Mobile apps and devices track a user's health statistics. But those data are sometimes sold and can end up in the hands of employers and insurance companies.

RENEE MONTAGNE, HOST:

And in today's Business Bottom Line: businesses that trade on information you wish could be kept secret and privileged. If you want to lose a few pounds after the holidays, you might download a smartphone app that counts steps climbed or calories burned.

And Aarti Shahani of member station KQED reports companies will be looking to use your fitness data to make money.

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AARTI SHAHANI, BYLINE: Here at the Quantified Self Meetup in downtown San Francisco, technology lovers are testing homemade do-it-yourself devices on people eager to measure their mind and body. Charles Wong straps a belt to me that vibrates when I slouch.

CHARLES WONG: You should get a nice, gentle vibration that'll remind you to...

SHAHANI: Ooh, it just happened. It vibrated.

WONG: How was that for you?

SHAHANI: It felt like a massage.

Jonathan Toomim slaps a Velcro headband on me to measure my concentration according to prefrontal cortex activity.

JONATHAN TOOMIM: Your brain activity is increasing, on average, at a fairly slow rate but still substantial - until you start talking.

SHAHANI: Heather Heine pokes my finger for a blood test.

It's going to hurt, right?

HEATHER HEINE: A little bit. This is fast. This is fast.

SHAHANI: It's a needle? It's a needle in there?

HEINE: Yeah. It's a little - yeah. Ah.

SHAHANI: OK.

HEINE: OK. You did it?

SHAHANI: According to Forrester Research, about 3 percent of online shoppers say they already use a self-tracking device. Tim Chang, a venture capitalist with the Mayfield Fund, is one of the money guys behind self-tracking. Chang raised $9 million for a new kind of tracker that he promises is...

TIM CHANG: The world's first very accurate heart rate monitor on just a wrist watch - no chest strap, no other device.

SHAHANI: Sensors and Bluetooth technologies have become so cheap and sophisticated, they can record more than steps taken and calories burned. The startup Basis plans to make money by selling the heart watch. But if the company turns a big profit, Chang says, it'll be from selling the data aggregated on a smartphone app and analyzing it for you, the user.

CHANG: People aren't really interested in raw data. If I just gave you your heart rate data, you wouldn't even know how to interpret it. In fact, it might confuse you or it might scare you, and say, what is this spike? Why is it low? Why is it high?

SHAHANI: Facebook and Google collect data on users and get advertisers to pay for access to those users. By contrast, Chang's first self-tracking company, Lumos Labs, sells data directly to hundreds of thousands of its own users. This paid subscriber base has more than doubled every year since 2007.

CHANG: The beauty of subscription business models, if you can retain your users, they can be naturally very profitable.

SHAHANI: Big data raises big privacy issues. Two years ago, some users of a leading self-tracking brand Fitbit were logging their sexual activity as exercise and found the sex logs somehow pop up on Google searches.

WOODY SCAL: We've now made those privacy settings more prominent.

SHAHANI: Woody Scal is Fitbit's chief revenue officer.

SCAL: So that people are more aware of what the privacy settings are. I think the area of privacy is an area that many companies like ours have learned a lot over the last couple of years.

SHAHANI: Fitbit is entering a brave new world in privacy as they start selling devices and data to a new market: employers. Scal says Fitbit is attempting to grow through corporate wellness programs.

SCAL: Companies can see how many of the devices they've given out have actually been activated, how many are being used. How is it actually changing employee behavior?

SHAHANI: Scal explains bosses typically don't get reports on an individual employee. They get aggregated data, and the worker must consent first. One of Fitbit's competitors, Body Media, says it's working with insurance companies to get its self-trackers into more workplaces. Scal says Fitbit is running an experiment with one insurer to see if employees who use the devices go to the doctor less.

SCAL: If we could make a direct connection to reduction in medical care costs, then I think the floodgates would be open.

MARK GOODMAN: People should be asking themselves: What happens with this data? What type of inferences can be drawn from this data?

SHAHANI: Mark Goodman is chair of policy law and ethics at the technology research hub Singularity University. He says users typically don't read disclosures. And while health and fitness devices are personal tools for now, Goodman warns, health insurers in the future could use incentives to pressure people to wear these devices.

GOODMAN: So that they could get a better perspective on how healthy or unhealthy you are. If your self-tracking health device shows that you lead a sedentary lifestyle, then maybe you will pay more for insurance.

SHAHANI: Goodman says consumers should be careful about letting any company track health data that can be used against them. For NPR News, I'm Aarti Shahani, in San Francisco. Transcript provided by NPR, Copyright NPR.