Letter: Combatting car fumes

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Sir: With regard to your article relating heart attacks to car fumes, when driving behind a vehicle with a catalytic converter one is frequently assailed by the smell of hydrogen sulphide (a smell of rotten eggs).

Hydrogen sulphide is only marginally less toxic than hydrogen cyanide, and both can cause cyanosis, leading to heart failure. The question is, which is more toxic in the concentration we breathe, hydrogen sulphide, or the sulphur dioxide which is emitted from cars not fitted with a catalytic converter?