Priority Existing Chemical

n.3Public health risk

n.3.1Acute effects

Regarding the potential for acute health effects following exposure to aftermarket products containing MMT, skin irritation is not expected from exposure to MMT at the concentrations present (< 10% w/w equivalent to approximately < 7% v/v). Assuming that toxicokinetics and toxicodynamics of MMT are similar in rats and humans after dermal exposure, a comparison of the dermal LD50s with the exposure estimates in Section 8.6.1 suggests that there is some potential for acute health effects resulting from dermal exposure to MMT in aftermarket products. However, the LD50 values in rats were obtained after a constant 24-hour exposure to MMT and in contrast, much shorter exposures are expected following spillage. Overall, the risk of acute health effects is low given the small amounts to which people are likely to be exposed, the concentration of MMT likely to be lower than 7% v/v (approximately 10% w/w) and any spill on the skin is unlikely to reside untreated for long periods.

The risk of acute health effects as a result of accidental ocular exposure to MMT in LRP and aftermarket products is considered to be low since exposure to very small amounts is expected to occur only infrequently and MMT is not expected to cause eye irritation at concentrations present in aftermarket products.

Acute health effects could also occur as a result of accidental ingestion by a child. Acute oral LD50s were in the range of approximately 9-905 mg/kg bw (see Table 11) with the rat being the most sensitive to the effects of MMT and having oral LD50s generally in the range of about 20-60 mg/kg bw. Assuming that toxicokinetics and toxicodynamics of MMT are similar in rats and humans after oral exposure and using the lowest LD50 of approximately 10 mg/kg bw, a child (10kg) ingesting about one mL of a product containing 10% w/w MMT could receive a potentially lethal dose. Children between one and a quarter and three and a half years of age can swallow approximately 4.5 mL of liquid (Gosselin et al., 1976), giving a potential dose several times higher than the lowest oral LD50 observed in laboratory animals. Aftermarket products are more likely to be stored in garages rather than in the home, these types of products are generally not "attractive" for ingestion by a child and the information provided by companies marketing these products states that they are supplied in packages with child resistant closures. These factors could reduce the potential risk associated with accidental ingestion of aftermarket products containing MMT. However, since very small volumes provide a potentially lethal dose, products containing MMT represent a significant acute health risk for children.

The risk to public health as a result of ingesting MMT in LRP is unlikely to be any greater than the public health risk associated with ingestion of petrol without the additive. Given the concentration of MMT currently in LRP (about 73mg/L), it can be estimated that a 70 kg adult would need to ingest about 10 L of MMT-LRP in order to receive a dose of MMT approaching the lowest acute oral LD50 observed in laboratory animal studies. Similarly, it can be estimated that a child of 10 kg bodyweight and a youth of about 35 kg bodyweight would need to ingest about 1 or 5 L respectively, of MMT-LRP in order to receive doses approaching the lowest LD50. Therefore, acute toxic effects as a result of accidental ingestion exposure to MMT in LRP are considered to be unlikely.

n.3.2Chronic effects

The most significant adverse health effect from chronic (inhalation) exposure to Mn is neurotoxicity. A variety of inhalation health standards and guidance values have been promulgated in different countries based, in many cases, on an occupational epidemiological study conducted by Roels et al. (1992). Workers examined in this study demonstrated poor eye-hand coordination and hand steadiness and poor visual reaction times after exposures to Mn dust in a battery factory.

From this study, the NOAEL for neurological effects in humans was established at 30 μg/m³ (Section 14.2.1) (WHO 1999). Converting intermittent exposures (5 days/week, 24 hours/day) to continuous exposures,

For Present Use scenario, where current LRP market share is maintained,

Margin of Exposure = (30 μg/m³ x 5/7 x 8/24)/4.9 ng/m³ = 1458

For 2004 scenario, where the LRP market share declines,

Margin of Exposure = (30 μg/m³ x 5/7 x 8/24)/2 ng/m³ = 3571

These margins of exposure are considered sufficient, taking into account conservative exposure estimates.

Australian estimated ambient exposures are also below overseas chronic reference values for Mn. Based on the study by Roels et al. (1992) and factoring for continuous exposures, interindividual variations and uncertain pharmacokinetic information on different Mn species especially regarding deposition in the brain, Wood and Egyed (1994) for the Environmental Health Directorate, Health Canada derived an air reference level of 110 ng/m³. Using uncertainty factors for continuous exposures, interindividual variations, lack of data on developmental toxicity and toxicity of different forms of Mn, the USEPA (1993) set an inhalation reference concentration (RfC) at 50 ng/m³ based on the study by Roels et al. (1992). In a re-evaluation of inhalation health risks associated with MMT in fuels in 1994, the USEPA derived chronic reference concentrations in the range of 90-200 ng/m³ using a variety of methodologies (USEPA 1994).

Factoring for continuous exposures, interindividual variations and developmental effects in young children, WHO (1999) derived a guidance value of 150 ng/m³ for Mn, based on the study of Roels et al. (1992). Similarly, and factoring for toxicity of different Mn species and possible reproductive effects in females, ATSDR (2000) devised a minimal risk level of 40 ng/m³. Although based on the same epidemiological study, differences in these guidance values reflect differences in applied uncertainty factors and derived starting values.

There is currently no Australian ambient air standard for Mn. Comparing the estimated ambient air concentrations in Table 9 with the range of health standards set overseas indicates that the estimated air concentrations for both the Present Use scenario and the 2004 scenario are unlikely to represent a significant risk to public health. Air concentrations are much lower than the USEPA RfC for Mn, the guidance value derived by the ASTDR and the standards developed by WHO and Wood and Egyed (1994) for Health Canada.

The estimated ambient air concentrations of Mn due to MMT combustion are lower than a range of ambient air standards and a number of apparently conservative assumptions were used in the exposure assessment. Therefore, it could be concluded that, the risk to public health as a result of the use of MMT (as outlined in the Present Use scenario, Section 8.3.3 of this report) as an AVSR is expected to be low. However, as outlined below, there is considerable uncertainty associated with this risk assessment and there are likely to be sub-populations that have higher exposures and hence are at greater risk than the general population. For example, exposure of people in Launceston is of potential concern since the ambient air concentration of total (but not respirable) Mn even without the contribution from MMT combustion is higher than some of the ambient air standards developed overseas and the use of MMT would add to environmental Mn levels in this region.

n.3.3Uncertainties

Like uncertainties associated with occupational risk assessment, uncertainties involved in the chronic health risk assessment are derived in part from significant database limitations. There is a lack of suitable Australian air Mn data upon which to base a realistic exposure assessment and there is very limited data that could be used to determine the contribution that MMT combustion might make to ambient air levels of respirable Mn. There are no Australian data on indoor concentrations of Mn and, since Australians generally spend a significant amount of time indoors, the indoor concentration of Mn could significantly influence personal exposures. In addition, there are no Australian data for ambient air concentrations of Mn that could be used to estimate exposures for individuals who live (or work) in areas with high traffic densities and there are no data on air concentrations of Mn inside cars or in homes that may have attached garages. Manganese concentrations in these microenvironments could significantly influence public exposures.