Overview of ecological assessment of substances: chapter 6


6. Risk Characterization

Risk characterization involves using a combination of qualitative and quantitative approaches to understand and describe the risk a substance poses to the environment. The various lines of evidence explored during the assessment are considered, in a weight-of-evidence approach, to evaluate the potential for harmful effects of a substance in the Canadian environment.

6.1 Lines of Evidence

The various lines of evidence that may be considered are discussed briefly below.

Evidence that a Substance is both Persistent and Bioaccumulative (or a Precursor of Such a Substance)

The Ministers of the Environment and of Health consider that evidence that a substance is both persistent and bioaccumulative (according to the Persistence and Bioaccumulation Regulations under CEPA 1999), when combined with evidence of toxicity and release or expected release into the environment, is sufficient to conclude that the substance can lead to harmful ecological impacts.

Although it is not possible, using current science, to accurately predict the long-term ecological effects of persistent, bioaccumulative substances, they are generally acknowledged to have the potential to cause serious and possibly irreversible impacts. Persistent substances remain in the environment for long periods of time, increasing the probability and the duration of exposure. Persistent substances that are subject to long-range transport are of particular concern because they can result in low-level, regional contamination. Releases of extremely small amounts of persistent and bioaccumulative substances may lead to relatively high concentrations in organisms over wide areas. Bioaccumulative and persistent substances may also biomagnify through the food chain, resulting in especially high internal exposures for top predators. Because they are widespread, several different persistent and bioaccumulative substances may be present simultaneously in the tissues of organisms, increasing the likelihood and potential severity of harm.

Evidence that a Substance is a Precursor of a Substance on the List of Toxic Substances

Evidence that a substance is a precursor of a substance on the List of Toxic Substances (in Schedule 1 of CEPA 1999) is a strong indication that the substance is "capable of becoming toxic," that is, that it may ultimately harm the environment, even if the untransformed parent compound does not cause direct harm on its own.

There are particular concerns about precursors of substances that are on the List of Toxic Substances and are candidates for virtual elimination. Evidence that a substance is a precursor of a substance that is on the List of Toxic Substances and is a candidate for virtual elimination indicates that release of the precursor into the environment can lead to harmful ecological impacts. This indicates that the precursor meets the criteria in Paragraph 64(a) of CEPA 1999.

Field Evidence that a Substance has Caused or is Causing Environmental Harm

Field evidence can provide a strong indication that a substance meets the criteria in Paragraph 64(a) of CEPA 1999. However, one of the main objectives of the Act is to prevent harm from occurring, rather than reacting after harm has occurred. Pollution prevention cannot occur if ecological assessments rely solely on this line of evidence, which documents environmental harm that has already occurred, although action could be taken to prevent further damage.

Evidence that a substance is causing environmental harm in another country should also be considered, particularly if the environmental conditions in that country are similar to those in Canada.

Results of a Quantitative Comparison of Exposure and Effects Data

Exposure and effects data can be compared in a number of ways. Possibly the simplest way is to derive a risk quotient (the ratio of the PEC and the PNEC). This is called a deterministic approach, as it uses only single data points for PEC and PNEC to derive a risk conclusion.

The use of the risk quotient method is a common approach in ecological assessment and typically provides an important component of the weight of evidence. A risk quotient is calculated for each assessment endpoint using the following formula:

where:

PEC/PNEC

RQ = risk quotient
PEC = Predicted Environmental Concentration (derived during exposure characterization)
PNEC = Predicted No-Effect Concentration (derived during effects characterization)

The risk quotient approach for characterization of ecological risk usually involves the use of reasonable worst-case PEC and PNEC values and provides a conservative estimate of risk.

A risk quotient greater than or equal to 1 suggests that a substance may cause harm, whereas a risk quotient less than 1 suggests that a substance is unlikely to cause harm unless environmental concentrations increase. Care is taken to ensure that the assumptions used to obtain these risk quotients are realistic.

If a risk quotient analysis based on total concentrations of a substance does not suggest a risk, then the assessment can be concluded without correcting for bioavailability. If, however, a risk quotient based on total PEC and PNEC suggests a likelihood of harmful effects, then the characterization of risk may be refined based on consideration of bioavailability of the substance (which is often dependent on the substance's solubility).

If data permit, a probabilistic risk assessment may be performed. This type of assessment involves analyzing distributions of exposure and/or effects and presenting the results of the analyses in a variety of ways. It allows an in-depth consideration of sources of variability and uncertainty in the risk analysis.

In its simplest form, a probabilistic risk assessment may involve estimating the probability of exceeding the PNEC derived in the assessment by comparing an exposure distribution with a single PNEC value. However, probabilistic assessments may be more complex, using joint probability curves to compare exposure concentration distributions with distributions of adverse effects (e.g., species sensitivity distributions or concentration-response curves for sensitive organisms) in order to estimate the probabilities of differing magnitudes of adverse effects.

Evidence that a Substance has the Potential to Harm Organisms at Relatively Low Concentrations

Substances that have the potential to harm organisms at relatively low concentrations and/or that have specific modes of toxic action (e.g., substances that are carcinogens or endocrine modulators) are of particular concern. Such substances may occur at harmful concentrations in the environment, but exposure to them may be undetected or may not be suspected if it results from low-level but continuous releases.

Evidence that a Substance is Widespread in the Environment and/or that Concentrations have been Increasing Over Time

Evidence from monitoring studies indicating that a human-made substance is widespread in the environment and/or that concentrations have been increasing over time may indicate an elevated exposure potential and/or an increasing potential to cause harm.

It may be possible to incorporate trend information in a quantitative comparison of exposure and effects data. For example, it may be possible to predict what exposure concentrations would be, for example, in five years if a trend of increasing exposure continues. These predicted exposure values could be used in a risk quotient analysis or could be incorporated into a probabilistic assessment (see above).

Evidence that a Substance is Used in Canada in Moderate to Large Quantities

Evidence that a substance is used in Canada in moderate to large quantities, in a variety of locations, and/or that use quantities are increasing may also be taken as an indicator of significant potential for exposure.

Evidence that a Substance is Likely to Affect an Endangered or Threatened Species in Canada or that a Substance may be Present in a Protected or Particularly Sensitive Area at or Near a Level that could have Adverse Environmental Effects

Endangered and threatened species are apt to be susceptible to any additional environmental stressors. It is therefore important to take into account the likelihood that a substance would affect such species, even if exposure is localized. Similarly, the likelihood of a substance being present or becoming present in a protected or particularly sensitive area should be given close consideration. Ecological communities in such areas might be disrupted by additional stressors.

Restrictions in Other Jurisdictions

The determination that a substance has been or is being banned or severely restricted in another jurisdiction is an indication that a substance may meet the criteria in Section 64 of CEPA 1999. This is particularly true if the substance's use pattern and environmental releases and the environmental conditions, such as climate, types of biota, etc., in the other jurisdiction are similar to those in Canada.

6.2 Dealing with Uncertainty

Sources of uncertainty in the ecological assessment are related to information gaps and conflicting data pertaining to entry, exposure, and effects for each assessment endpoint. Within the ecological assessment, key areas of uncertainty are considered and may be discussed in detail when arriving at a final conclusion.

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