The sudden reduction in house sparrow populations in large cities came as a surprise. World sparrow authority Denis Summers-Smith suggested that unleaded petrol was to blame:

• The sparrows demise began with its introduction, and accelerated as unleaded replaced four star
• The demise occurred in large cities, but hardly at all in small towns

Denis suspected that the chemicals which replaced lead in petrol - MTBE (methyl tertiary butyl ether) and benzene (used extensively in super unleaded originally but now greatly reduced) - had killed the sparrows, either directly or by killing the insects they fed on. Benzene is a known carcinogen and MTBE a suspected carcinogen. Both are being phased out in many countries.

In the UK, despite official (but unpublished) concern from the Department of Health, no measurements of MTBE air pollution are conducted, and there are no plans to use a less dangerous alternative.

Some researchers question whether leaded petrol really was the principal source of lead poisoning, the reason given for the move to unleaded petrol.

• Research in Frankfurt found that reducing the lead content of leaded petrol by two thirds for five years made no difference to lead levels in the blood

• Australia‘s National Energy Advisory Committee reported that "no single case of clinical lead poisoning has ever been demonstrated to be due to automobile emissions of airborne lead"

• Professor Lowthur of the University of London pointed out that the lead in car emissions settled as a very heavy dust at the edge of roads. It doesn’t get absorbed through the lungs and doesn’t even get dissolved in the diluted hydrochloric acid of the stomach

• Professor Roger Perry of Imperial College, London explained that lead levels in blood were not related just to leaded petrol, but also to lead in paint, water, dust and solder. He held that lead in our water and in the solder used to make tin cans was the major cause. He also stated that lead levels in the blood fell consistently since 1935, at the same time as the use of leaded petrol was increasing