You picks your fuel and you takes your chances when it comes to pollutantsfrom diesel and petrol. Science Editor Dick Ahlstrom reports.
Should the Government raise excise and VAT on diesel to curb consumption of a fuel that is suspected of causing cancers? Or is use of petrol the real danger because of the complex chemicals released when it burns?
You pays your money and you takes your chances - this is the general advice from experts on vehicle pollution. Both fuels emit harmful combustion products, but not the same ones.
"If you're talking about fossil fuels, it's six of one, half a dozen of another," says Dr John Simmie, an expert in combustion in NUI Galway's department of chemistry. "I don't know if there is a clear-cut argument that says diesel good, petrol bad. I think this is a trade-off."
Mr Michael McGettigan, senior scientific officer with the Environmental Protection Agency, agrees: "Overall there wouldn't be an awful lot of difference between the two, in terms of the pollution they cause, other than the chemistry. They would probably be similar. Both produce significant impacts from one or more pollutants."
A great deal of attention is being focused on these issues at the moment because of an important piece of EU legislation in the form of the National Emission Ceiling Directive, says McGettigan. The directive sets limits for four key pollutants, three of them strongly linked to transport.
They include new ceilings on national outputs of nitrogen oxides, volatile organic compounds, sulphur dioxide and ammonia. "There are quite low levels to be met for total emissions from the country," says McGettigan.
Current annual NOx output stands at about 130,000 tonnes, he says. The new directive expects us to pull this down to just 65,000 tonnes when the legislation comes into effect in 2010.
For years the Government has allowed lower prices on diesel fuels as a way to help heavy transport remain competitive. Many European countries did the same, including Britain, and the car manufacturers were quick to spot an opportunity, explains Simmie. They began offering smaller and increasingly reliable diesel engines that cost more to buy initially but eventually cost less because of lower fuel costs.
The clunky diesels - the ones that sounded like trucks and needed a long preheat before they would fire up - are long gone. Turbochargers make them purr, also eliminating the sluggish acceleration that typified the older large displacement car diesel engine.
However, this in turn pushed demand for diesel fuel, Simmie points out. And with this demand came an increase in the down side of what diesels have to offer, exhaust particulates.
Diesel engines discharge tiny particles of soot too small to be seen but large enough to have a health impact. German figures suggest that country has thousands of deaths a year linked to diesel particulates, with soot particles lodged in the lungs strongly suspected of causing cancers. "Diesel is the cause of some concern because of the particulates and the growth of the diesel market," says McGettigan. "Our national sales of diesel are also a bit exaggerated."
Britain increased taxes on diesel and its fuel pricing strategy led to higher prices for all motor fuels in Britain relative to the Republic. This has lead to a booming cross-border trade in all fuels with the flow currently into the North. McGettigan estimates that between 15 per cent and 20 per cent of all the fuel sold in the Republic is actually burned in the North or abroad. This localised distortion will make it even more difficult to adhere to the EU's Ceiling Directive because it boosts apparent consumption at a national level.
None of this gives petrol a clean bill of health however. It doesn't suffer from particulate discharge, but does lace the environment with a nasty collection of cancer causing pollutants, the great irony being many arose as a direct result of attempts to clean-up the atmosphere.
Lead in petrol was the old enemy, put there to boost octane levels and control engine knock. Unfortunately it is also readily absorbed into our tissues and is a particular danger for children exposed to high lead levels.
Leaded fuel has been banned in the EU since 2000 says Simmie, although the move to eliminate it began much earlier across Europe and in the US. Other chemicals were added to boost octane including a collection of complex organic compounds.
These, however, also increased the release of benzene in car exhausts, a recognised carcinogen. Benzene levels began to rise in urban areas around the world, leading to health risks from this new threat. Benzene has now been banned in the EU and will go out of service as a fuel additive by 2005, says Simmie.
"Oxygenates", substances that carry oxygen to help petrol to burn better were also introduced to replace lead. The most successful of these, MTBE or methyl tertiary butyl ether, has subsequently been shown to cause as much trouble as it was meant to clean up.
It has been added to petrol in the US since 1979 as an octane enhancer and over the years has become a huge business in its own right, with chemical and oil companies producing 200,000 barrels per day in the US by 1999. "It went from being a total laboratory curiosity to the fourth largest chemical produced in the world," says Simmie.
It has since been found to be a major source of water pollution, caused when leaking underground petrol tanks released water soluble and highly persistent MTBE to pollute wells and the water table. It is now banned in California, and only two weeks ago New Hampshire in the US announced it was suing 22 refiners for their use of MTBE.
So where are we to go for fuel? "I can't think of a single human activity that isn't polluting," says Simmie. "What you really need is better efficiency, more miles per gallon. Efficiency is the key." All fuels will pollute so we should use less fuel, a reason he hates SUVs. "These SUVs are an abomination," he believes, given their 10 mpg mileage.
If the Minister for the Environment and Local Government seeks to introduce a carbon tax it should be based on engine displacement and not on the fuel itself, he adds. An increase in cost per litre is an immediate disadvantage for rural dwellers, whereas charging people on the basis of how much fuel they use - or waste - is a fairer way to go, he argues.
Petrol does not produce the high level of particulates, minute soot particles, that are typical of diesel engines. It is also much easier to wash sulphur out of petrol during the refining process, leading to lower sulphur dioxide emissions, the substance that when combined with precipitation produces acid rain.
This does not mean that petrol is better than diesel from an environmental point of view however. The traditional motor fuel has plenty of its own chemical problems that started in the 1930s with lead.
Lead was added to petrol to help slow combustion. It helped to prevent early ignition, the cause of engine knock and not a few blown piston heads.
When lead was banned because of the health risks associated with lead absorption, the oil companies were forced to add other substances, both to control ignition but also to boost the octane rating and ensure smooth burning. These include among other things aromatics and oxygenates.
Aromatics are hydrocarbons based on benzene that when burned release volatile organic compounds (VOCs) or polycyclic aromatic hydrocarbons (PAHs). These are serious pollutants and some of these substances are known carcinogens. Benzene measurements are taken as a relative indicator of the level of urban pollution.
Oxygenates are organic compounds that contain oxygen molecules. One of the most notorious is MTBE, methyl tertiary butyl ether, a popular petrol additive that is now banned in California and the subject of lawsuits in many other US states.
MTBE is water soluble and can produce a foul taste at very low concentrations. It is also very persistent, making it a genuine nuisance pollutant. Leaking underground petrol tanks have caused the pollution of many wells in the US, particularly in California, hence its ban on the substance.
The big problem with the use of diesel is particulates, tiny soot particles that are discharged by all diesel engines. The real danger is not the big chunky particles seen as smoke spewing from a rickety lorry or old bus. Rather it is the invisible particles that pass easily into the lungs to lodge in the deepest tissues.
Here they are suspected of causing cancer and other lung complaints. Depending on whose figures you look at, diesel emissions kill an estimated 14,000 a year in Germany. US figures suggest that diesel particulates lead to 125,110 new cancers a year in the continental US, with the worst figures in Los Angeles, where the car is king. The city of Angels sees more than 16,000 cancers a year arising from particulates.
The US National Institutes of Health indicates that diesel particulates are "reasonably anticipated to be a human carcinogen", although the link is not definitive despite the figures.
Diesel exhaust is a complex combination of products, with the mix highly dependent on the type of engine, the speed the engine is running at and the load. The fuel also contributes to this cocktail. Low loads produce fewer particles but more organic compounds and engines struggling under high loads produce fewer organics but more particulates.
The small invisible particles measured less than a millionth of a metre across, get into the alveolar tissues, carrying with them polyaromatic hydrocarbons and nitroarenes. One answer is to install passive filters or active filtration that includes a reactive chemical that neutralises the particles. Of course, diesel fuels also cause the release of other chemicals.
Nitrogen oxide, NOx is one, a characteristic shared with petrol. Diesel also used to be a significant source of sulphur dioxide, a chemical that produces acid rain when washed out of the atmosphere. Environmental controls have forced down sulphur levels in diesel to counter this.