The restaurant was crowded for lunch.

About 10ft from us was a table with four or five couples, their babies and toddlers. It was a noisy gathering and, much as I love children, the babies seemed to scream a lot.

Anyway, we ordered coffee and one of our friends produced her shisha - an electronic cigarette. It was small and elegant with a diamond cut end and hers contained zero nicotine. I felt like applauding. I'm talking about a long-standing and dedicated smoker who, just months before, might have spent half the lunch standing outside puffing on a real cigarette. Instead, she was with us throughout, courtesy of her fruit-flavoured substitute.

Steven Camley's cartoon

For her the shisha mimics the action of having a cigarette with her coffee (she even flicks imaginary ash off the end). Her hands and mouth are performing familiar actions as she "vapes" but her daily nicotine consumption is much reduced because she smokes fewer real cigarettes.

It seemed so harmless, probably even beneficial, that we were taken aback when a waitress approached and asked her to put it away. The use of e-cigarettes was not permitted, she said. It was against company policy. As we struggled to hear her over the din of the children, I paused to wonder which table was producing the greater environmental disturbance. I also wondered about the busy-body attitude. Wasn't the company's policy illiberal? What harm was my friend doing anyone else?

I support Scotland's stance on smoking. A year from now, shops will stop displaying tobacco products by law. With smokers costing the NHS more than £320 million a year, it's a sensible move. Translate that into the saving in human misery and untimely loss of life and who could argue against the policy?

But is there justification for banning a nicotine-free substitute, as in my friend's case, or even one with nicotine that helps smokers to quit? Just how far should we encroach on the right of the individual to harm themselves if their actions cause us no harm?

Quite far, if we are to follow in America's footsteps. New York, Chicago, San Francisco and Los Angeles have banned the use of e-cigarettes in public places. Public Health England has debated whether the use of e-cigarettes risks "re-normalising smoking in public places". Should Scotland follow suit, even though one of the strongest arguments for banning cigarettes in public places was the risk of passive smoking and, as far as we know, that doesn't apply with e-cigarettes?

According to a recent YouGov survey commissioned by ASH Scotland, the use of e-cigarettes has soared. More than half of smokers have used one but, significantly, that figure drops to below 1% among people who have never smoked. So there is no evidence that "vaping", as it is called, attracts new recruits. There is, however, anecdotal evidence that addicted smokers cut back or give up cigarettes.

Instead of individual establishments banning e-cigarettes, should they be tolerated for the greater good?

The World Health Organisation concedes that they are safer than cigarettes. The Journal of Public Health says available evidence shows them to be comparable in toxicity to conventional nicotine replacement products. You don't see restaurants frisking clients for patches or asking them to stop chewing gum. So are they right to ban e-cigarettes because they mimic the act of smoking?

Critics point out one danger. Although the liquid used to fuel my friend's shisha was nicotine-free, it doesn't have to be. Opponents say that even when diluted to 8% nicotine, a tablespoon of it can kill, and they worry that, because it is unregulated, people have it at home to re-fuel their e-cigarettes. Some of it is in brightly coloured bottles with fruit flavours that could attract children.

The liquid typically comes in strengths from the zero nicotine to around 4.8 milligrams per millilitre, but higher concentrates can be bought on the internet. It contains other chemicals, like propylene glycol.

There is no doubt that it is a toxic brew and, in America, there have been cases of children being hospitalised after drinking it. But there is, as yet, no evidence that smoking it causes harm. It produces an odourless water vapour. (Advocates say that if users hold their breath for a few seconds it produces no vapour at all.) Admittedly, the product hasn't been around for long enough for its side effects to emerge or for there to have been long term studies.

Sheila Duffy of ASH Scotland has called for a vigorous public debate about the use of e-cigarettes.

She wants to see regulation of the market, an under-18 age bar for Scotland and restrictions on how the products are promoted. She notes that nicotine is highly addictive and that the companies involved are under commercial pressure to recruit young users.

But she adds: "Including e-cigarettes in the smoke-free enclosed public spaces legislation would require scientific evidence that harm from second hand e-cigarette emissions is likely. This is not the situation to date. But we support venues that have banned vaping to protect smoke-free environments."

A new study from Mississippi involving more than 3000 adults found that young parents use e-cigarettes believing them to be safer than the real thing. Eighty-one per cent thought they were less harmful to others, 76% thought they were more acceptable and almost as many said they could use them where real cigarettes were not allowed.

This last point is important. I look at the effort my friend is making to cut down and I worry that, if she has to step outside to "vape", she'll revert to her old ways.

I have some sympathy with the view of Mark Littlewood, the director general of the Institute of Economic Affairs, who said: "My concern is that public health bureaucrats are now more interested in whether there should be a cultural norm of individuals putting small cylindrical devices between their lips than whether the consequences of doing so are life threatening."

Smoking is a curse that lines the pockets of tobacco companies and destroys the health of its addicts, a disproportionate number of whom are also poor. In Scotland's most affluent areas, 10% of people smoke. That rises to 39% in our most disadvantaged areas.

It kills more than 10,000 a year and puts five times that number into hospital. I wouldn't and couldn't defend it for a second.

I was once a smoker but I was one of the lucky ones who found it easy to quit. Others are not so fortunate and one of the problems they have is missing the ritual of lighting up.

If they can do that less harmfully with an e-cigarette, I'd say let them. I agree with ASH that the devices should be regulated. But while we have no scientific evidence that they cause their users or others harm, why ban or discourage them? While the statistics show that non-smokers do not use them, are we tempted to be punitive just because we can be?