White mineral oil (petroleum)

Endpoint summary

Additional information

Discussion on methodology

In undertaking environmental risk assessments the best use of all available ecotoxicity data should be made. However, the assessment of ecotoxicity data for many petroleumproducts is complicated since several different test methods and procedures have been used. As petroleum products contain a mixture of substances with a range of solubilities a critical aspect with respect to interpreting the validity of ecotoxicity tests is how the test media is prepared. Although not always explicitly stated most of the data generated in the period up to the early 1990s originated from experiments in which a "water soluble fraction" (WSF) was tested. WSFs are prepared by mixing the petroleum product with the aqueous test medium (e. g. 25 to 50 mL product with 1 L of medium). After mixing the test solutions are then allowed to stand, the aqueous phase is separated and dilutions of this medium are used in testing the species under study. The results areexpressed either as (a) the dilution, or % WSF, or (b) the concentration of dissolved hydrocarbons expressed in mg/L (CONCAWE, 1992). A disadvantage with these WSF studies is that it is not possible to convert the quoted result to the amount of product that must be added to a given volume of aqueous medium to produce the effect.

The problems of preparing test media for oil products were recognised in the early 1990s. As a consequence the recommended method, which enables ecotoxicity assessments of petroleum products to be interpreted, was to determine the amount of test substance that must be equilibrated with the test medium to produce a specified level of effect. This is the so-called "loading rate" or Water Accommodated Fraction (WAF) methodology as developed by Girling et al. (1992) and reported in CONCAWE (1992). Even with these laboratory based studies there are doubts about their value in the context of risk assessment owing to the fact that once a petroleum product is released to the environment its constituent substances will partition to the various compartments (water, sediment, soil and air) in accordance with their physico-chemical properties. The assumption being that in the receiving environment the substances will be degraded and transformed in accordance with their individual susceptibilities to physical, chemical and biological degradation processes and will exhibit effects in accordance with their individual toxic potencies.

 

Discussion on mechanisms of toxicity and PNEC derivation

In an attempt to better understand the potential for adverse effects of a product, the effects of a product's constituent substances (hydrocarbon blocks) can be integrated in such a way that an overall assessment of their combined effects can be made. For the assessment of toxic effects it is important that the method of integration meets the assertion that effects can only be integrated for substances that share the same mode of toxic action. All components of petroleum products exhibit non-polar narcosis effects on organisms.

Under ideal circumstances a PNEC for a hydrocarbon block would be derived from ecotoxicological test data for one or more components that are representative of that block. The TGD sets out how this can be done either by applying an Assessment Factor to the lowest acute ecotoxicological effect or chronic no effect concentration or by applying statistical extrapolation methods to a number of data points. For petroleum products this was not a practical option since the majority of its mass is comprised of chemical components that cannot be accurately described by a chemical structure (and which may not have a unique CAS number) and for which there is an absence of ecotoxicological data. Under such circumstances the only practical option is to estimate a PNEC using a relationship between physico-chemical descriptors of a component or a hydrocarbon block and concentrations resulting in ecotoxicological effects or absence of an effect. This is the hypothesis encompassed by the Target Lipid Model (TLM) described by McCarthy et al. (1991).

The theory underpinning the TLM is that the concentration of a substance in a lipid that is responsible for the onset of a non-polar narcosis effect is the same when expressed on a molar basis for a range of taxonomic groups e. g. fish, invertebrates and algae. Consequently the toxic potency of a substance depends upon its capacity to achieve the threshold concentration within an organism. There are a number of variables that determine this capacity, key of which are the solubility of the substance in water and lipid and its molecular size. In an application of the theory, DiToro et al. (2000) have published a non-polar narcosis-based QSAR for predicting the aqueous concentration of a hydrocarbon substance that induces a specified level of biological effect. The QSAR relates biological effect to the log Kow of the substance. Log Kow is a function of the solubility of a substance in water and lipid (octanol) but is limited by molecular size because large molecules cannot pass through biological membranes.

In the absence of measured ecotoxicity data for a substance the TLM and associated QSARs provide a theoretical basis for predicting the ecotoxicity of a substance. By extension of the theory it should also be possible to evaluate the toxicity of a mixture of substances provided that they have the same mode of toxic action. McGrath et al. (2004) have validated the theory by characterising the aquatic toxicity of six gasoline blending streams and have showed that predicted and measured toxicity were in good agreement.

Having established procedures that enable the toxicity of a mixture of hydrocarbons to be predicted, McGrath et al. (2004) have also utilised statistical theory developed by a number of workers to define an acute species sensitivity distribution for narcotic chemicals. A relationship has been established enabling the concentration of a hydrocarbon substance to be determined that will affect a specified proportion of the species present in a community. By setting the proportion to a notional low level (e. g. 5%), a hazard concentration (HCx where x is the proportion that might be affected i. e. 5%) is obtained. The HCx has similarities with a hazard concentration derived by applying statistical extrapolation procedures described in the TGD to a set of test substance data. It can also be considered analogous to, and used for risk assessment in the same way as, a PNEC derived by applying an Assessment Factor (AF) specified in the TGD to a lowest acute EC50 or LC50 value in a data set.

Read across justification

Highly refined base oils are viscous fluids derived from petroleum lubricant base oils through a sequence of fractionations and chemical purifications to ensure that the final product is comprised of saturated acyclic and cyclic hydrocarbon molecules. To gain insights into the composition of highly refined base oils, it is critical to understand the nature of other lubricating base oils since these substances serve as the feedstock for the highly refined mineral oils. In evaluating toxicological properties for highly refined base oils, other lubricant base oils which have undergone less refinement provide a conservative basis for read-across.  

Short-term toxicity to fish: 

In a key static 96 -hour short-term Golden ide (Leuciscus idus melantous L.) limit test (OECD 203; KS = 2), 10 animals/loading were exposed to the WAF of a highly refined base oil (RL 89 -1973) at a nominal concentration of 10,000 mg/L. The LL50 was > 10,000 mg/L and the NOEL was ≥ 10,000 mg/L (IWL, 1992).

In five supporting 96-hour acute rainbow trout (Oncorhynchus mykiss) limit tests (OECD 203; KS = 2), 10 animals/loading were exposed to the WAFs of highly refined base oil (HT70, HT100, HT200, HT350, and N60LHT) at nominal concentrations of 100 mg/L. The LL50 was > 100 mg/L and the NOEL was ≥ 100 mg/L (Petro-Canada, 2008a, b, c, d, e). 

Long-term toxicity to fish: 

The long-term toxicity for fish endpoint is addressed by read across to long-term toxicity for aquatic invertebrates (Daphnia magna). Read across to OLBOs has been applied for the long-term toxicity to aquatic invertebrates endpoint, which is justified based on similarities in composition and physical/chemical properties between highly refined base oils and OLBOs. Toxic effects of hydrocarbons are primarily caused by narcosis, and occur in a narrow range of molar concentrations across aquatic taxa; hence, read across between species is justified.

Results of computer modelling to estimate aquatic chronic toxicity in a 28-day freshwater fish study show no chronic toxicity of highly refined base oil to freshwater fish at or below its maximum attainable water solubility (Redman et al., 2010b). This supports the applied interspecies read across.

Short-term toxicity to aquatic invertebrates:

In a key static 48 -hour short-term Daphnia magna limit test (OECD 202; KS = 2), 10 animals/loading were exposed to the WAF of a highly refined base oil (HT350) at a nominal concentrations of 100 mg/L. The 48 hour LL50 was >100 mg/L and the NOEL was ≥100 mg/L (Petro-Canada, 2008f). 

In four supporting 48-hour short-term Daphnia magna limit tests, 10 animals/loading were exposed to highly refined base oils (HT70, HT100, HT200, and N60LHT) at nominal concentrations of 100 mg/L. The 48 hour LL50 was >100 mg/L and the NOEL was ≥ 100 mg/L (Petro-Canada, 2008g, h, i, j). 

Long-term toxicity to aquatic invertebrates: 

In a read-across key semi-static 21-day long-term Daphnia magna toxicity test (OECD 211; KS = 2), 10 animals/loading were exposed to the WAF of other lubricant base oil LVIN 38 (CAS #64742-53-6) at nominal concentrations of 1, 10, 100 and 1000 mg/L. The NOEL was 10 mg/L based on reproduction. The loss of all daphnids in the 100 mg/L WAF was attributed to a non-treatment related effect, the cause of which was unknown. Further testing would be required to clarify the consequences of exposure to a 100 mg/L WAF of the base oil (Shell, 1995).

In a read-across supporting semi-static 21-day long-term Daphnia magna reproduction test (OECD 211; KS = 2), 10 animals/loading were exposed to the WAF of solvent-refined heavy paraffinic distillate (PSG 1860; CAS # 64742-04-7) at nominal concentrations of 0, 10, and 1000 mg/L. The EL50 was > 1000 mg/L and the NOEL was ≥ 1000 mg/L based on the lack of mortality or reproduction impairment (BP Oil Europe, 1995).

In a read-across supporting semi-static 21-day long-term Daphnia magna reproduction test (OECD 211; KS = 2), 10 animals/loading were exposed to the WAFs of other lubricant base oils HVI 60, XHVI 4.0, HVI 65, and LVIN 38 (CAS #64742-53-6) at nominal concentrations of 1 and 1000 mg/L. The NOELs for other lubricant base oils HVI 60, XHVI 4.0, and HVI 65 were ≥ 1000 mg/L based on reproduction. The NOEL for LVIN 38 was ≥1 mg/L based on reproduction (Shell, 1994); this substance was retested across a wider range of nominal concentrations and a NOEL of 10 mg/L was determined (as described above in Shell, 1995).

Supporting data estimated from the PETROTOX computer model show no acute toxicity of highly refined base oil to aquatic invertebrates at or below its maximum attainable water solubility (Redman et al., 2010b). These predictions support the applied read across.

Toxicity to aquatic algae:

Algal toxicity was examined using a weight of evidence approach.  In six static 72-hour algal (Pseudokirchneriella subcapitata) limit tests (OECD 201; KS = 2), the freshwater aquatic alga was exposed to the WAFs of highly refined base oils (HT70, HT200, HT350, N60LHT; N650HT; and HT100) at nominal concentrations of 100 mg/L. The NOEL was ≥ 100 mg/L for HT70, HT200, HT350, N60LHT; N650HT based upon average specific growth rate and cell yield (Petro-Canada, 2008l, m, n, o, p). TheLOEL for HT100 was ≤ 100 mg/L based upon average specific growth rate and cell yield. Further testing across a wider range of nominal concentrations would be needed to determine a specific LOEL or NOEL value for this substance (Petro-Canada, 2008k).

Based on five studies resulting in no effects and one study resulting in a LOEL ≤ 100 mg/L, highly refined base oils are not toxic to aquatic algae. 

Toxicity to microorganisms:

In a key microorganism toxicity study (KS = 2), the Microtox ™ test was used to determine the toxicity of mineral oil contaminated soil samples during bioremediation processes, at a nominal concentration of 2000 mg/dry weight soil. The initial IC50 following application of mineral oil to the soil was 100% of the reference dose. After 93 days, the Microtox ™ test showed 50 to 75% inhibition. The LOEL was <2000 mg mineral oil/dry weight soil (Lee et al., 2002).

Supporting data estimated from the PETROTOX computer model show no acute toxicity of highly refined base oil to microorganisms at or below its maximum attainable water solubility (Redman et al., 2010b).

Some information for this category has been generated using the models PETROTOX and/or SPARC. The QMRFs for PETROTOX and SPARC are attached in IUCLID Section 13, with the associated QPRF.