Technical approach for "rapid screening" of substances of lower ecological concern



The following approach has been developed by Environment Canada for the rapid screening of low volume substances. The approach, as illustrated in Figure 1, consists of multiple steps that address different factors relating to the potential for a substance to cause ecological harm. The approach is intended to be pragmatic, protective and fairly rapid, largely making use of available or easily obtainable data and either "mechanical" or simple "manual" evaluation of this data. The approach ensures consistent handling of all substances and is based in art on estimation techniques similar to those used by the New Substances Program when evaluating substances proposed to be introduced into commerce in Canada.

The first step consists of identifying substances that belong to categories that are considered higher priorities for further evaluation as part of a category assessment. For example, this includes substances whose chemical structure is similar to those of substances that were identified as substances that are persistent and bioaccumulative and inherently toxic to non-human organisms (PBiT) through the categorization process. These substances are identified at this step as requiring further assessment (beyond the rapid screening process), and do not proceed further through rapid screening.

The second step in the approach involves applying different exposure scenarios through use of environmental fate models. First, two generic aquatic exposure scenarios are applied (described further on as scenarios A and B) to identify potential concerns near the point of discharge of a substance to the environment. This involves comparing conservative estimates of exposure in receiving waters with an effects threshold to evaluate whether a chemical is expected to cause harm to the local aquatic environment. A regional multi-media model named RAIDAR (Risk Assessment, IDentification And Ranking) is also applied. This fugacity-based model (described further on as scenario C) takes into account the combined characteristics of the substance in estimating potential harm in different environmental media, as well as in food chains.

There are two possible outcomes from Step 2:

  • if the scenarios indicate a potential harmful effect to aquatic or terrestrial organisms, the substance is identified as requiring further assessment; or
  • if the scenarios indicate a low likelihood of harm to these organisms, then the substance proceeds to the third step of rapid screening.

Figure 1: Overview of the rapid screening approach

Overview of the rapid screening approach (see long description below).
Description of figure 1

Figure 1 is a process chart diagram consisting of candidate substances and a three step process. Step 1 consists of assessment categories, step 2 consists of generic aquatic exposure scenarios and step 3 consists of either mechanical filters or a manual process. Both lead to unlikely meeting the criterion of paragraph 64(a) of the Canadian Environmental Protection Act, 1999. All three steps may require further assessments.

The third step of the approach uses "filters" (i.e., various information sources) and involves identifying whether or not a substance appears on different lists or sources of information relating to hazard or exposure (including quantity in commerce). This flags substances that have been identified by domestic or international sources as being of greater concern due to their hazard properties, or which may now be in commerce at greater quantities than was believed to be the case based on the available information.

Depending on the nature of the information sources, substances flagged by the filters may be further evaluated within rapid screening using a "manual process". This step involves case-by-case evaluation to decide, for example, whether the information in the source that flagged the substance is relevant to the situation in Canada. This may also involve collection and review of information from other sources that are not as amenable to evaluation using a mechanical approach. The manual process involves evaluation of the weight and relevance of information obtained from the full range of sources identified.

Depending on the conclusion from consideration of all information that was obtained, substances are either identified as requiring further assessment (beyond the rapid screening approach), or as being unlikely to cause harm.

A more detailed description of each of these steps is provided in the following sections.

First step: Assessment categories

The first step of the rapid screening approach is to determine whether candidate substances have chemical structures similar to those for substances identified for priority actions as a category. At this time, such chemical categories have been identified based on substances that were concluded to be PBiT under the categorization process. These include discrete organics and UVCBs (Unknown or Variable composition, Complex reaction products or Biological substances) that meet the PBiT categorization criteria. At this stage of the rapid screening process, substances that fall into one of the identified chemical categories are directed to the group of substances that require further screening assessment.

Second step: Exposure estimation

Three different exposure scenarios are applied as part of the rapid screening approach. Figure 2 illustrates these exposure estimation approaches. These approaches are used to conservatively estimate local and regional ecological exposure, making use of available data from the Domestic Substances List (DSL) notification and from DSL categorization activities.

Data from DSL notification includes use and quantity information from each reporting facility. Data collected or estimated during categorization includes "pivotal" values for acute aquatic toxicity (iT), persistence and bioaccumulation, as well as physical/chemical properties.

While the generic aquatic exposure scenarios (A and B) have been developed to be conservative overall, the level of conservatism applied to individual parameters is moderate, since it is recognized that:

  • a high level of conservatism applied to each parameter can easily compound into an excessively conservative overall exposure scenario;
  • it is very unlikely that each parameter would be "worst case" at the same time; and
  • interdependency of some parameters exists.

Rather, values in keeping with an overall realistic worst case scenario have been used.

Figure 2: Exposure scenarios

Exposure scenarios (see long description below).
Description of figure 2

Figure 2 is a process chart diagram consisting of the life cycle stage, the release, the exposure and the evaluation and of three possible scenarios.

The first scenario consists of manufacture & blending leading to aquatic release (5 %) leading to local aquatic exposure leading to compare to aquatic effects threshold leading to requiring further assessment or proceeds to step 3.

The second scenario consists of manufacture & blending leading to product use & disposal (down-the-drain) leading to aquatic release (100 %) leading to local aquatic exposure from multiple municipal sewage treatment plants leading to compare to aquatic effects threshold leading to requiring further assessment or proceeds to step 3.

The third scenario consists of manufacture & blending leading to product use & disposal (lifecycle release) leading to multimedia fugacity model – RAIDAR (critical emission rate) leading to compare to quantity in commerce leading to requiring further assessment or proceeds to step 3.

Scenario A - Industrial point-source aquatic release

Scenario A is based on release from an industrial facility that is manufacturing the substance, or blending it into products. A conservative estimate of exposure resulting from the release of the substance to the aquatic environment from such an industrial point source is calculated as shown in the following equation. The aquatic estimated no-effect level is derived as shown in the second equation. Parameters used in Exposure Scenario A are described in Table 1.

Exposure (mg/L)

Qty × Loss × (1-Remov)
Dur × (Rflow + Sflow)

1 000
86 400


Aquatic Estimated No Effect Value (mg/L)



The exposure value is then compared to the estimated no effect value to determine a risk quotient (Exposure / Effect). If the risk quotient is greater than one, this indicates that the conservatively estimated concentration in water exceeds the aquatic estimated no-effect level and that there exists a potential to cause harm in the aquatic ecosystem. On the contrary, a value below one indicates that concentrations that may cause an effect to sensitive aquatic organisms are not reached and therefore harm to aquatic organisms is unlikely under this scenario.

Table 1 - Parameters used in Exposure Scenario A
QtyMaximum quantity of substance used at one facility100 or 1000kgSubstance specific
LossLoss of substance during manufacturing or handling5%Based on New Substances Program and US EPA conservative estimates of loss from cleaning of container residues (3%), transfer lines (1%) and reactors (1%)
RemovSewage Treatment Plant (STP) removal efficiency70%Conservative value for secondary treatment, recognizing biodegradation and sludge adsorption
DurDuration over which substance is released150daysAssumes seasonal use of substance
SflowSTP flow rate0.04m3/s10th percentile of municipal STP flow rates
RflowFlow of receiving watercourse1.84m3/s15th percentile of the distribution of receiving watercourse flows in the country (based on the distribution of the 50th percentile of flow rates); weighted by number of industries releasing to the receiving watercourse
-Factor combining conversion from kg to mg and m3 to L1000  
-Conversion factor from days to seconds86 400  
CTVCritical Toxicity Value mg/LSubstance specific; acute aquatic toxicity from categorization (iT pivotal value)
AFApplication factor100 Acute-to-chronic; lab to field; inter-species

Scenario B - Down-the-Drain Aquatic Release from Products

Under Scenario B, a value for aquatic exposure from down-the-drain release of a substance contained in products (such as soaps) is calculated, as well as a value for the aquatic estimated no-effect level, as defined in the equations below. Parameters used in the Exposure Scenario B are described in Table 2 below.

Exposure (mg/L)

Qty × Loss × (1-Remov) × Pop
Dur × RPE × (Rflow + Sflow)

1 000
86 400


Aquatic Estimated No Effect Value (mg/L)



As was the case for Scenario A, the exposure value and the effects value are combined to determine a risk quotient (Exposure / Effect), which indicates a potential risk if the value is above 1 in this conservative scenario.

Table 2 - Parameters used in Exposure Scenario B
QtyTotal quantity of substance used in CanadaUp to 1 000kgSubstance specific
LossLoss of substance from product during use100%Complete loss for down-the-drain products
RemovSTP removal efficiency70%Conservative value for secondary treatment, recognizing biodegradation and sludge adsorption
PopPopulation of representative community100 000personsValue corresponding to the 10th percentile of the distribution of receiving watercourses weighted by population
DurDuration over which substance is released150daysAssuming seasonal use of substance
RPERegional product effect2 000 000personsValue set to represent population of a Canadian region in which total quantity of product could be used
SflowSTP flow rate0.66m3/sValue corresponding to the 10th percentile of the distribution of receiving watercourses weighted by population
RflowFlow of receiving watercourse3.58m3/sValue corresponding to the 10th percentile of the distribution of receiving watercourses weighted by population
-Factor combining conversion from kg to mg and m3 to L1 000  
-Conversion factor from days to seconds86 400  
CTVCritical Toxicity Value---mg/LSubstance specific; acute aquatic toxicity from categorization (iT pivotal value)
AFApplication factor100 Acute-to-chronic; lab to field; inter-species

Note that river flow distributions used in the two scenarios are different. Likelihood of harm from industrial releases (scenario A) is dependent on the number of industrial facilities releasing to a water body. Therefore, for that scenario, a distribution of the dilution capacities of receiving waters (receiving watercourse flow rates) was generated with a weighting by the number of industrial facilities releasing to the water body. Likelihood of harm from down-the-drain release of consumer products (scenario B) is dependent on the human population that may be releasing a substance to a municipal sewage treatment plant. In this scenario, a distribution of the ratio of population of the community to the dilution capacity of the receiving water body was generated. As a result, the parameters "population of representative community", "STP flow rate" and "flow of receiving water course" are inter-connected. In this scenario, it is this ratio that is important, not the actual values of the population or flow rates.

Scenario C - Life-cycle release

Scenario C uses a fugacity-based multimedia modelling approach to address possible release of the substance over its full life-cycle. Such models allow substances released to the environment to be distributed throughout a unit world - and are thus suitable for a disperse release scenario from all stages of the substance life-cycleFootnote 2.

This modelling approach also provides a "safety net" scenario, since it accounts for combined effects of a substance’s physical/chemical and hazard properties as well as considerations for different environmental media (water, air, soil, sediment) and organisms.

Description of the model

Risk Assessment, Identification And Ranking (RAIDAR) is a peer-reviewed fugacity-based model developed by the Canadian Environmental Modelling Network (CEMN) to assess chemicals for risk by estimating environmental fate and transport, bioaccumulation and exposure to organisms, and determining a critical emission rateFootnote 3.

Representative food webs are included to assess chemical exposure routes to organisms in the environment. The food web models take the output from the fate and transport calculations for the substance (the concentration in the different environmental media) and estimate internal concentrations in some 20 biotic groups including plankton, vegetation, domestic animals, fish and wildlife. This uses data on the nature and quantity of diets, and respiration and growth rates. Essentially, each organism absorbs the chemical by respiring air (or by exchange at the gill-water interface in the case of fish) or by consuming water and other organisms (plants or animals). The concentration of the substance in each organism is generally calculated using these rates, absorption efficiencies, and the concentration in the respective media. The steady-state concentration in the organism is calculated from an input-output mass balance. The result is an estimate of fugacity and concentrations in the biota.

Using a multi-level, multi-media foodchain, the most sensitive endpoint is identified (based on toxicity and exposure potential) and a "critical emission rate" is then calculated based on that sensitive endpoint. The estimated critical emission rate is then compared with an estimated potential emission rate (based on quantities in commerce) to determine a "risk assessment factor" or RAF.

Substances are ranked according to their critical emission rates and their RAF values. Substances identified as having greater potential for harm are thus identified as requiring further assessment. The model output also indicates substances which are unlikely to be of concern owing to release to the environment through their life-cycle.

Features and limitations

RAIDAR can be applied to substances for which little or no empirical property data are available and emission rates are known only approximately. Although the uncertainties in output may be high, the results may be used to sort substances into groups of similar concern and thus compare lower and higher risk potential.

RAIDAR can be applied as a Level II fugacity model (L II; where results do not depend on media of release) or as a Level III model (L III; where results are affected by the media of release). Typically, for the Level III model, four release scenarios are considered, as the principle medium of release is often not known: 1) 100% release to air (A); 2) 100% release to water (W); 3) 100% release to soil (S) and 4) 33% release to each of air, water and soil (AWS). The scenario that has been chosen for detailed evaluation in rapid screening is the LIII with releases to air, water and soil (AWS). It is the individual scenario that is most sensitive to releases to any of the three media (which are generally not known without detailed evaluation), and is never more than a factor of three away from the most conservative scenario (i.e., assumed 33% release to a medium, versus a maximum possible release of 100% to the medium).

RAIDAR also allows two options for addressing possible chemical biotransformation in food webs. The first approach assumes no metabolic biotransformation. The second includes estimated rates of metabolic biotransformation in fish, birds and mammals. For the purpose of rapid screening, it is assumed that there is no metabolism of the substance, as this is the more conservative approach.

It must be recognized that for RAIDAR, as for all models, there are limits to the range of suitable application of the model. As with other fugacity models, RAIDAR is intended for use at a regional geographic scale, and its results can not be meaningfully interpreted at a local scale (i.e., in the area immediately surrounding a point-source of discharge). As such, its results are complementary to those provided by generic exposure scenarios A and B. Further, as outlined in a report on the application of RAIDAR in rapid screeningFootnote 4, there are some classes of substances (e.g. inorganic substances) for which application of the model was not designed or may not be appropriate. Substances belonging to such classes are identified and the model is not applied to them.

Applicability for rapid screening

The RAIDAR model is considered to be relevant in the context of the rapid screening approach. Estimates produced by the model are based on partitioning of substances between environmental media as a function of their physical/chemical properties, food chain transfer, persistence, bioaccumulation potential and toxicity (critical body residue). This allows the integration of different fate and exposure considerations that cannot necessarily be, or have not been, included in the single-medium exposure scenarios A and B (which were focused on aquatic releases and effects). RAIDAR can therefore provide additional information on each substance, by providing a multi-media representation of fate and exposure potential for aquatic and terrestrial organisms.

For the purpose of the rapid screening approach, the critical emission rate, the RAF and the media of concern are the most important outputs of RAIDAR. The use of the critical emission rate and RAF allow identification of chemicals that are unlikely to be of concern because of their limited potential for exposure. For example, even if the currently available quantity in commerce information for certain chemicals is uncertain, if the critical emission rate for that substance is well above the maximum potential emission rate (therefore showing a low RAF), the substance is unlikely to be released in quantities that could lead to ecological effects. Additionally, the identification of the most sensitive ecological endpoint allows consideration of environmental media and/or types of organisms that may not have been previously addressed in the rapid screening exposure scenarios A and B.

Third Step: Mechanical filters and Manual process

The purpose of this step is to locate substance-specific information that would either support or question the assumption that the candidate substances for rapid screening are in commerce in low quantities, or that indicate that there may be particular concern associated with the hazard characteristics of some substances, and would indicate whether it is justified, given the available information, to conclude that:

  • the substance is unlikely to cause harm; or
  • further assessment of the substance is required (beyond rapid screening).

A two-part approach was developed that involves searching through different available information sources relating to quantities of substances in commerce or to their hazard properties. This approach involves initially the "mechanical" use of filters to flag substances of potential concern, and secondly a "manual" approach in which further details are sought on selected substances.

Mechanical filters

The mechanical filters stage involves comparison of Chemical Abstract Service (CAS) numbers of substances subject to rapid screening, with those that have been extracted from a wide range of lists or information sources on chemicals. Use of the filters involves a number of different steps. Firstly, different domestic and international sources of information or different lists of substances (relating to quantity in commerce, release quantity, hazardous substances, regulations, etc.) were identified. Information sources were identified based on past experience in the Existing Substances Program as well as through discussion with various stakeholders. This initial list of information sources is aimed at representing a good cross-section of the types of available information.

The second step involved understanding the basis of each list or information source - for example, the criteria that a substance must meeting in order to be included on a specific regulatory list. This information was used to determine if the list or information source is relevant to the rapid screening exercise. Lists or information sources that are judged to provide appropriate information are retained for use whereas other sources are put aside. Information sources relating to quantity in commerce or industrial information pertinent to Canada (or the US) are judged particularly relevant; however, information from other countries is also taken into account. Hazard information (e.g., lists of substances of concern) is judged on whether the endpoints are relevant to ecological receptors, and on whether new information is provided that was not necessarily considered at a previous stage of rapid screening. Other sources of information, such as technical databases, are also considered as indicators of the amount of information potentially available for a substance, as this may reflect the level of commercial interest in it.

Many sources of information have been evaluated. In selecting which lists or information sources to apply in rapid screening, there was an effort to limit the amount of overlap between lists. For example, clear secondary sources of information were removed if the primary source of information was also included. Thus, some sources of information have been retained for the purpose of rapid screening (see appendix A) and others have not (see appendix B). Each appendix explains the basis for retaining or excluding a list or source of information for the purpose of rapid screening.

A number of information sources were judged to be relevant for rapid screening, but were not amenable to being searched mechanically. These sources were directed to the manual process stage, discussed below.

The last step was to develop, based on the available information concerning the basis of each list or information source, a weighted approach to account for overall significance of the information. In order to make these decisions, lists and information sources were separated into three categories: 1) exposure - quantities, releases and industrial information, 2) hazardous substances lists or substance profiles, and 3) miscellaneous databases on physical-chemical and hazard properties of substances.

  • "Exposure - quantities, releases and industrial information" - These lists or sources of information are considered very relevant in the context of the rapid screening approach, because they could provide new information indicating whether or not the assumption that a substance is currently in use in Canada at low quantities is correct.
  • "Hazardous substances lists or substance profiles" - The presence of a substance on one of these lists indicates the possible existence of information relating to toxicological endpoints or other concerns that deserve additional consideration. Hazard information is judged on whether the endpoints are relevant to ecological receptors, the level of their applicability to Canada and whether they provided new information not considered at a previous stage of rapid screening.
  • "Miscellaneous properties and hazard databases" - The presence of a substance on one of these lists or information sources will in most cases only indicate that some information concerning it is available. However, presence on such a list or data source is still considered as a flag, because, typically, data are available for substances of higher interest. Several different sources of information have been selected to represent a good overview of what information is available.

The use of the mechanical filters only indicates whether a list or source of information includes a given substance. The overall significance of hits that a substance receives from the mechanical filters is evaluated according to a simple weighting system. The weight given to a list or source of information is based on the level of relevance of the information it contains. Some lists or information sources are considered to be of sufficient relevance that it is immediately concluded that a substance that appears on them requires further assessment (beyond the rapid screening process). Other lists or sources of information in the first and in the second category of filters are given a weight of one. One "hit" in one of these lists or sources is sufficient to conclude that further evaluation of the information is needed as part of the manual process. The third category ("Miscellaneous properties and hazard databases") are given a weight of one half, recognizing their more anecdotal nature. Two hits in this category are needed to proceed to the manual process for further evaluation. Appendix C summarizes the path resulting from a hit, for each of the mechanical filters.

Manual process

Substances subject to the manual process have already been flagged by one or more of the information sources at the mechanical filters stage, as described above. The manual process provides an opportunity to look in more depth and on a substance-by-substance basis at:

  • the relevance of the information source to the specific substance,
  • any substance-specific data contained in the information sources that flagged it (e.g., evaluation of temporal trends from international data on quantities of a substance in commerce), and
  • whether the substance is listed in an information source that was not amenable to mechanical search.

Other, more general sources of information are also considered at the manual process stage.

Information sources used at this stage are grouped as follows:

  1. Information on substances from specific sources of information that were flagged at the mechanical filter stage (e.g. NPRI data, information collected under S.71 of CEPA 1999)
    1. Exposure - quantities, releases and industrial information
    2. Hazardous substances lists or substance profiles
    3. Miscellaneous properties and hazard databases

  2. Information from additional sources that were not part of the mechanical filter stage:
    1. General quantities and release information for substances
    2. General use and industrial information (e.g. industry or use sector) and other related information
    3. Hazard, properties and other

Not all sources of information listed for the manual process are consulted for all substances. The approach involves examining applicable sources of information from group A, and then moving to group B only if sufficient information to make a conclusion has not yet been identified. Using this approach, relevant information is obtained without the need to consult all the listed information sources for all substances - only substances for which very little information is identified are searched on all databases.

Appendix A describes the groups of information sources that are used at the mechanical filters stage to first flag substances. These sources are also used at the manual process stage to evaluate any substance specific information contained in them. Appendix D lists the groups of additional sources of information that are consulted at the manual process stage only.

Once relevant information sources have been consulted at the manual process stage, the information is evaluated and weighed, and a conclusion is reached that:

  • the substance is unlikely to meet the criterion set out in paragraph 64(a) of CEPA (1999); or
  • further assessment of the substance is required.
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