If you hadn’t heard of ad blockers before, the chances are you will soon. With Apple now allowing apps that block ads in its software store and the New York Times reporting how significantly ads slow down many websites, these applications are going to receive a lot more attention as we all consume more online content.

Such services come with a significant downside, however, and it’s another issue that more consumers will have to be aware of in future: surrendering private data.

By their very nature, ad blockers have to take note of the sites that you visit and the content on each page. Doing this allows them to weed out the advertising slots and prevent them from loading. So far, so good, at least if you have no qualms about denying revenue to the site in question, which is a separate debate to hold. But what happens to the data collected by the ad blocker from there?

That’s certainly the question being asked by users of the Adblock Chrome Extension, which last week was sold to an unknown buyer, along with any data it has collected on the individuals who use the service, possibly including their browsing habits. To add insult to potential injury, the software suddenly opted users into its default “Acceptable Ads” setting, which attempts to convince users that some ads on a site should be allowed through its filter.

Obviously this raises all kinds of questions over what makes for an ad that isn’t deemed objectionable, whether the new owner is being paid for the privilege of letting certain ads through, and whether users of such services could be persuaded to consider acceptable ads as anything other than an oxymoron.

More importantly, it raises questions over whether ad blockers are trustworthy at all. Is the upside of a quicker, cleaner browsing experience worth the asking price of logging the path of that browsing, with no guarantee that this information won’t be sold on at some point in the future?

In some sense, this could be a case of better the devil you know than the one you don’t.

For all the potential of an ad block service being sold on, at least users can research the creators when they first sign up and try to monitor changes in the software. The flipside is to leave ourselves open to the mixed-up, sometimes murky web of online ad networks, through which all kinds of ad can flow and where we’ll rarely know who is behind them.

As the Digital Citizens Alliance continues to report, the purpose of these ads can easily be to fund piracy sites or deliver malware, neither of which are attractive alternatives to trusting an ad blocker and preventing those dangers hitting our devices in the first place. Blocking everything is a big step, however, and can cut off funds to legitimate sites that rely on ads to claw back some of what they spend on creating the content we read. The Washington Post underscored that issue last month, when it activated ad block detection methods that prevent users reading its content until they turn off their blocking software.

This game of ad avoiding cat-and-mouse is limited for now to more tech-savvy users and how the sites that they visit handle their aversion to advertising. As more consumers opt for online content only and the rewards for advertisers and publishers become greater, however, ad blockers will appeal to a much wider audience. Before that happens, we will all want to be well versed in not only what they offer, but what they take in return.