The British Medical Somethingorother did a study on this a while back. The carbon monoxide levels & general crud levels in the blood of the tin-box people is a fair bit higher than that of the cyclist, despite them being 'protected' by their box, cos the regular cyclist's cardio vascular systems are more efficient and, as Seanholio sez, they're sucking each other's fumes while we're above it all <smug>