3 October 1995 What Is The Fuss About Leaded And Unleaded Petrol And Automobile Emissions? - Are They Really Dangerous?

Dr Ravi Gooneratne, a Lincoln University toxicologist, explains that at the levels of exposure to petroleum chemicals and automobile emissions in New Zealand, health risks to the public are negligible. The most practical way to alleviate motor car emission related environmental pollution is to introduce legislation for manufacturers to fit catalytic converters to all new motor cars sold in the country.

The recently renewed debate about leaded and unleaded petrol has highlighted the carcinogenic effects of benzene, a chemical found in petrol and emitted from car exhausts. Advertising from one side of the debate has emphasised that more benzene could be released from cars which use unleaded premium petrol.  Is this true?  Many people are unaware of the dangers of exposure to petrol during refuelling, and to the chancels in car exhaust fumes. This article looks at the chemicals released into the atmosphere from petrol and motor cars, the exposure of the human population to them and whether current levels of these pollutants in the atmosphere are a real health risk.

What is petrol?  Petrol is a refined petroleum product used as fuel.  It contains more than 200 hydrocarbons (compounds of hydrogen and carbon) and depending on the type and grade of petrol, traces of chemicals such as lead, aromatic hydrocarbons like benzene, and alcohols like methanol and ethanol are added to improve engine performance rating (octane rating) and as 'antiknock' agents (agents which ensure smooth running of the engine).

Petrol can exist in two forms, liquid and vapour.  Approximately 1% of petrol vapour is emitted when a car is refuelled.  This petrol vapour can be inhaled during refuelling.  Although we are exposed to a high concentration of petrol vapour, it is only once or twice a week for 1 or 2 minutes, and this level of exposure is unlikely to be a significant health hazard.  But some sectors of the population, such as pump attendants, petrol tanker drivers and those who work in oil refineries and service underground petrol storage tanks, are exposed to much higher concentrations and constitute a 'high risk' group.

We can also be exposed to liquid petrol. This may be by unintentional drinking of petrol (during siphoning), accidental skin contact, or misuse.  Using petrol to clean floors, tools, hands and machine parts represents some risk to health.  Petrol kept in the home for degreasing and as motor fuel for motor cycles, lawn mowers and boats is both a fire and toxicity hazard.  Petrol contaminated water, although not common, is a potential source of exposure for the general public.  In future, leakage of petrol from underground storage tanks or pipelines is likely to become an environmental problem worldwide. Most of the present storage tanks and pipelines were built and installed many years ago and are due for replacement.  In the USA, it is estimated that as many as 100,000 tanks leak millions of litres of petrol to groundwater each year. In the past few years there have been a few news reports of petrol leakage from underground tanks in New Zealand.  Chronic intake of petrol through contaminated drinking water would be a potential health concern, but the extent and risks are unknown.

The two most toxic components released into the atmosphere from petrol are lead and benzene. Toxic gases like carbon monoxide and several hydrocarbons (in addition to benzene) are also emitted from car exhausts. Lead gives petrol a high octane rating and is an efficient 'antiknock' agent. In the 1970's the lead content in petrol was 0.084%. Because so much lead-containing paint and leaded petrol was used, lead is now widespread in the environment. Several studies showed lead poisoning was a major public health hazard and this led to a reduction of lead in paints to a maximum of 600 mg per litre. In petrol, lead was reduced in the mid- 1980's to 0.045%. In cities with heavy traffic such as Chicago and Los Angeles the lead concentration in the environment still remains high. Because of experiences there, many countries including New Zealand are striving to move completely away from leaded petrol and switch over to unleaded petrol which has a maximum of 0.013% lead.  About 1.35 billion litres of petrol is sold annually in New Zealand and it is hoped that by the year 2000 virtually none of it will be leaded.

Lead is a naturally occurring toxin, but in petrol it is in a form which is easily absorbed. Pregnant women and children, for example absorb high levels of it and, therefore, the unborn child and children under 6 years of age are especially vulnerable.  The effects of lead on children are nasty. It affects the developing brain and to a lesser extent blood cell formation. Children exposed to toxic levels of lead develop nervous and behavioural disorders, they exhibit low IQ scores, difficulties in learning, writing, reading and hearing, poor hand-eye coordination, lower attention span and hyperactivity.  It is not surprising therefore to find these children showing poor classroom behaviour at school.

Because of these effects on children, an advertising campaign to promote unleaded petrol began in 1980's, with a view to reducing environmental lead content. Since then, poisonings related to lead from petrol have declined and the few lead poisonings we hear about today in New Zealand are in children breathing in lead dust during renovation of old houses or chewing lead paints on old toys, cots and beds.  Blood lead level is the most useful screening and diagnostic test for lead exposure.  Although the absolutely safe level of lead for children is not known, currently the consensus is that it should not be above 100 micrograms of lead per litre of blood.  The average blood lead level in New Zealand is well below this figure (about 70 micrograms per litre).

Another toxic chemical in petrol is benzene. Benzene is a natural component of petrol and is a well known carcinogen and blood poison with effects on bone marrow which can cause leukaemia.  Recently there has been considerable speculation that a complete switch from leaded petrol will mean more use of petrol containing high benzene levels.  Is this correct? It is true that aromatic hydrocarbons are intentionally added instead of lead to give unleaded petrol a high octane rating and anti-knock properties. The combustion of these aromatic hydrocarbons in the engine can give rise to higher emissions of benzene and other benzene like chemicals.

Natural benzene emissions are very low, with levels in pristine rural areas seldom exceeding 5 micrograms per cubic metre of air. The major source (over 85%) of man­ made benzene in urban areas arises from exhaust emissions of automobiles and losses during refuelling.  These can raise the benzene in urban environments.

Benzene concentration in the New Zealand environment is not known.

Concentrations found in urban areas of Europe are between 35 to 100 micrograms per cubic metre of air depending on traffic density and local industries.   That is 7 to 20 times higher than in rural areas.

Since petrol is the major source of environmental benzene, many countries have a legislated upper limit for benzene in petrol. In New Zealand this limit is 4.2% (by volume) and is lower than in Europe and Australia (5%), but higher than in the USA (1%) and Japan (3%). But this legislation may not be strong enough and may need to be extended to account for benzene emissions from the exhaust pipe. This is because it has been shown that over 50% of benzene emissions from car exhausts are formed during combustion in the engine and only about 40% of benzene originally present in petrol is emitted as exhaust material.  In that case, should there be limits on benzene emissions from the tail pipes of motor vehicles? It is possible to monitor this during a warrant of fitness test, but how relevant is this to our total exposure of benzene?

It is now accepted that environmental measurements of benzene concentration are poor predictors of individual exposure to benzene. This is because most people spend 80% of their time indoors. Indoor concentrations of benzene can be quite high due to emissions from building materials, surface paint, some consumer products and importantly, tobacco smoke.

Cigarettes can contain benzene up to 30 micrograms per cigarette and therefore, smoking is the single most important source of exposure to benzene.  A person who smokes 20 cigarettes a day has a daily intake of benzene over twice that of a non-smoker.   Passive smoking and side stream smoke can increase the exposure of non-smokers slightly.

Do current levels of exposure of the general population (including smokers) to benzene in petrol and exhaust emissions pose a significant risk to health?  Exposure to high concentrations of benzene (above 150 milligrams per cubic metre of air) can cause bone marrow depression and leukaemia, but no adverse effects have been seen in humans exposed to benzene at concentrations below 30 milligrams per cubic metre of air. This is at least 20-fold above the exposure of the highest risk group in the general population, the smokers. There is no information on levels of benzene in New Zealand urban environment but they are probably much lower than values reported from big cities in Europe which can reach up to 0.1 milligrams per cubic metre of air. This is less than 1/1000 of the carcinogenic level.  So, replacing leaded petrol is not going to increase benzene emissions to a level where they will have an impact on health.

In addition to lead and benzene, the public are exposed to a whole range of other additives in petrol including alcohols, ethanol and methanol.  But these are found in such small amounts in petrol, that ethanol and methanol toxicity in humans is rarely attributed to petrol.

To date the debate in New Zealand on the subject of motor vehicle emissions has centred on toxicity of benzene and lead, and ignored the effect of gases such as carbon monoxide and nitrogen oxides, and hydrocarbons. These major environmental pollutants are emitted from vehicle exhausts mostly due to inefficient burning of hydrocarbon fuels in the engine.

Carbon monoxide is a major pollutant in Christchurch, as well as Auckland and Wellington. It accounts for about 75% of the total pollution emitted (over 90,000 tonnes per year) in Christchurch and will continue to escalate as the number of cars increase. Recent tests in Christchurch showed carbon monoxide levels in the environment in excess of WHO (World Health Organisation) guidelines of 10 milligrams per cubic metre in air during heavy traffic flow between 8 am and 8 pm.

Carbon monoxide is a colourless, odourless gas that interferes with the delivery of oxygen throughout the body. At low levels it causes tiredness and in people with chronic heart disease, chest pains. At higher concentrations, it can cause headaches, dizziness, weakness, nausea, confusion and disorientation. Pregnant women, and their unborn babies, infants, and elderly are particularly sensitive to carbon monoxide.

Hydrocarbons and nitrogen oxides are the second and third largest air pollutants in Christchurch, with emissions around 9,000 and 7,000 tonnes per year respectively. It has been shown that hydrocarbons react with nitrogen oxides and oxygen under conditions of photochemical smog as you find in Los Angeles and in larger European cities to form ozone. Ozone is harmful to human health. Prolonged inhalations of ozone leads to damage of lung tissue and a weakening of the immune system.

According to some scientists ozone is as destructive to the lung as some chemical weapons.   In Los Angeles the ozone problem is so acute that drastic measures such as banning products which release large amounts of hydrocarbons are currently being taken to clean up the atmosphere.   Although smoke in Christchurch air can exceed the WHO standard of 125 micrograms per cubic metre, for New Zealand as a whole the conditions are not such a problem.

To prevent more damage to the environment, in USA and Europe, catalytic exhaust converters are now fitted to most cars in order to meet stringent requirements for low emissions.

The Minister of Energy Doug Kidd earlier this year warned of the Government's intention to introduce more stringent exhaust emission legislation to New Zealand. One proposal is for manufacturers to fit catalytic converters to all new vehicles. This is good news for the environment. Recent adverts by the Holden Motor Car Company showed that they have already started fitting catalytic convertors to their cars. These devices remove up to 90% of exhaust emissions, not only benzene, lead, and carbon monoxide, but all potentially harmful emissions.

In conclusion, petrol contains many chemicals; among these lead and benzene are regarded as dangerous.  Lead is a nervous system toxin and exposure to benzene can produce cancer leukaemias. But at normal levels of exposure in New Zealand these chemicals are not considered a health risk to the general public.  Smokers are exposed to a higher concentration of benzene and a huge range of other noxious chemicals including carbon monoxide. Exposure to carbon monoxide emissions at above WHO health guidelines in heavy traffic streets in urban Christchurch remains a concern. Fitting catalytic converters to new motor vehicles is a practical solution to controlling not only petroleum chemicals, but all exhaust emissions.

Dr Ravi Gooneratne is Senior Lecturer in toxicology at Lincoln University, Canterbury. He has recently been admitted to the Register of Toxicologists in the United Kingdom, administered by the British Toxicology Society and Institute of Biology, UK.

Ian  Collins, Journalist, Lincoln  University, Canterbury, New Zealand.