A number of large vehicles are believed to be running on a hazardous combination of light heating oil and diesel to cut down on costs, The Sunday Times has learnt.

Though the Malta Transport Authority could not confirm whether heavy vehicles, especially buses, are making use of light heating oil (LHO), experts say that the quality of smoke emissions on the roads, together with the ensuing stench, clearly pointed to abuse. The government, however, is monitoring the situation.

LHO, a name given to a type of light cycle oil, is harmful because it is packed with a high dosage of sulphur. It can even be potentially fatal if inhaled in certain concentrations.

Before the price of kerosene was raised, it was an open secret that bus drivers were mixing this fuel with diesel. Now the government is studying what system to put in place to weed out operators of heavy vehicles who are using LHO.

Since most fuels and oils do not vary in colour, it is often difficult to establish if owners of vehicles are using LHO. It is also difficult to control it at the point of sale.

Sources close to the government told The Sunday Times: "We know they're resorting to LHO because of the volume of sales. We know what's happening from the tests carried out, and we also know what the operators are saying ."

Asked if the ADT had discovered LHO in buses during any inspections and if so, what action has been taken, a spokesman replied:

"The ADT tests fuel for its properties... not necessarily for the actual fuel products that are used. Tests carried out by authority personnel and Customs' officials on route buses have revealed that, in certain cases, the level of sulphur in the fuel exceeds the required level. This is probably due to a fuel mix which is below the standards commercially sold from fuel pumps. The high sulphur content is in itself a breach of the law and action is taken."

The procedure for conducting tests on buses varies according to the vehicle's year of manufacture. The test is based mainly on the emission levels, which should also indicate the type of fuel used in the vehicle.

Statistics obtained from the ADT show that 73 buses were tested between June 2007 and last June after they were reported through the SMS emissions scheme. Somewhat surprisingly, 77 per cent of reported vehicles passed the test.

Follow-up inspections were "continuously" carried out on those that failed the test until the problem was rectified, an ADT spokesman said.

Since the Public Transport Association had objected to the test procedure last May, the ADT and the association had agreed to engage the Malta Standards Authority to review the test on buses.

"The results and advice of the MSA will be taken on to ensure the correctness of the test procedure. The report by the MSA will be made public when issued. Until the MSA review is completed, current enforcement procedures continue to be applied."

Prof. Alfred Vella, dean of the university's Faculty of Science and Chemistry Department head, explained the dangers normally associated with fuels like LHO. Such sulphur oxides turn into sulphuric acid in the air which irritates the respiratory system, not to mention the environmental damage inflicted.

"LHO is like plonk - it's the loose odds and ends which remain when you're refining fuels," he said.

LHO has a higher concentration of sulphur compared with diesel - once it's burnt it emits a lot of sulphur dioxide in the air.

Any heating of LHO, even by industry, will generate emissions, but vehicles pose a bigger danger since they are operating so close to the public while industry emissions could be somewhat diluted when they are being let out in the air through chimneys.

Prof. Vella acknowledged that it is difficult to identify LHO, though a solution would be to colour it with certain compounds.

Ultimately, owners are not realising the damage they are inflicting to their vehicle by using LHO since it forms acid fumes which could damage the engine, Prof. Vella added.

Though Malta only permits the importation of low sulphur diesel, which is considerably more expensive, the average annual ozone levels went up by 19 per cent between 2005 and 2006, according to the latest State of the Environment report.