The number of vapers in Great Britain has topped three million for the first time, while e-cigarette use in the United States continues to fall.

From a survey of 12,000 British adults, charity group Action on Smoking and Health (ASH) estimates that e-cigarette users increased 10% between 2017 and 2018, up from 2.9 to 3.2 million.

3.2 million vapers represents almost five percent of Britain's population.

Across the Atlantic, meanwhile, the number of e-cigarette users is falling.

The Journal of the American Medical Association (JAMA) found that the number of US adults who tried vaping rose between 2014 and 2016, but ongoing use fell.

The number of adults who used e-cigarettes every day or on some days fell from 3.7% in 2014 to 3.5% in 2015 and 3.2% in 2016, JAMA found.

The report's authors wrote: "These trends suggest that some individuals are trying but not continuing use of e-cigarettes, but further investigation with individual longitudinal on use of both e-cigarettes and other nicotine products is needed."

Although it's difficult to get a perfect comparison between the data sets, there appears to be a general downward trend in vaping in the US and an upward trend in Great Britain.

So how do we explain this difference? Well, there are a number of factors in play.

Cost

The cost of smoking in Great Britain is higher than in the United States. A lot of this is due to high tobacco taxes in Britain.

In Great Britain, smokers pay a Tobacco Duty of 16.5% of the retail price of cigarettes, plus £4.34 ($5.71) on a pack of 20 cigarettes.

The average price for a pack of 20 cigarettes in the UK is £10.40 ($13.68).

In the US, tobacco prices vary widely from state to state. In New York, the state with the highest per-pack taxes, smokers pay an extra $4.35 in taxes on their cigarettes and the base price for cigarettes in NYC is up to $13.

But the national average for a pack of cigarettes is still just $6.16, significantly less than in Great Britain.

This means that British smokers can save a lot of money by making the switch to e-cigarettes, which cost about the same in the US and the UK.

According to the ASH survey discussed above, 10% of vapers use e-cigarettes to save money.

Policy

In some ways, policy in Great Britain is less vaper-friendly than it is in the United States.

As a member of the European Union, the UK has been forced to institute the Tobacco Product Directives (TPD). This law puts limits on the nicotine content in e-liquid and the size of e-cigarette tanks among other things.

But e-cigarette policy has grown more hostile in the United States. E-liquid flavor bans have already been implemented in some cities in America. The FDA and members of the Senate have also threatened nationwide bans on flavored e-cigs.

Meanwhile, in Great Britain, lawmakers have been more inclined to talk up the benefits of e-cigarettes.

Recently, members of parliament suggested that rules on vaping should be relaxed. They also called for a debate on the use of e-cigarettes on public transport, something that's banned in large parts of the US.

Differences in how policy makers talk about e-cigarettes on both sides of the Atlantic has fed into the public's perception of vaping, particularly relating to safety.

Safety

We can't say that e-cigarettes are any safer in Great Britain, but the public perception of harm is different, and that's important.

According to the ASH study in the UK, 25% of the public wrongly think that e-cigarettes are as bad or worse than smoking combustible cigarettes.

A comparable study in the US in 2015, however, found that almost 40% of adults believed e-cigarettes to have about the same level of harm of be more harmful than cigarettes. This proportion actually increased from around 12% in 2012.

Dr Leonie Brose, from King's College London said that the mistaken belief that vaping is as harmful as smoking was "worrying".

"Campaigns from Public Health England and others to challenge these views are important and must continue," she said.

E-cigarettes have formed a central part of the UK's push to reduce the smoking rate and 62% of e-cigarette users said they use the products to quit.

Public Health England has published landmark research which shows that vaping is around 95% less harmful than regular combustible cigarettes.

The health authority has also said that e-cigarettes should be made available on prescription because of how successful they were in helping people give up smoking.

In the United States, however, the FDA has refused to approve e-cigarettes as a stop-smoking aid and much of the rhetoric around e-cigarettes focuses on their harm, rather than how they can be used as a safer alternative to cigarettes.