There's one more reason to think twice about who you give your email address to.

A new service from Google called "Customer Match" uses email addresses to help marketers track and target users more precisely than ever before.

The service, launched in September, allows any business with a database of customer email addresses — from an email newsletter, or collected from customers at the point of purchase, for example — to upload the addresses into Google's new advertising system.

The search company will then use that information to target online ads, including those that users see while web browsing, while using YouTube, or in other places where Google sells advertising.

"For marketers and businesses, the story is that Google has given you more ways to zero in on specific, targeted audiences," said CBC technology columnist Dan Misener.

"On the flip side, for consumers, this means if you have ever given your email address to any company, there's now one more way they can track and target you online."

Opens door for personalized ads across devices

Misener said the service opens up new possibilities for highly segmented personalized ads and is an example of "retargeting," which results in users seeing the same ads over and over again, across multiple sites.

Retargeting has traditionally been accomplished using cookies — small pieces of code that websites leave behind on your computer.

But while cookies are tied to a user's device, such as a specific computer or smartphone, Customer Match will target ads to specific individuals, regardless of what device is being used.

Nicki Lamont, an internet marketer and search engine optimization specialist, told CBC this means more targeted ads showing up in more places.

"Anytime that you open up Google, when you're watching YouTube, they're going to be popping up on your phone and your tablet," she said.

"These are the devices that as a user, by nature, we don't log out of frequently. So we have more access to reach those customers."

Customer Match raises privacy concerns

Google says when companies upload email addresses for Customer Match, they're handled in a "secure and privacy-safe way."

Lamont acknowledges that customers may not expect a business to share an email address with a company like Google.

"I do see the concern of, 'It's your email address, where's it going to go?'" she said. "But that's kind of the world we live in now. If you put something online, it's a little bit fair game."

Misener said people who are concerned about their privacy have a few options, including being selective about who they share their email address with.

He also said users can avoid logging into sites that use a Google login, and users can opt out of Customer Match ads in their Google settings.