Dinitro-o-cresol final screening assessment: chapter 2

Characterization of ecological risk

As part of risk characterization, one line of evidence includes consideration of risk quotients to identify potential for ecological effects. Other factors that affect current or potential risks, such as persistence, bioaccumulation and trends in ambient concentrations, are also considered.

Risk quotient analysis

Critical exposure and effects results and risk quotients are summarized in Table 12 and described in more detail below.

Table 12: summary of data used in risk quotient (RQ) analysis of 4,6-Dinitro-o-cresol (DNOC)
Scenario Predicted environmental concentration (PEC) Critical toxicity values (CTV) Application factor Predicted no-effect concentration (PNEC) RQ
(PEC/ PNEC)
Pelagic organisms: Industrial release; rainbow trout 0.0014 mg/L 0.26 mg/L 100 0.0026 mg/L 0.54
Pelagic organisms: Rainfall; rainbow trout 0.0025 mg/L 0.26 mg/L 10 0.026 mg/L 0.096
Soil organisms: Earthworm 0.1 mg/kg 15 mg/kg dry weight 100 0.15 mg/kg dry weight 0.67
Wildlife consumers: Mink 0.0004 mg/kg-bw per day 0.35 10 0.035 mg/kg-bw per day 0.011
Wildlife consumers: River otter 0.000 007 mg/kg-bw per day 0.047 10 0.0047 mg/kg-bw per day 0.0015

Pelagic organisms

For pelagic organisms, a risk quotient was developed using the average 96-hour LC50 values of rainbow trout reported by Mayer and Ellersieck (1986) (0.066 mg/L) and Sewell et al. (1995c) (0.45 mg/L). The average of the two studies, which is the CTV, is 0.26 mg/L.

For the industrial release scenario, if sewage treatment plant (STP) treatment is considered (27% removal efficiency), the PEC will be 0.0014 mg/L. Using an application factor of 100 on the CTV to account for acute to chronic extrapolation and intra- and interspecies variations, differently sensitive biological endpoints and laboratory to field extrapolations, the PNEC is calculated to be 0.0026 mg/L.

The risk quotient is therefore calculated as:

PEC    = 0.0014 mg/L = 0.54
PNEC = 0.0026 mg/L

Even with STP removal considered, this represents a conservative scenario due largely to the very high quantity of DNOC assumed to be used by a single facility.

The maximum PEC under the defined rainfall scenario was determined to be 0.0025 mg/L with no STP treatment due to the assumption of a heavy rainfall. As rainfall represents an acute exposure scenario, the application factor does not need to account for acute to chronic extrapolation. Therefore, using an application factor of 10 and the same CTV of 0.26 mg/L for rainbow trout, a PNEC of 0.026 mg/L is calculated. The risk quotient is therefore:

PEC    = 0.0025 mg/L = 0.096
PNEC = 0.026 mg/L

Soil organisms

There are no quantified amounts of DNOC concentrations in Canadian soils. OMEE (1994) did not detect DNOC in 161 soil samples collected from soils in Ontario. The method detection limit of 0.1 mg/kg (100 ng/g) will be used as a surrogate for the level of DNOC in Canadian soil and is selected as the PEC.

One study was located in the literature on the effects of DNOC on terrestrial organisms. The LC50 from a 14-day acute toxicity study on the earthworm is 15 mg/kg of soil. This value is selected as the CTV for exposures of soil organisms to DNOC. Dividing the value by a factor of 100 to account for extrapolation from laboratory to field conditions, acute to chronic ratio and interspecies and intraspecies variations in sensitivity gives a PNEC of 0.15 mg/kg.

The risk quotient for soil organisms is therefore:

PEC    =  0.1 mg/kg  = 0.67
PNEC = 0.15 mg/kg

Aquatic wildlife

The PECs for the mink and river otter were estimated to be 0.0004 mg/kg-bw per day and 0.000 007 mg/kg-bw per day, respectively. The PNEC for the mink was estimated to be 0.035 mg/kg-bw per day, and the PNEC for the river otter was calculated to be 0.0047 mg/kg-bw per day.

The risk quotients for aquatic wildlife are thus calculated to be:

 

The risk quotients for aquatic wildlife

Benthic organisms

No monitoring data for DNOC in sediments in Canada were identified. Level III multimedia fate simulation estimated that only about 1% of DNOC is expected to partition to sediments. It is therefore believed that there will be minimal exposure of benthic organisms to DNOC.

Weight-of-evidence analysis

The risk quotient analyses for pelagic and soil organisms and wildlife have shown that it is unlikely that organisms are currently exposed to concentrations of DNOC above known effect thresholds. This conclusion is based on import levels and locations where DNOC was used industrially in the year 2000, and the current state of knowledge of its atmospheric chemistry.

A conservative scenario based on concentrations of DNOC in precipitation that could be expected to enter Canadian receiving water indicated that the potential for risk to aquatic organisms from this source is low.

In addition, modelling estimates of industrial releases to the St. Clair River indicate that DNOC is not likely to have adverse effects on pelagic or benthic organisms. This is based on a conservative release scenario developed for a facility located in the same region as the one company that reported use of DNOC in 2000 in response to a notice published under section 71 of Canadian Environmental Protection Act (1999) (CEPA) and that reported to the National Pollutant Release Inventory (NPRI). It is noted that the reporting facility ceased use of DNOC in late 2002.

Although sorption is low at environmentally relevant potential of hydrogen (pH)s, little leaching to groundwater has been found, likely due to biodegradation.

Potential sources of release of DNOC to the environment are to air and water. Based on its properties, DNOC is persistent in air and water but is not bioaccumulative. Long-range transport modelling estimates that it will be transported over moderate distances, and a decreasing concentration with increasing latitude is expected.

Uncertainties in evaluation of ecological risk

There are uncertainties associated with development of the PNECs used in this assessment. However, a moderate number of empirical studies from different sources were identified, and this increases confidence in the values. Application factors of 10-100 were used to account for information gaps relating to chronic toxicity, effects in the field, and effects on potentially more sensitive species.

Very few Canadian monitoring data are available for DNOC, and those that were identified were fairly old. To both support the limited amount of empirical data and provide greater insight into the potential range of levels of DNOC in the environment, releases were estimated and fate and exposure were modelled. Entry of DNOC into the environment from two sources was considered--industrial releases and precipitation containing DNOC scavenged from the atmosphere. To address the significant uncertainty in these estimations, conservative assumptions were used to ensure that errors would be protective of the environment.

Although there have been no reports of direct releases of DNOC to water from industrial facilities, a conservative scenario was developed to estimate possible releases from an industrial source. This conservatively assumed an upper-limit estimate of the quantity of DNOC potentially used by a single facility; a slightly conservative estimate of the fraction of substance typically released due to handling practices for a substance used in bulk; and a low-percentile estimate of river flow for the receiving water body used in the scenario. Flow characteristics of the St. Clair River were used in the exposure scenario, as the only facility that had reported use of DNOC was located close to this water body. This river is extremely fast flowing and consequently disperses effluents very rapidly. Were there to be facilities with substantive releases to smaller water bodies, then the assumptions used in this scenario might not be sufficiently protective. However, it is believed that there are currently no large users of DNOC in Canada, and it is possible that the substance is no longer in commercial use in Canada.

Estimation of possible exposure from atmospherically generated DNOC in precipitation conservatively assumed that the concentration in the atmosphere in Canada would be similar to that in more heavily populated regions of Europe; that the rainfall event would be particularly heavy; that a high percentage of precipitation from a census subdivision would be released to the receiving river body through a single discharge point; and that there would be no removal of DNOC by the municipal STP. In particular, the assumption that atmospheric concentrations in Canada would be the same as average to high concentrations in Germany, which is much more heavily populated and industrialized, is uncertain. While it is believed that use of monitoring data from Germany in the scenario is conservative, the origins of atmospherically generated DNOC are at present not well understood, and no Canadian atmospheric monitoring data were identified for comparison.

Potential to cause harm to human health

Exposure assessment

The upper-bounding estimate of exposure to DNOC for the general population is 0.06 µg/kg-bw per day for the 0- to 6-month (formula-fed) age group, based on very limited data from Canadian surveys of drinking water and soil (OMEE 1994; City of Toronto Water and Wastewater Services Division 2002a, 2002b, 2002c, 2002d) and an estimated concentration of DNOC in air in Switzerland (Leuenberger et al. 1988) (see Appendix 1). No quantitative data on levels of DNOC in food were identified. Confidence in the database for estimating exposure is considered moderate, since there is information for conservative estimation of exposure through drinking water and air, the likely principal media of exposure. The levels of DNOC in drinking water were below the detection limit; thus, estimates based on the detection limit likely overestimate exposure. The concentration of DNOC in air was estimated from rain samples but is considered to be conservative, as it is higher than levels measured in automobile exhaust, a source of DNOC (Tremp et al. 1993).

Health effects assessment

A health assessment of DNOC was published by the International Programme on Chemical Safety (IPCS) in 2000 (see Appendix 2 for an overview of the toxicological database, in which confidence is considered to be high, in view of the wide range of toxicity studies available). Although the IPCS did not select a critical study for use as a basis of a tolerable intake or guidance value, the lowest-observed-effect level (LOEL) identified in that review that is considered to be the critical effect level is 2.5 mg/kg-bw per day in a 90-day rat dietary exposure study, with resulting dose-related decreases in blood pyruvate and triiodothyronine levels (Den Tonkelaar et al. 1983). Although several lower effect levels were reported in the IPCS assessment, there was less confidence in these studies due to the fact that insufficient details were available; however, these lower values were generally within an order of magnitude of the effect level considered to be critical. Similarly, in very early clinical investigations of the potential application of DNOC in the treatment of obesity, effects associated with increases in basal metabolic rate were observed in individuals administered doses in the range of this critical value. DNOC was not carcinogenic in the only long-term study identified (Broadmeadow 1991), and the weight of evidence for genotoxicity was considered to be equivocal by the IPCS (2000), as positive results were observed in some but not all in vivo assays in which rodents were administered doses generally greater than the critical effect level for non-neoplastic effects. Similarly, the results of modelling of in vivo and in vitro genotoxicity endpoints are also equivocal.

Confidence in the database upon which the critical effect level is based is considered to be high in view of the wide range of toxicity studies available (that is, acute toxicity, repeated dose toxicity, chronic toxicity/carcinogenicity, genotoxicity, reproductive and developmental toxicity and immunotoxicity). There is some uncertainty concerning lower effect levels reported in secondary accounts of studies for which original reports could not be obtained; however, since these values are generally within an order of magnitude of the effect level considered to be critical, they would not alter the conclusion of the screening assessment. There is also uncertainty with regards to the potential genotoxicity of DNOC, as the IPCS (2000) concluded it to be equivocal.

Characterization of risk to human health

Comparison of a conservatively selected lowest effect level (that is, 2.5 mg/kg-bw per day) for slight changes in biochemical parameters in a 90-day study in rats to the highest of the upper-bounding estimates of exposure for all age groups in the population (that is, 0.06 μg/kg-bw per day) for the 0- to 6-month (formula-fed) age group resulted in a margin of exposure of approximately 41 700. In light of the moderate to high confidence in the databases on exposure and effects upon which this assessment is based and the conservative nature of this evaluation, including the use of an upper-bounding exposure estimate and lowest effect level, this margin is considered adequate to address elements of uncertainty associated with limitations of the database for health effects and population exposure and intraspecies and interspecies variations in sensitivity, as well as the biological adversity or severity of the effects deemed critical.

Conclusion

Based on the information presented in this screening assessment, it is concluded that DNOC is not entering the environment in a quantity or concentration or under conditions that have or may have an immediate or long-term harmful effect on the environment or its biological diversity or that constitute or may constitute a danger to the environment on which life depends. In addition, it is concluded that DNOC is not entering the environment in a quantity or concentration or under conditions that constitute or may constitute a danger in to human life or health.

It is therefore concluded that DNOC does not meet the criteria in section 64 of the Canadian Environmental Protection Act, 1999. Additionally, DNOC meets the criteria for persistence but does not meet the criteria for bioaccumulation set out in the Persistence and Bioaccumulation Regulations (Canada 2000).

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