Step-counting apps could be doing harm by driving people to chase overambitious goals, a leading computer scientist says.

Dr Greg Hager, of Johns Hopkins University in the US, maintains "very few" of the estimated 165,000 available healthcare apps are based on scientific evidence, yet after being downloaded more than a billion times, they were likely to have an enormous impact on public health.

Dr Hager was especially critical of apps and devices that set the user a target of 10,000 steps. "Some of you might wear Fitbits or something equivalent, and I bet every now and then it gives you that cool little message: 'You did 10,000 steps today'," he said.

"But why is 10,000 steps important? What's big about 10,000? Turns out in 1960 in Japan they figured out that the average Japanese man, when he walked 10,000 steps a day, burned something like 3000 calories (12552 kilojoules) and that is what they thought the average person should consume so they picked 10,000 steps as a number.

"But is that the right number for any of you in this room? Who knows? It's just a number that's now built into the apps."

A survey of several hundred mental health apps used for coaching and diagnosis found only five that could be linked to an evidence base, he said. None of those were available to the public; they were all research tools.

"I think apps could definitely be doing more harm than good. I am sure that these apps are causing problems," he added.

"Without any scientific evidence base, how do you know that any of these apps are good for you? They may even be harmful.

"The 10,000 steps example typifies the problem in many ways. We all know that probably the more you exercise, the better it is for you. But if you are elderly or infirm then this is not going to be good for you."