Unleaded Petrol Reduces Dioxin Levels in Air

by Dr Chris Winder

One interesting spin-off from taking lead out of petrol has been the reduction in other contaminants being emitted from cars. For example, unleaded cars produce less dioxins and related compounds, recognised as being highly toxic compounds.

Because the lead in petrol can end up in the engine oil, additives were added to petrol to encourage the lead to discharge from the engine in the exhaust gases. These additives, called scavengers, include ethylene dichloride, pentachlorophenate and ethylene dibromide. The lead oxides formed in internal combustion are converted to more volatile lead chlorides and bromides, and out into the exhaust they go.

In 1986, dioxins and related compounds were measured in used motor oil in Germany.' In 1987 came the first direct evidence of dioxins and related compounds in car emissions, based on tailpipe measurements taken from four Swedish cars running on petrol containing 0.15 g/L (grams per litre) tetramethyl lead and 0.1 g/L ethylene dichloride as a scavenger. Levels of 20-220 picograms of toxic equivalents of dioxins were emitted for every kilometre driven. Levels of dioxins in two other cars running on unleaded petrol were below the level of detection (that is, less than 13 picograms of toxic equivalents of dioxins for every kilometre driven).

Two other studies repeated and confirmed these measurements, finding:

1 picogram of toxic equivalents of dioxins for every kilometre driven for cars using unleaded petrol, and up to 39 pg/km in cars running on leaded in a New Zealand study; '0.36-6.3 pg/km (unleaded), 1.1-3.6 (leaded) and not detected in diesel vehicles, in a study in Sweden.'

While these numbers are variable, they indicated that the use of chlorinated scavengers in leaded petrol do increase emissions of dioxins and related compounds to air. The original Swedish study suggested that cars using leaded fuel in Sweden emitted 10-100 g of toxic equivalents of dioxins a year. The incremental decrease that will occur from cars using unleaded petrol is one unforeseen benefit from phasing out leaded petrol.

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