Google users might want to be a little more careful about who's looking over their shoulder when they use the popular search engine or browse videos on YouTube. The ads that pop up on results pages and the clips recommended on the video-sharing site soon will be based on keywords and phrases that appear in messages sent through the company's popular mail service, revealing a lot more about the person behind the keyboard than some users might be comfortable sharing.

The new privacy policy, which goes into effect March 1, allows Google to track and share user data across its services, which include popular - and free - mail, calendar and document-sharing applications. Under its current policy, Google tracks user data but compartmentalizes the information within each service. The company claims the new policy creates a simpler, "intuitive user experience across Google" by combining more than 60 current privacy documents for its services into one new overarching policy.

When it announced the changes Jan. 24 , Google immediately faced withering criticism from Internet privacy policy experts, watchdog groups, Congress and some Google users. Critics argue Google's new policy takes privacy away from users, who should have more control over what they share online. But others applaud Google's willingness to share its plans ahead of time and point out the company's ability to offer so many free services depends on the information they can sell to advertisers.

Charles Palmer, associate professor of new media at Harrisburg University of Science and Technology, says users should be concerned about what he dubs "the creepy factor" in Google's new policy. Users like to think their e-mail is private, so when Google Search picks up on keywords and interests from user's e-mails to generate targeted ads, it could make them uncomfortable, he said.

"We like to get riled up about privacy," Palmer said. "We want to give it away whenever we want to, not at some other company's discretion."

Under the new policy, Google's ads will become much more targeted toward users' interests because of the sharing between applications. For example, if a user consistently e-mails about repairing laptops, Google may generate ads for computer parts and more narrowly tailor its search results based on specifics mentioned by that user in private communications.

Although privacy experts rail against targeted ads, research shows that they work, largely because users respond to them, Palmer said.

"Targeted ads are one of those weird things that people complain about, but they don't really mind them," he said. "They would much rather have ads that relate to their interests."

Wesley McGrew, lecturer for the National Forensics Training Center at Mississippi State University and computer security and forensics consultant, said Google is a clear frontrunner among companies that gather user data, with its business model centered on profiting from user information.

"Google's services are nice, and, by all appearances, free," he said. "The reality is, though, that you pay for the use of those services with the personal information that Google mines to use in marketing, target advertising, and whatever else they might do."

Phil Simon, technology expert, speaker and author of "The Age of the Platform: How Amazon, Apple, Facebook, and Google Have Redefined Business," defended Google's business model, noting that like any other company, Google has to make money. Other Internet giants like Amazon and Facebook already mine user data to generate income, so it's not surprising that Google would ask users to pay for services by sharing information, Simon said.

"Google isn't selling your information on your favorite movies to the mob," he said. "I'm a big tennis and golf fan - if I'm emailing a friend about playing tennis over the weekend and later see an ad in the corner for tennis, is that really so bad?"

Having overstepped boundaries and laws with its Books, Buzz and StreetView services, Google has a history of recovering from privacy mishaps. By openly announcing its policy changes ahead of their implementation, Simon said he thinks the company has learned from its mistakes.

Paul Rubell, equity partner at Meltzer, Lippe, Goldstein & Breitstone and special professor of law at Hofstra University Law School, said while the new policy works for Google and its business partners, it may not be as beneficial for the users.

Google users are customers, Rubell said, and customers should know to be careful when shopping online.

"With Google, you are 'shopping' all the time. Whenever you search or mail or watch a video or use any Google-owned product, you are Google's customer, even though you don't know it," he said. "Because Google treats you as a shopper all the time, you should exercise caution with every keystroke that you type."

McGrew recommends users who really want to protect their privacy opt to use other services or adjust what they share through Google.

"It's the individual's responsibility to choose what information they give out for the privilege of using a service," he said.

Palmer urges his students to be careful with everything they share online because information that starts off private has a tendency to be passed around for others to see.

"Putting anything online is like writing it on the bathroom wall," he said. "It's going to be out there. If you really don't want someone to know something, don't put it in digital form."