It was the man in the Aylesbury bike shop who started it. I'd popped in to buy a lock, and he asked where I was from. "Ah, the Smoke," he intoned, sagely, when I confessed to being a Londoner. "So you'll need a mask, then."

The hay fever season was upon us and the plane trees were preparing to flutter their fluff and it's true that London's streets, thick with traffic, can be heavily polluted.

You don't see many masked riders in London, but they seem to be on the increase. Could the shop manager have been right? Online, I discovered a 2004 newspaper report quoting British Heart Foundation cardiologist David Newby, which rather scarily revealed that pollutants in congested streets damaged cyclists' blood vessels in a manner associated with the early stages of heart disease.

If that study was done four years ago, how much more might have emerged since? I clearly needed to know, so I emailed Newby, who turned out to be a professor at Edinburgh university. "Not sure which piece you are referring to," he replied. "There was a rather mischievous article in the Sunday Times that misrepresented our work."

In reality, he said, "The exposure to air pollution is about the same whether you sit in a car or cycle. The concentration of air pollutants inside a vehicle is four times that outside the vehicle, but cyclists breathe four times harder because of the exertion. Overall, the exposure is about the same."

Does that mean that really we should all be wearing masks, whether we travel by car or by bike? In principle, says Newby, a mask should reduce exposure to pollutants, "although it has to be tight-fitting, with no leaks. Modern masks are pretty efficient, but their efficiency can be a problem with time, especially when clogged up."

I anxiously forwarded this correspondence to my partner, the Bike Fixer General, who has been known to cycle from one end of London to the other and back again in the course of a day's work. The BFG immediately fixated on the word "congested".

"It's the motorists' lot to be stuck with congestion," he sniffed, "any city cyclist with half a brain will find ways of avoiding it."

If in doubt, check the Transport for London journey planner and then customise your route, he added, because "to give TfL its due, it does tend to go to extremes in offering slightly circuitous but offroad cycle routes."

Fumes are one thing, but what about all those other allergens? Asthma UK, whose remit includes seasonal allergic rhinitis - aka hay fever - categorically states that "there is no strong evidence that wearing a face mask will be helpful". But then it confuses things by issuing a statement that Olympic athletes heading for Beijing shouldn't be put off using masks because of the British Olympic Authority's decision not to issue any. A spokeswoman explains that some of its members swear by masks. "Our line is to do whatever you personally find helpful."

So, what are the options if you decide that the most helpful thing would be some sort of protection? It's not the habit of this column to offer consumer advice, but I'm rather taken by the idea of a Bandit Anti-Pollution Scarf, which uses a charcoal filter. It has a lifespan of six months, so you don't have to remember to change it every few weeks like most mask filters, but the thought of wearing it in the current heat makes me feel faint.

As for the future, Brunel university seems to have it in hand. Over the past couple of years it has produced two new designs - a rather intimidating mask/helmet combination and a lightweight mouth-held breathing contraption that looks like an oversized baby's dummy and claims to be able to filter particles as small as bacteria. Neither has yet found a manufacturer, but it's surely only a matter of time.

Meanwhile, as Newby says, don't let fear of pollution put you off: "Cycling and physical exercise are good for your health, other members of society and the environment."