You always thought diesel stank? Well, you were right - now they tell us

It's official: diesel cars are dirtier than petrol cars. Or, rather, that is the official view now.

Given that "official" thinking on environmental transport issues seems to change about as often as transport secretaries, there's no guarantee that this latest thinking - as decreed last week by government scientists - will be any more permanent than the old thinking.

Old thinking? Yes, you may remember that only a few years back, we were being urged, by environmentalists (who naively believed that just because a diesel car used less fuel, it must be greener), car makers (keen to propagate a new trend and therefore sell more new cars) and politicians (determined to jump on to the green bandwagon) that diesel was good - despite the sooty evidence to the contrary. It seems that our eyes and our noses were right all along. What you saw (and smelt) was indeed what you got. The government scientists are now telling us that between 2,000 and 10,000 people a year, specifically those with respiratory or heart problems, may be being killed by diesel fumes.

The worst offenders are not cars at all, but buses. Yes, those characterful old double-deckers are the foulest diesel polluters of all. Does this come as any surprise? Just look at the filth that comes out of the back of a Routemaster or a Metrobus when it accelerates down a crowded high street.

It is far from being the first transport environmental volte-face of recent times. Not so long ago, we were all being urged to convert to unleaded fuel - a few years before another one of these "official" reports said, well, actually, er, the extra benzene added to unleaded fuel to compensate for the loss of lead may be causing cancer, so it's probably best not to have converted your car at all. So sorry.

It really is hard to be an environmentally conscious motorist these days. Just as it's very hard to believe anything uttered on the subject from environmental lobbyists, car makers, the oil industry, or politicians.

This confusion has been so unnecessary. If only we'd taken notice of international trends, and forgotten about local issues, much of the recent misguided diesel and unleaded contention could have been avoided. Britain was one of the few countries in the world to encourage the widespread use of unleaded fuel before the simultaneous use of catalytic converters. In America, Japan, Germany, Australia and other countries the two went hand in glove, as they are designed to. The catalysts kill off the benzene. No converters mean more air-borne benzene. Sounds simple. But it's a fact that environmentalists overlooked, and experts in the car and oil industries have clearly suppressed.

Our recent embrace of diesel, having sensibly spurned the stuff for years, was equally misguided. Experts in America and Japan, the two countries that traditionally have had the toughest emission regulations, dismissed diesel as an environmentally unfriendly fuel for their cars years ago. Diesel died in America because it couldn't meet the pollution regulations. That surely should have been enough of a warning for us.

Europe is the only developed part of the world where diesel cars are popular, and even here it is mainly the French who promote them. And the French are keen for the EU to continue to encourage their use.

Why? The French first encouraged the use of diesel to reduce the amount of oil it imported (diesel cars are more frugal). Because of the resulting popularity of diesel in France, the French have become the world leaders in its technology. Some other European makers have followed suit and invested heavily in diesel research and production. Europe's car makers now have a vested interest in its popularity.

Given that the European car market is about to be opened to all comers at the end of this decade, doesn't it make sense to bias taxation in favour of a technology that Europe dominates, and one in which Europe's main automotive rivals - Japan and America - are weak? That has been the French position. There are many other European governments who like the sound of the argument and, indeed, in most EU countries diesel is marginally cheaper in garage forecourts than petrol (in France, massively so).

So where does that leave you, the concerned green motorist keen to minimise his or her damage on the environment? The current thinking is that you'd do best to drive a small, new petrol car (being new, it will have a catalytic converter as standard). If you can't afford new, go for a second-hand petrol car with a catalyst. Mind you, that's today's thinking. It may be different next week.