Is ambient ethene a cancer risk factor?

Ethene is, on a molar basis, a major urban air pollutant. It has been shown beyond doubt that a fraction of inhaled ethene is metabolized in mammals (including humans) via ethylene oxide, an electrophilic reagent that has been shown to be mutagenic and carcinogenic. To the extent that the linearity hypothesis for dose-response relationships at low levels is accepted, exposure to ethene is therefore expected to lead to a risk increment. In order to judge whether ethene as a single compound should be considered a risk factor, it has to be evaluated whether this risk increment is negligibly small or of concern to individuals or societies. The magnitude of the cancer risk from ethene cannot be inferred from animal experiments. Because of saturation of the metabolism of ethene, sufficient statistical power cannot be attained in long-term animal tests with about 100 animals per dose. By application of the radiation-dose equivalent of the unit of target dose of ethylene oxide and using the best (although still uncertain) value for the conversion factor (about 5%), exposure to 10 ppb ethene--a level occurring in urban areas--is expected to lead to a lifetime risk of cancer death amounting to approximately 70 per 100,000. According to a recent estimate the average exposure in Sweden to ethene is some six times lower. These figures are uncertain by a factor of at least three. They indicate ethene to be a risk factor of concern.