Professor Ray Kearney was at it again this morning, in a particularly uncritical SMH Drive piece: The truth about diesel. I guess Fairfax has a responsibility to its advertisers to persuade it loyal readers to occasionally open a supplement that normally goes straight into the bin.

Pushing the alarm button, the professor claims that one-fifth of all lung cancer is caused by fossil-fuel emissions. Actually, Sir Richard Doll entertained the same belief when he began his landmark studies in smoking among Brits, who bathed whole winters in London’s coal-fired smogs, and came upon the remarkable finding that smoking causes lung cancer.

Active smoking causes 90% of lung cancer and according to the American Cancer Society, responsibility for the remaining 10% is split among second-hand smoke, previous exposure to asbestos, exposure to radon gas, and other causes, including diesel exhaust.

So where does Kearney get his headline grabber? If not from a fertile imagination, then possibly from studies among railway workers and truck drivers exposed to levels greatly in excess of what we see on the roads. Or from studies where mice virtually bathed in the stuff. Kearney’s own record of publications shows no evidence that he has independently studied the risk. I guess when one flourishes an academic title, the critical faculties of journalists are rendered stuporous.

The American EPA concluded in 2002 that there was persuasive evidence that diesel fuels could cause lung cancer, but felt that exposure-response data are too uncertain to derive a confident quantitative estimate of cancer unit risk to the general population.

The best they could guess was a lifetime risk of around one in a hundred thousand. For comparison sake, the risk to a smoker is one-in-six over a lifetime.

No one is suggesting that people use diesel as air freshener — it contains some nasties — but in this game numbers matter, otherwise we end up chasing butterflies and not tackling disease. But from the evidence, oil companies do far more to kill people from lung cancer when they lure them to the cigarettes displays at service station checkouts. The stuff they put in the tanks isn’t in the race.