I have long thought that governments should mix an innocuous coloured dye into fuel to make all exhaust fumes visible to the eye.

That way people would be forced to think about every mile they drove, and on short urban journeys cycling may become a much more popular choice.

The thing about modern air pollution is that you can’t really see it. A century ago, the air in cities was a viscous murk of coal smoke. The Great Smog of 1952 led to the Clean Air Act (1956), which dramatically cut levels of sulphur dioxide in the air.

By the 1980s, the watchword was ‘particulates’, found in vehicle exhaust fumes. These tiny pieces of matter suspended in the air are associated with asthma, lung cancer and premature death. Lead was linked to problems such as lowered IQs and schizophrenia, and was phased out from the early 1990s.

In the last 20 years, CO2 has taken over the debate about emissions. In spite of the prevalence of catalytic converters that remove some particulates, vehicles still emit a lethal cocktail.

Nitrogen dioxide is a toxic reddish-brown gas which causes lung disease. By far the biggest source of nitrous dioxide in the UK is diesel fumes.

In Oxford, levels of nitrogen dioxide have failed since 2005 to fall below the target of 40ug/mg (micrograms per cubic metre of air). In fact, this invisible killer is on the rise, and alarmingly fast. By 2009 in Oxford the level of nitrogen dioxide in the air was 50ug/mg, and by 2011 (the most recent available figures) a shocking 61ug/mg.

Just because you can’t see it doesn’t mean it’s not there. Dogs have 100 times more olfactory receptors in their noses than humans. When my dog and I walk down Cowley Road, she sneezes incessantly.

It is time for both city and county councils to take this problem seriously and act. Trying harder to move urban journeys to the bike would be an easy start.

Driving is perceived by most as a reasonable and blameless way of getting about. Certainly, few feel guilty about it. Perhaps it’s time that councils made drivers much more aware. If everyone could see the invisible poisons that engines are pumping into the atmosphere, they and passers-by might feel differently about the blamelessness of driving.

Tens of thousands of people in Oxfordshire choose to drive every day – the more rural, the less it’s a real choice, of course. But in the city and its environs, there are decent alternatives. Everyone has to breathe. It is now time that unnecessary car journeys were viewed as antisocial and unacceptable as drink-driving.

Thousands of travellers to and in Oxford have been persuaded to abandon their car in favour of the bus. With air pollution getting worse, it’s now time to persuade drivers that they can cycle or walk to work and to school instead of fumigating everyone.