Dinitro-o-cresol final screening assessment

Official title: Screening Assessment for Phenol, 2-methyl-4,6-dinitro- (DNOC), Chemical Abstracts Service (CAS) Registry Number 534-52-1

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Environment Canada
Health Canada

November 2009


Under the Canadian Environmental Protection Act, 1999 (CEPA), the Ministers of the Environment and of Health have conducted a screening assessment of Phenol, 2-methyl-4,6-dinitro-, also known as 4,6-dinitro-o-cresol (DNOC), Chemical Abstracts Service Registry Number 534-52-1, which was selected as one of 123 substances on the Domestic Substances List for a pilot project for screening assessments.

DNOC is used predominantly in the plastics industry as an inhibitor of polymerization in styrene and vinyl products. Results from a section 71 Notice with Respect to Certain Substances on the Domestic Substances List (DSL) conducted for the year 2000 indicated that, although DNOC was not manufactured in Canada, 100 to 1000 tonnes were imported at that time. DNOC is included in the National Pollutant Release Inventory (NPRI), and facilities manufacturing, importing or otherwise using more than 10 tonnes per year of the substance must report their releases. The one company that reported to the NPRI for the years 1994 to 2002 ceased use of DNOC in late 2002. There have been no reports to the NPRI for this substance since 2003.

DNOC was detected in surface water and sewage sludges but not in sediment in Canadain the early 1980s. No more recent monitoring data for these media were identified. DNOC was not detected in rural, urban or agriculture soil from various locations across Canada. No Canadian air or groundwater monitoring data were identified.

It is believed that industrial uses of DNOC could result in releases of the substance to surface waters. A conservative scenario developed to account for potential releases from industrial process losses indicated a low potential for risk to aquatic organisms.

Scientific studies have shown that DNOC may form in air by reaction with reactive species such as OH and NO radicals, although the extent and mechanisms of formation are not well understood at present. 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.

Toxicity data for DNOC are available for microorganisms, bacteria and mammals, and for aquatic and terrestrial plants, invertebrates and vertebrates. No data on toxicity were identified for amphibians or marine organisms.

DNOC is persistent in water and air but is not bioaccumulative. The substance therefore meets the persistence criteria but does not meet the bioaccumulation criteria set out in the Persistence and Bioaccumulation Regulations. Modelling indicates that it is not likely to be transported over very long distances, and a decreasing concentration with increasing latitude is expected.

Based on available information, 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 effect on the environment or its biological diversity or that constitute or may constitute a danger to the environment on which life depends. Therefore, it is concluded that DNOC does not meet criteria set out in paragraphs 64(a) and 64(b) of the Canadian Environmental Protection Act, 1999.

Sources of human exposure to DNOC in Canada are likely to be limited to fugitive releases from industrial sites and the combustion of fossil fuels. There is no indication that DNOC is present in consumer products.

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, 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.

The outcome of this screening health assessment is that DNOC does not meet the criterion set out in paragraph 64(c) of CEPA, that is, it is not entering the environment in a quantity or concentration or under conditions that constitute or may constitute a danger in Canada to human life or health. This determination is based on the adequacy of the sufficiently health-protective margin between a conservatively selected lowest effect level and upper-bounding estimates of exposure of individuals in the general population.

Based on the information available for environmental and human health considerations, it is concluded that DNOC does not meet any of the criteria set out in section 64 of the Canadian Environmental Protection Act, 1999.


This screening assessment report was conducted pursuant to section 74 of CEPA. This section of the act requires that the Ministers of the Environment and of Health conduct screening assessments of substances that satisfy the categorization criteria set out in section 73 of the act in order to determine of they meet or may meet the criteria set out in section 64 of the act.

Screening assessments focus on information critical to determining whether a substance meets the criteria for defining a chemical as toxic as set out in section 64 of CEPA (Canada 1999). Screening assessments examine scientific information and develop conclusions by incorporating a weight-of-evidence approach and precaution.

A screening assessment was undertaken on Phenol, 2-methyl-4,6-dinitro-, also known as 4,6-dinitro-o-cresol (DNOC; CAS RN 534-52-1), on the basis that this compound was included in the Domestic Substances List (DSL) pilot project for screening assessment as a substance likely to be prioritized because it met the criteria for persistence and/or bioaccumulation and inherent toxicity to non-human organisms and as a substance likely to be prioritized on the basis of greatest potential for human exposure.

Owing to the chemical nature of DNOC, it readily forms water-soluble sodium, potassium and ammonium salts, and virtually 100% of dissolved DNOC will be in the ionized form at environmentally relevant potential of hydrogens (pHs) 6-8. Based on this information, Environment Canada reviewed the use of DNOC salts to determine if they should also be included in this screening assessment. The sodium, potassium and ammonium salts of DNOC are not on the Domestic Substances List (DSL), although DNOC sodium salt is on the Non-Domestic Substances List (NDSL). If a company were intending to manufacture or import these substances, they would be considered to be new to Canada and subject to notification under the New Substances Notification Regulations (Chemicals and Polymers). Therefore, although the screening assessment focused on the uses of DNOC, a review of the fate and effects of its salts was also carried out.

This screening assessment includes consideration of information on chemical properties, hazards, uses and exposure. Data relevant to the screening assessment of this substance were identified in original literature, review and assessment documents, stakeholder research reports and from recent literature searches, up to August 2004 for ecological sections of the document and June 2003 for human health sections of the document. In addition, an industry survey was conducted in 2000 through a Canada Gazette notice issued under authority of section 71 of CEPA.

The screening assessment report does not present an exhaustive or critical review of all available data. Rather, it presents the critical studies and lines of evidence pertinent the conclusion. One line of evidence includes consideration of risk quotients to identify potential for ecological effects. However, other concerns that affect current or potential risk, such as persistence, bioaccumulation, chemical transformation and trends in ambient concentrations, are also considered.

Evaluation of risk to human health involves consideration of data relevant to estimation of exposure (non-occupational) of the general population, as well as information on health hazards. Decisions for human health are based on the nature of the critical effect and/or margins between conservative effect levels and estimates of exposure, taking into account confidence in the completeness of the identified databases on both exposure and effects, within a screening context. The screening assessment does not represent an exhaustive or critical review of all available data. Rather, it presents a summary of the critical information upon which the conclusion is based.

This screening assessment was prepared by staff in the existing substances programs at Health Canada and Environment Canada. The substance matter in this report pertaining to ecological aspects has been subjected to external review. The report for a screening health assessment was reviewed externally by V.C. Armstrong (Consultant) and staff of Toxicology Advice and Consulting Limited. While external comments were taken into consideration, the final content and outcome of the screening risk assessment remain the responsibility of Health Canada and Environment Canada. Additionally, the draft of this screening assessment was subject to a 60-day public comment period from June 23, 2007, to August 22, 2007. The State of the Science Report for a Screening Health Assessment has been posted on the Health Canada website since January 30, 2006, and the draft ecological screening assessment report on 4,6-dinitro-o-cresol (DNOC) has been posted on the Environment Canada website since July 2006.

Information on ecological and human health screening assessments under CEPA may be linked from the Canadian Environmental Protection Act Registry.

The critical information and considerations upon which the assessment is based are summarized below.

Substance identity

Substance name

For the purposes of this document, this substance will be referred to as DNOC, a common name for this substance.

Table 1: substance identity for Phenol, 2-methyl-4,6-dinitro- (DNOC)
Chemical Abstract Service Registry Number (CAS RN) 534-52-1
DSL name Phenol, 2-methyl-4,6-dinitro-
National Chemical Inventories (NCI) namesa Phenol, 2-methyl-4,6-dinitro- (TSCA, AICS, PICCS, ASIA-PAC, NZIoC) 2-methyl-4,6-dinitro-phenol (EINECS) 4,6-dinitro-o-cresol (ENCS) 2,2'-methylene-bis-(4-methyl-6-tert-butylphenol) (PICCS)
Other names 2,4-dinitro-6-methylphenol; 3,5-dinitro-2-hydroxytoluene; 4,6-dinitro-2-methylphenol; 6-methyl-2,4-dinitrophenol; Antinonin; Antinonnin; Arborol; Degrassan; Dekrysil; Detal; Dillex; Dinitro; Dinitro-o-cresol; Dinitrocresol; Dinitrodendtroxal
Chemical group (DSL Stream) Discrete organics
Major chemical class or use Phenols
Major chemical sub-class Aromatic phenols
Chemical formulab C7H6N2O5
Chemical structure Chemical Structure CAS RN 534-52-1
SMILESc [O-][N+](=O)c1cc([N+]([O-])=O)cc(c1O)C
Molecular massb 198.1348 g/mol

Physical and chemical properties

Experimental and modelled data are available for DNOC.

Table 2 summarizes data on experimental physical and chemical properties of DNOC that are relevant to its environmental fate.

Table 2: summarized experimental physical and chemical properties for the neutral form of DNOC
Property Value Temperature (°C) Reference
Physical characteristics Yellow, crystalline solid, odourless   HSDB 2000; IPCS 2000
Melting point (ºC) 85.8-87.5   PhysProp 2003; Verschueren 2001; NLM 2000; IPCS 2000
Boiling point (ºC) 220-378   PhysProp 2003; Verschueren 2001; NLM 2000; IPCS 2000
Density (kg/m3) 1.58 20 HSDB 2003; NLM 2000; IPCS 2000
Vapour pressure (Pa) 1.6 × 10−2 - 4.79 × 10−2 20-35 HSDB 2003; ATSDR 1995; IPCS 2000
Henry’s Law constant (Pa·m3/mol) 2.490 × 10−2- 1.4 × 10−1 25 HSDB 2003; NLM 2000; IPCS 2000
Log Kow (Octanol-water partition coefficient) (dimensionless) 2.12 (neutral species) - 2.564 (neutral species); 1.78 at pH 4; 0.087 at pH 7; 1.32 at pH 10   HSDB 2003; NLM 2000; IPCS 2000; Schwarzenbach et al. 1988; UNEP/FAO 2002
Log Koc (Organic carbon-water partition coefficient) (dimensionless) 2.35-2.77; 1.3 (DNOC Na salt)   IPCS 2000
Water solubility (mg/L) 1000-198; 15-20 ChemFinder 2003; Schwarzenbach et al. 1988
Water solubility (mg/L) 21.3-3300 (pH 4-10)   UNEP/FAO 2002
Water solubility (mg/L) 100 000 (DNOC Na salt)   Vogue et al. 1994
Other solubilities (g/g) 4.3/100 (ethanol)    
Other solubilities (g/g) 100/100 (acetone)    
Other solubilities (g/g) 37/100 (benzene)    
pKa (Acid dissociation constant) (dimensionless) 4.32   PALLAS (v. 4.0)
Conversion factor 1 ppm = 8.10 mg/m3   NLM 2000; IPCS 2000


DNOC occurs in the environment primarily as a result of human activity, whether through direct release or through secondary transformation of atmospheric pollutants. It may form in the atmosphere following the reaction of 2-methylphenol with NOx present in ambient air (ATSDR 1995). DNOC may also form in the atmosphere during the combustion of fossil fuels or as a result of photochemical reactions between precursor compounds (for example, benzene, toluene) and hydroxyl radicals and nitrogen oxides (Tremp et al. 1993).

An industry survey was conducted by Environment Canada for the year 2000 (Environment Canada 2003a). Under section 71 of CEPA, the Notice with Respect to Certain Substances on the Domestic Substances List (DSL) applied to any person who, during the 2000 calendar year, manufactured or imported DNOC, whether alone or in a mixture or in a product, in a total quantity greater than 10 000 kg. The survey results indicated that DNOC is not manufactured in Canada; however, between 100 and 1000 tonnes of DNOC were imported in 2000 by a single company (Environment Canada 2003a). Information received more recently indicates that the only company that had reported using DNOC in response to the section 71 notice ceased use of DNOC as of late 2002 (NOVA Chemicals Corporation 2007).

Two Canadian companies reported manufacture or import of DNOC in 1986, with amounts in the range of 100 to 1000 tonnes (Environment Canada 1990).


Historically, DNOC was used in Canada as an antioxidant, corrosion inhibitor, tarnish inhibitor and antiscaling agent, for a total of 99.9% of the Canadian market (Environment Canada 1990). Globally, the principal uses of DNOC are in the plastics industry as an inhibitor of polymerization in styrene and aromatic vinyl products; it is also used as an intermediate in the synthesis of fungicides, dyes and pharmaceuticals (International Programme on Chemical Safety (IPCS) 2000; United Nations Environment Program (UNEP)/United Nations Food and Agriculture Organization (FAO) 2002). Sources of exposure in the general environment are likely to be limited to fugitive releases from industrial sites and the combustion of fossil fuels. There is no indication that DNOC is present in consumer products.

DNOC was registered as an active ingredient in ten pesticides in Canada. Registration of the last two products that contained this active ingredient expired on December 31, 1990 (Pest Management Regulatory Agency (PMRA) 2009). Three of the DNOC salts (sodium, ammonium and potassium) have been used as pesticides internationally but were never registered under the Pest Control Products Act (PCPA) in Canada.

Releases to the environment

Only one company reported releases of DNOC to the NPRI (Environment Canada 2003b). The facility, NOVA Chemicals, located in Sarnia, Ontario, reported only off-site transfers and no releases to water, air or soil. The company states that all process water from its facility is collected, analyzed and sent to another facility for treatment in a biological oxidation unit, and that other quantities of waste DNOC would have been incinerated or sent for disposal. As such, NOVA Chemicals reports that there were no releases of DNOC to water from its facility (NOVA Chemicals Corporation 2007). Information about releases was requested in the survey conducted pursuant to section 71 of CEPA, but no releases were reported (Canada 2001).

As indicated above, the use of DNOC by the NOVA Chemicals Corporation facility ceased as of late 2002 (NOVA Chemicals Corporation 2007). No facilities have reported to the NPRI for DNOC since 2003.

Environmental fate

Environmental fate analysis combines information on the chemical behaviour of the substance with the properties of the receiving environment. The objective of fate analysis is to determine the multimedia distribution of the substance after its release into the environment. This includes consideration of the persistence and bioaccumulation of the substance in the environment.

The results of Level III fugacity modelling (Equilibrium Criterion Model (EQC) 2003) indicate that, if the chemical is released into water, the majority of DNOC would remain in water with a minor amount partitioning to sediments and less than one percent to air and soil (Table 3). With emissions solely to air, the majority of DNOC would partition to air, soil and water, with a negligible amount adsorbing to sediment. If DNOC were to be released equally to all three major environmental compartments (air, water, and soil), it would mainly partition to water and soil, with some DNOC partitioning to air.

Table 3: results of the level III fugacity modelling (EQC 2003)
Substance released to: Percentage of substance partitioning into each compartment
Air Water Soil Sediment
Air (100%) 47.7 20.1 32.0 0.212
Water (100%) 0.04 98.9 1.04 0.02
Soil (100%) 0.04 1.93 98.0 0.02
Air, water, soil (33% each) 6.65 63.3 29.3 0.66

Persistence and bioaccumulation potential

The information below was considered in evaluating whether DNOC meets the criteria for persistence and bioaccumulation as defined under the Persistence and Bioaccumulation Regulations under CEPA (Canada 2000). Persistence criteria are half-lives of greater than or equal to 2, 182, 365 and 182 days for air, water, sediment and soil, respectively. Bioaccumulation criteria are bioaccumulation factors (BAFs) or bioconcentration factors (BCFs) of greater than or equal to 5000 or a log Kow of greater than or equal to 5.0.

Environmental persistence

When DNOC is released to the environment, measured data demonstrate that DNOC will persist in air with an atmospheric oxidation half-life of 129 days (Table 4a). Predicted values indicate that photoxidation half-lives range from 8 hours to 53 days (Table 4b).

Empirical and predicted half-lives of 7, 37.5 and 58 days indicate that DNOC is expected to biodegrade in surface water, but at a relatively slow rate. The substance was reported to be difficult to degrade in activated sludge (< 20% degradation under aerobic conditions) (Tables 4a and 4b).

DNOC is expected to disappear from soil within 14 hours to 2 months (Callahan et al. 1979). DNOC is not expected to volatilize from dry or moist soils, based on its vapour pressure of 1.6 - 4.79 x 10-2 Pa (Table 2). After an 80-day lag period, DNOC rapidly degraded at a rate of 2.1 μg/L/day in groundwater and sediment (Table 4a).

Table 4a: empirical data for degradation of DNOC
Medium Fate process Degradation value Degradation endpoint / units Reference
Air Photoxidation 129 Half-life (days) Howard et al. 1991; Atkinson 1987
Water Biodegradation 7-58 Half-life (days) Capel and Larson 1995; Mabey et al. 1981; IPCS 2000
Soil Biodegradation 14 hours to < 2 months   Callahan et al. 1979
Waste water Biodegradation < 20% 28 days Zahn and Wellens 1980
Sediment Biodegradation 2.1 μg/L/day Tuxen et al. 2000
Table 4b: modelled data for degradation of DNOC
Fate process Model and model basis Model result and prediction Extrapolated half-life (days or hours)
Atmospheric oxidation Atmospheric Oxidation Program for Windows (AOPWIN) 2000 t½ = 35-53 days >2 days
Ozone reaction AOPWIN v. 2000 Half-life (days) Not reactive
Biodegradation Biodegradation Probability Program for Windows (BIOWIN) 2000, Sub-model 3: Expert Survey (ultimate biodegradation) 2.4 “biodegrades fast” Half-life (days) <182
Biodegradation BIOWIN 2000 Sub-model 6: MITI non-linear probability 0.0009 >182

The empirical and modelled data (tables 4a and 4b) demonstrate that DNOC meets the persistence criteria for air (half-life in air > 2 days) and water (indicated by a degradation rate of less than 20% within the 28-day test period) (Zahn and Wellens 1980).

Potential for bioaccumulation

A bioaccumulation factor of 25 was estimated by Gobas and Arnot (2003) for DNOC. This is significantly lower than the bioaccumulation criteria of BAF or BCF greater than 5000 as laid out in the Persistence and Bioaccumulation Regulations (Canada 2000).

Based on the criteria in the Persistence and Bioaccumulation Regulations (Canada 2000), DNOC is persistent in air and water and is not bioaccumulative.

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