"I'm afraid I call that 'the London taste'," says Ian Colbeck, a little apologetically. We are in Queen Street in Colchester, where the professor of environmental science at the nearby University of Essex has one foot in the roadway, clogged with buses and afternoon traffic, and both hands gripped around a portable aerosol spectrometer, a device about the size of an old-fashioned car radio, which he is using to measure the atmospheric pollution levels.

The results it is recording, as the sour tang at the back of the professor's throat might suggest, are not reassuring. "Eighty! No, 90!" says Colbeck, referring to the number of micrograms per cubic metre that the device's laser detects in the air it is sucking in with a faint hiss. At one point he gets a reading of 118. The World Health Organisation's recommended maximum particle level, averaged over a 24-hour period, is 50.

As a medium-sized town within commuting distance of the capital, Colchester is hardly rural, but neither is it so densely populated or heavily industrialised that its residents might expect to encounter dangerous air quality on their way home from school or work.

But while the choking smog has receded a bit from its peak on Wednesday when "the air felt thick", levels in the park surrounding the Essex town's Norman castle, which the professor calls "the lungs of Colchester", are about double what he would normally expect on a Thursday afternoon. It is a bright and largely cloudless day but buildings in the distance are a smudge while the sun remains mostly hidden behind a greyish white blur.

This week's pollution episode, having begun to pass relatively quickly, is likely to prove not as serious as Colbeck initially feared, he concedes. Nonetheless, he says, there will be deaths as a result. These are less likely to be prompted by the Saharan sand, traces of which stand out clearly in rain patterns on the bonnet of the professor's navy blue car, than the microscopic particulates generated by road traffic, in particular diesel engines.

"It's the small particulates which are causing most of the health problems and they are the ones we can't see. The smaller they are the deeper they get into your lung and the easier it is to get into your bloodstream." As many as 30,000 people may die prematurely each year in the UK (4,000 of them in London) as a result of air pollution, says Colbeck, most killed by heart attacks or strokes caused by particulates absorbed into the bloodstream.

As a result, during peak times "I think certainly if you were in an urban area it would be a good idea to stay indoors." The London Marathon should be postponed if there is a repeat of similar levels next weekend, he believes.

Despite the foul air, Colbeck hasn't greatly moderated his own behaviour in the past few days, "mostly because I have spent most of them indoors in meetings". But while staying closing the doors and windows will always help when pollution is high, "it doesn't have as much benefit as you think if you live in a well-ventilated house".

So what's the answer? "I suspect we will eventually have to move away completely from diesel vehicles, because they are the main source of particles." In the meantime, a change of wind would help.