Why is lead added to petrol?

Lead is added to petrol for boosting the octane number, which increases anti-knocking properties. Lead also acts as a lubricant for valves, particularly valve seats. Lead is added to petrol as mixtures of tetra-ethyl lead and tetra-methyl lead.

What are the reasons for phasing out lead in petrol?

Notwithstanding the advantages of using lead additives in petrol, several countries have phased out leaded petrol, while others are in the process of doing so on account of the following reasons:

Over the years, the results of medical research have revealed that even in low concentrations, lead can be harmful. According to medical experts, the ingestion andaccumulation of lead in living systems can affect the cell division (gametoxic), cell character (carcinogenic), embryonic growth and production of normal species (embryotoxic and teratogenic).

Lead levels of 15 to 30 micrograms per decilitre (1/10 litre) of blood may result in nervous system abnormalities and behavioural defects. Formation of red blood cells is impaired by intake of lead, which leads to anaemia, irreversible brain damage and mortality. Children are at a greater risk than adults because of the higher intake of lead on a per-unit-body weight basis and incompletely developed defence mechanisms. According to medical experts, lead levels of 10 micrograms per decilitre of blood can cause the lowering of IQ in children. Expectant mothers are particularly susceptible to lead pollution, which can result in miscarriages, deformities and still births. Animals that feed on lead-contaminated vegetation also show the symptoms of lead poisoning. The accumulated lead in plants and animals eventually entersour food chain.

Through vehicular exhausts, the lead added to the petrol is emitted back into the atmosphere in the form of small particles, less than one micron (1/1000 mm) in size. Part of this air-borne lead is deposited along the roadside, which finds its way into surface water; the rest is transported long distances by the wind. More than 85 per cent of the air-borne lead is attributed to emissions from petrol-driven vehicles.

What is the status of lead phase-out in India?

In India, it has taken us some time to start phasing out lead in petrol. As against the earlier practice of having as much as 0.56 gm/litre of lead in petrol, it has been possible to introduce low-lead (0.15 mg/litre) petrol in four metros in June 1994, which was followed by the introduction of unleaded petrol from April 1995. From June 1998, unleaded petrol was made available in all state capitals, union territories and other metros. By the year 2000 AD, unleaded petrol is likely to be available throughout thecountry.

Is it a fact that unleaded petrol gives rise to harmful emissions containing benzene?

Benzene, being a hydrocarbon, is one of the constituents of petrol, which is derived from crude oil through fractional distillation and blending of different fractions. As such, benzene is present in the petrol whether it is leaded or unleaded.

Unleaded petrol does not necessarily increase the benzene level. According to refinery experts, benzene levels can increase if the fractions containing higher benzene content are blended for petrol. It is possible to contain the benzene level in unleaded petrol and maintain the desired octane rating through various methods like feed trimming for removal for benzene precursors, reformate splitting, benzene extraction, isomerisation and addition of oxygenates such as MTBE (methyl tertiary butyl ether) or ETBE (ethyl tertiary butyl ether).

Will it cause any harm to the engines if vehicles using leaded petrol switch over to unleaded petrol?

It will notcause any harm to the engines. Earlier, there was an apprehension that unleaded petrol could adversely affect the valve seats of vehicles since lead acts as a lubricant. Such problems relate to only those vehicles that have valve seats made up of soft metals. However, extensive testing of vehicles with soft valve seats has shown that the use of petrol with high lead content is not required for lubrication of valve seats and lead can be totally replaced by additives for the purpose of lubrication. Such additives have been used in a number of countries that have phased out leaded petrol even though their vehicles contain soft valve seats.

To ascertain the facts with respect to Indian vehicles, we have checked up the matter with the Association of Indian Automobile Manufacturers (AIAM) and the association has confirmed that the use of unleaded petrol in all petrol driven vehicles (2-wheelers, 3-wheelers and 4-wheelers) will not affect the performance of the vehicles or their engines.

Leaded petrol, as itcontains lead salts and halogen acids, causes corrosion of automobile exhaust systems and requires frequent oil and spark plug changes. A switchover to unleaded petrol may increase engine life.

Is it possible to use unleaded petrol in vehicles that are not fitted with catalytic converters?

Yes, it is possible. Ideally, the vehicles should not only use unleaded petrol, but also have catalytic converters for the treatment of tailpipe emissions. However, with respect to those vehicles where fitting of catalytic converters is not feasible, unleaded petrol is still recommended since it will help in getting rid of lead emissions which are a lethal pollutant. Alongside, measures should be taken for lowering the benzene content in petrol.