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1. Methyl tertiary butyl ether (MTBE) is used as an oxygenating agent in petrol as it increases the octane rating of the fuel and reduces production of some air pollutants. In the UK, petrol typically contains up to 5% MTBE, the EC regulatory limit being 15% (from January 1^st 2000: EC Directive 98/70/EC).

2. We are aware that the use of MTBE in the United States has given rise to two groups of complaints:

(a) Complaints of symptoms, including headache, dizziness and nausea, after short-term exposure in communities in which MTBE had been introduced into gasoline as an oxygenating agent. It was alleged that these complaints were due to the increased production of air pollutants: this has not been confirmed. Complaints of this nature were recorded in only one part of North America.

(b) Complaints relating to contamination of water supplies from storage of petrol containing MTBE in underground tanks.

3. We are aware of the considerable volume of toxicological research that has been done on the possible effects of MTBE in experimental animals. We comment first on the results of these studies and then turn back to consider the reports of effects in man.

4. In terms of both acute and chronic dosing in experimental animals MTBE is a compound of low toxicity. Whilst there is evidence that large doses can produce cancer in animals, there is no indication that this occurs by mechanisms relevant to humans; effects are unlikely to be mediated by a genotoxic process. This being so, there is no case for assuming that inhalation of low concentrations of MTBE would be associated with a risk of cancer. Studies designed to investigate the possibility of reproductive and developmental toxicology are also reassuring.

5. Attention has been focussed on animal studies that have revealed neurotoxic effects at very high dose levels. We note the points made by the HEI panel who concluded that motor activity in rats was affected on exposure to 800 ppm MTBE and that at higher levels of exposure sedation and ataxia occurred. These are very high concentrations: 800 ppm is 3142 mg/m³. Ambient concentrations are likely to be < 10 µg/m³. Levels at petrol stations are significantly higher than this, reaching a peak of about 30 µg/m³ during refuelling. Details are provided on pages 10 and 11 of the HEI report and in the table attached at Appendix IV. Given that concentrations lower than 800 ppm were not studied in animal models and that effects were found at this level, it is not possible to identify a No Observed Adverse Effect Level as far as neurotoxic effects are concerned. We also note that the HEI report records exposure of workers involved in handling levels of MTBE of > 1000 ppm. This as a cause for concern and we return to this point in our conclusions.

6. Evidence of effects in populations exposed to petrol treated with MTBE has been reviewed in detail by the Health Effects Institute in their report: The Potential Health Effects of Oxygenates added to Gasoline (published in 1996). The community studies are difficult to evaluate because of problems of design and statistical power and also because of confounding produced by awareness that MTBE had been added to gasoline. We also note that experimental studies involving volunteers have failed to produce effects similar to those described in some community-based studies. No evidence of effects on psychomotor or psychometric functioning was produced in the volunteer studies. This is reassuring in view of the findings of neurotoxic effects in rats exposed to very high concentrations of MTBE.

7. Our conclusions from this review of the evidence and more recent literature are similar to those of the Health Effects Institute report:

a) The addition of MTBE to petrol is unlikely to increase significantly the health risks associated with exposure to ambient air in the UK.

b) We recognise that there is a lack of data on concentrations of MTBE in air in the UK and recommend that measurements be made. These need not be made routinely but establishing a baseline and monitoring, at intervals, possible changes in concentration would be sensible.

c) We recognise that there is a lack of data relating to possible occupational exposure to MTBE amongst workers handling petrol. In view of the findings reported in the HEI report, which suggests that in some cases exposure to high concentrations of MTBE may occur, we recommend that the Health and Safety Executive be made aware of this.

d) We also recognise that concerns have been expressed regarding the leakage of MTBE into water supplies. We are not competent to comment on the implications of this for health but recommend that the problem should be considered by an appropriate body.

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