Priority Existing Chemical

n.2Occupational risk

1. 
   Identification of the critical effect(s);
   
2. 
   Identification of the most appropriate/reliable NOAEL (if available) for the critical effect(s);
   
3. 
   Where appropriate, comparison of the estimated or measured human dose or exposure (EHD) to provide a Margin of Exposure:
   

4. 
   Characterisation of risk, by evaluating whether the Margin of Exposure indicates a concern for the human population under consideration.
   

n.2.1Critical health effects

MMT is acutely toxic by all routes of exposure. The critical effects from acute exposure to MMT are neurological and pulmonary dysfunction. In humans, giddiness, headache, nausea, chest tightness, dyspnea and paresthesia are reported in anecdotal cases of acute occupational exposure. Acute lethal exposure to MMT in animals is associated with damage to the lungs, kidney, liver and spleen effects, tremors, convulsions, dyspnea and weakness. In both animals and humans, slight skin and eye irritation results from dermal and ocular exposure respectively.

Only limited repeated dose toxicity data are available. Repeated inhalation exposure to MMT is reported to result in degenerative changes in liver and kidneys. A NOAEL of 0.0062 mg/L for inhalation exposure was reported. MMT did not cause teratogenic or embryotoxic effects in developmental studies in rats.

Manganese

Given their production and widespread dissemination resulting from MMT combustion, the present assessment also considers the health effects of Mn and inorganic Mn compounds.

In animal studies, the critical effect following acute exposure to inorganic Mn compounds is neurological dysfunction. Decreased activity, alertness, muscle tone, touch response and respiration have been reported with oral administration. Pulmonary effects are also reported in inhalation studies, but these may at least in part reflect an inflammatory effect following inhalation of particulate matter rather than a result of pulmonary toxicity of Mn.

In repeated dose animal studies, the critical effect is also neurological dysfunction. Effects range from decreased motor activity to increased activity, aggression and movement tremors. In humans, chronic occupational exposure to respirable dusts (2-22 mg/m³) leads to manganism, a progressive neurological disorder. Subclinical nervous system toxicity has been detected in repeated occupational exposures ranging from 0.14-1.0 mg/m³. Reproductive effects including impotence and loss of libido in male workers have also been associated with high Mn exposures. However, a dose-response relationship for reproductive effects has not been established.

A principal epidemiological study of occupational inhalation exposure to Mn in which neurobehavioural endpoints were examined (Roels et al., 1992) was used to determine a dose-response relationship for neurological effects. A lower 95% confidence limit was estimated at the level of Mn exposure expected to result in a 5% response rate. This value (30 μg/m³) was considered a surrogate for a NOAEL for neurological effects. It is important to base the risk characterisation on effects seen due to inhalational exposure, as there appears to be differences in toxicity based on route of exposure and absorbed dose.

n.2.2Occupational health and safety risks

The blending of LRP is essentially an enclosed, automated process. Although exposure via the dermal and ocular routes is possible from slops, spills and residue during decanting of the imported MMT solution prior to LRP blending, exposure is expected to be infrequent, minimal and of short duration. Overall, the potential for exposure to MMT and hence Mn is low. Consequently, the risk to refinery workers from handling MMT is assessed as low.

Formulation of imported MMT into aftermarket fuel additives is essentially also an enclosed, automated process that occurs even more intermittently than LRP blending. In a similar fashion to LRP blending, exposure is possible via the dermal and ocular routes from slops, spills and residue, but exposure would normally be minimal and of short duration. The potential for exposure to MMT and hence Mn during formulation is low. Consequently, the risk to formulation workers from MMT is assessed as low.

Tanker drivers, petrol station attendants and auto mechanics may be exposed occupationally to MMT through contact with additised fuels and less frequently with aftermarket fuel additives containing MMT. Although exposure to fuel vapours may be expected during a typical working day, the low vapour pressure of MMT and its high dilution in fuel renders inhalation exposure to MMT unlikely. Exposure to MMT via dermal and ocular routes is expected to be infrequent, minor and of short duration and also limited due to its dilution with solvents and other additives in the fuel and fuel additives. Therefore, the risk to these workers from MMT is assessed as low.

As reflected by overseas exposure data, auto mechanics at petrol stations and at dedicated maintenance workshops may have particular potential for repeated exposure to Mn from MMT and from Mn particulates associated with auto exhaust. Of overseas studies examining Mn exposure of garage mechanics, the highest mean personal exposure measured was 448 ng/m³ (Sierra et al., 1995) for auto mechanics working in Montreal in mostly closed workshops (Section 8.5.1). Zayed et al. (1994) measured mean personal exposure levels of 314 and 152 ng/m³ for garage mechanics in closed and open workshops respectively.

Bearing in mind the limitations associated with extrapolating these overseas data to local workplaces and given the likelihood that in the Sierra et al (1995) study only up to one third of airborne Mn resulted from MMT combustion (Section 8.5.1), 148 ng/m³ (448 x 33%) is considered a worst case for Mn exposure from the use of MMT.

Assuming that all of the above Mn measured in the breathing zone of workers in the closed garages will be deposited and absorbed, then:

Margin of Exposure = 30,000/148 = 203

This is considered a sufficient Margin of Exposure as it is probable that for Australian auto mechanics the exposure to Mn would be much lower than calculated due to lower use of MMT in fuel, differences in working conditions (i.e. less closure of workshops) and the lower background levels of Mn in Australia. Therefore, the risk to these workers from Mn exposure is considered to be low.

Car park personnel, professional drivers and road maintenance workers

Occupational exposure to Mn particulates from automotive exhaust may occur for these workers but exposure is likely to be highly variable depending on the level of separation from the exhaust sources and traffic densities. Personal exposure data for Montreal taxi drivers show exposures that are lower than garage mechanics in the same study by an order of magnitude (Zayed et al., 1994). Therefore, despite the lack of personal exposure data for local workers, given the more restricted use of MMT and lower environmental levels of Mn locally, exposure of Australian professional drivers to MMT and Mn is likely to be significantly less than automechanics and so the risk to local workers is considered low. Similarly, exposure to and risk associated with Mn for car park and road maintenance workers is considered low.

n.2.3Uncertainties

There is also substantial uncertainty associated with the limitations in the amount and quantity of toxicological data. For example, as there was no threshold for neurological effects identified in the study by Roels et al. (1992) there is some uncertainty associated with the derivation of surrogate NOAELs from this study. In addition, the study of Roels et al. (1992) has some disadvantages in that it was an occupational epidemiological study involving young or middle aged males and therefore applicability to the general population, including women, infants, the elderly, and those more susceptible because of illness, diet, or genetic predisposition, can only be achieved by the use of uncertainty factors. The study of Roels et al. (1992) involved exposure to Mn₃O₄ dust whereas recent data indicates that Mn sulphates and phosphates as well as oxides are likely to be present in exhaust emissions as a result of MMT combustion. Recent animal studies by Dorman et al. (2001) show that soluble Mn salts such as Mn sulphate have toxicokinetics that differ from insoluble Mn compounds such as Mn₃O₄, Studies by Vitarella et al. (2000b) and Brenneman et al (2000) show that Mn can be transported to the brain via the olfactory bulb in rats. These authors note significant differences in nasal and brain anatomy and physiology between rats and humans that question the toxicological significance of this manganese absorption pathway for humans. Therefore, there is also uncertainty related to potential differences in toxicokinetics and potential toxicity of different Mn salts that may be associated with human exposures to airborne Mn resulting from MMT combustion.