo rigorous analysis to determine if they are "toxic" under CEPA. This approach allows for a substance to be found not to be "toxic" after a Tier 1, Tier 2, or Tier 3 assessment, and eliminated from further study. The determination of whether or not a substance is "toxic" must be based on sound, scientifically reliable data.
A Tier 1 assessment typically uses very conservative estimates of exposure and effects to eliminate from further study substances that have almost no chance of causing environmental harm. For example, a Tier 1 or worst-case scenario (also known as "hyperconservative") analysis could use for the exposure concentration the highest concentration of a substance ever measured in Canada. If no adverse effects are anticipated at this concentration, the substance would be judged not to be "toxic" under CEPA.
Tier 2 uses conservative, but more realistic, estimates of exposure and effects to eliminate from further study substances where the risk of environmental damage is very slight. For example, if releases of a substance have declined in recent years, the Tier 2 assessment could be based on the highest concentration measured in Canada in recent years.
In a Tier 3 assessment, the distributions of exposure and/or effects may be compared to estimate the likelihood that the substance will cause environmental harm. For example, a frequency distribution of environmental concentrations may be compared with a concentration-response curve from a toxicity test.
Environmental Risk Assessment Process
Problem Formulation is the planning stage of the process. The goals and focus of the assessment are established, an initial scoping is carried out, data gaps are identified, and a strategy for conducting the assessment is developed. Problem Formulation is an iterative process, and is updated when necessary as additional information is gathered during the assessment.
To determine how a substance enters the Canadian environment, a pathways analysis is conducted. Information on amounts manufactured, imported, exported, used, and released is reviewed to predict a substance’s geographic distribution and its entry into various compartments of the environment, such as water, air, soil and sediment. These data are then used to identify the types of organisms that are potentially exposed to the substance and hence could potentially be harmed.
Data from toxicity tests conducted in the laboratory or in the field are used to identify species that are particularly sensitive to the substance. Assessors must develop an understanding of the ecological relevance of exposed organisms that are particularly sensitive to the substance in order to predict possible indirect effects on other components of the ecosystem, such as predators or prey species.
The next step is to select assessment endpoints, pertaining to organisms that are likely to be most at risk from exposure to the substance. Assessment endpoints may pertain to populations of wild mammals or birds, aquatic organisms, soil organisms, etc. (for example, a decrease in numbers of fish). Measurement endpoints, are used to estimate effects on assessment endpoints (for example, the results of toxicity tests on rainbow trout). A conceptual model is then developed, summarizing the ways in which a substance acts in the ecosystem and its possible effects.
Once a first draft of the Problem Formulation is completed, scientists from academia, industry, the federal government, provinces and territories, environmental groups and other interested parties are invited to comment on it.
The Analysis Phase Consists of three parts: Entry, Exposure, and Effects Characterization
The objective of entry characterization is to identify the various sources of the substance in Canada, the quantity of the substance released from each of these sources, and how the substance is released over time, to air, water or soil. Entry characterization includes all phases of the substance’s life cycle, from manufacture or importation, through transportation and use, to final disposal. Information gathered during this phase is the first step in determining exposure. If the substance is found to be "toxic" under CEPA, this information is also used to guide development of risk management options.
During exposure characterization, the exposure of each assessment endpoint to the substance is determined. Factors considered include where a substance is released, where it ends up, how long it stays there, what it breaks down into, and how or whether it is taken up by sensitive organisms. A numerical value, known as the Estimated Exposure Value (EEV), is calculated for each assessment endpoint. In a Tier 1 assessment, the EEV is usually the highest concentration of the substance in air, soil, water, etc. For wildlife species, the EEV may be expressed as the maximum total daily intake through inhalation, drinking water and/or food.
The objective of effects characterization is to determine a Critical Toxicity Value (CTV), or the lowest concentration of a substance that will cause a certain adverse effect for each assessment endpoint. For example, the concentration of a substance causing a 25% reduction in growth or reproduction of test organisms in a toxicity study could be selected as a CTV. A CTV is usually calculated from the results of short-term (acute) and long-term (chronic) laboratory toxicity tests on one or more species, or, less often (due to their lack of availability), from the results of field studies.
CTVs for Tier 1 assessments are typically based on toxicity to the most sensitive organism tested. For Tier 2 assessments, it may be possible to refine the CTV. For example, warm-water fish may be less sensitive to a substance than cold-water fish. If the substance is released only to warm waters, the Tier 2 CTV should be based on toxicity to the most sensitive warm-water species tested. A Tier 3 assessment may consider the entire concentration-effects curve derived from a toxicity study in order to determine potential effects at various concentrations of the substance.
Risk characterization, the third stage in environmental risk assessment, compares exposure concentrations and concentrations causing effects for each assessment endpoint in order to determine whether adverse effects are likely.
The first step in risk characterization is to determine a level at which the substance would not be expected to affect the assessment endpoints. This is known as the Estimated No Effects Value (ENEV), and is usually derived by dividing the CTV by an Application Factor. An application factor is used to account for the uncertainties inherent in extrapolating between measurement and assessment endpoints, including variables such as: the difference between laboratory animals and species found in the wild; fluctuations in environmental parameters, such as temperature, which may cause different effects; or other environmental stresses that organisms may face in their natural habitat.
A Tier 1 quotient is calculated by dividing the EEV (see above) by the ENEV. If the resulting value is less than 1, the substance is judged not to be "toxic" under CEPA for the assessment endpoint, and the assessment does not have to proceed further. If the value is greater than or equal to 1, the substance moves into a Tier 2 assessment.
A Tier 2 assessment may involve the use of more realistic, less conservative values for the EEV, or the ENEV, or both. It may also be possible to justify using a smaller application factor. There must be sound scientific reasons to support any changes that are made, and these reasons must be presented in the assessment report. If the numerical result of the Tier 2 assessment quotient is less than 1, the substance may be judged not to be "toxic" for the assessment endpoint. It is stressed that Tier 1 and 2 quotients equal to or exceeding 1 do NOT result in a designation of "toxic" under CEPA for any substance.
If the quotient is greater than or equal to 1, the assessment moves to Tier 3. Tier 3 involves consideration of all available data to characterize environmental risks to the greatest possible extent. For example, it could involve comparing the distribution of exposure concentrations with concentrations causing effects, in order to determine whether adverse effects are likely. A special analysis is used for naturally occurring substances, such as metals, which may cause harmful effects. This analysis considers natural background concentrations and takes into account the possibility that organisms found in naturally enriched areas may have developed a special tolerance to the substance. If it is concluded from a Tier 3 assessment that adverse effects are unlikely, the substance is judged not to be "toxic" based on the assessment endpoint.
If a Tier 3 analysis of any assessment endpoint indicates that adverse effects do occur or are likely to occur, the substance will be considered to be "toxic" under CEPA, paragraph 64(a) and will be recommended for addition to the List of Toxic Substances (schedule 1 of CEPA), which will trigger actions to determine how best to reduce or eliminate the risk.
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