The following correction was printed in the Guardian's Corrections and clarifications column, Tuesday May 22 2007

We mistakenly said that the smoking ban in public places in Scotland had commenced in 2004 in the article below. The legislation was passed in 2004 and implemented in May 2006.

The smoking debate is a bear pit of lobbyists. In the one corner you have the government, doctors and a whole raft of anti-smoking groups. In the other you have the industry and Forest, the industry-funded group that defends the right to smoke. They argue bitterly about the impact of smoking bans.

Peter Terry, chair of the British Medical Association in Scotland, says: "There's now reasonably good evidence that definitely describes the health benefits of smoking bans." He mentions a study by Dundee University which showed that bartenders' lung function had improved by as much as 10% just two months after smoking was banned from pubs in Scotland. But what about whether bans actually encourage people to cut down or give up? "There is also anecdotal evidence from GPs that they have had more approaches from people who want to give up smoking," he says.

Neil Raffety, his foe at Forest in Scotland, does not buy this: "Smoking bans don't affect whether people smoke. Smokers will adapt. You can't force someone to stop smoking."

The hardest evidence of the impact of the bans in Scotland and Ireland are cigarette sales, which suggest some success. Imperial Tobacco, which holds the largest share of the tobacco market in the UK, reported an 8% drop in sales in Scotland following the ban in 2004. A year on, that had relapsed to between 3% and 4% down on pre-ban smoking levels. The industry says this is partly because of the long-term downturn in smoking, and partly because people are smoking a bit less as the opportunity to smoke decreases.

There may also be unexpected outcomes, however. One study from University College London suggested that as people move their smoking habit - along with their drinking habits - out of bars and into their homes, they are more likely to smoke around their children.

Martin Dockrell, of the anti-smoking charity Ash, says that smoking bans have an initial impact: in New York that meant 1.7% of the city's smokers quitting in the first year of a ban coming into effect in April 2003. But Dockrell says that effect will only keep growing as long as people don't think that a smoking ban is all it takes to make people quit.

"You have to keep tax on cigarettes high, provide help for people to give up smoking and promote that help, otherwise people will relapse," he says. "Cigarettes are extremely addictive and people will start again, like they did in Ireland."

In New York, however, where the ban is four years old, half a million people have given up because prohibition was followed up with targeted support, he says.

Terry accepts that the smoking ban on its own is no magic bullet. But "If it does nothing other than maintain the long-term trend of fewer people smoking, then it has worked," he says.

In California there has been a steady decline in smoking since 1988, when the state first introduced its anti-smoking policy, from 22% then to 15% now. The state also provides the first evidence that smoking bans can reduce rates of cancer. A study in San Francisco concluded that smoking bans contributed to a 6% decrease in lung cancer.

Raffety says the Californian zero-tolerance approach to smoking won't wash here, though. "The intention of the anti-smoking lobby is to create a California mindset which demonises smoking. I don't think people here will go for that. We won't ever be that fanatical."

But people in Scotland are already "looking at smokers differently in the street", according to Terry. "Smoking will become as socially unacceptable here as it is in California".