By KYW tech editor Ian Bush

PHILADELPHIA (CBS) — Much of what we do online is free — but that comes at a cost to our privacy. Services like Google Mail and Facebook are expensive to provide, and companies recoup their investments by selling ads targeted to our web browsing habits. How to stop that digital trail is the subject of a Pulitzer Prize winner’s new book — and a free talk on Monday in Old City.

Julia Angwin spent more than $2000 in a year trying to wall off her life from prying eyes.

“I got a separate cell phone, not in my own name. I tried to remove my information from as many data brokers as I could find,” says Angwin, a senior reporter at ProPublica and author of ‘Dragnet Nation: A Quest for Privacy, Security, and Freedom in a World of Relentless Surveillance.’

Still, more than half of those hundreds of firms marketing her address, phone number — and whether she votes, smokes, likes cooking or deep sea fishing and on and on — refused to let her remove her information. Only a handful would even let her see her own data.

“Similarly, I was pretty unsuccessful with my cell phone, because you have to carry it around everywhere and it’s always transmitting,” Angwin says. “You’re always kind of findable by the cell phone company or the maker of the phone or even the apps — many have visbility into your location and behavior.”

She bought a bag to block cell signals and paid for services that claimed to safeguard her personal information, but found free success with a switch from Google to DuckDuckGo, a search site based in Paoli, Chester County.

“Your search results are stored by Google for a very long time,” Angwin explains. “I found mine dating back to 2006 were stored by Google. Simply by switching, there are no more logs. So the company itself can’t sell the data about me and the government can’t get it. ”

She worries that those coming of age in the era of social media are especially at risk.

“My kids — I’m sure like most kids — live online,” Angwin says. “They don’t even think about the world beyond online as far as I can tell. And so they more than anything need to know what the skills are to navigate that world while not leaving a permanent digital trail.”

Angwin will talk more about her quest and what we — and the courts — can do at the National Constitution Center at noon on Monday, March 10th. Her website has more information about what you can do to protect yourself online and off.

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