The Network Advertising Initiative will unveil a new tool on Thursday that allows people who want to avoid behavioral targeting to permanently preserve their opt-out cookies.

Currently, Web users who don't want to receive targeted ads can opt out via cookies. But those cookies have notoriously short lives -- often because users who want to avoid tracking frequently delete all of their cookies, including the opt-out cookies. Once the opt-out cookies disappear, behavioral targeting companies revert to tracking users and serving them targeted ads.

The new NAI Consumer Opt-Out Protector, available as a Firefox add-on, preserves the cookies that opt out of behavioral targeting by NAI members even when consumers attempt to delete all of their cookies. The organization currently has more than 35 members, including Advertising.com, Audience Science, BlueKai, Google, Microsoft, Yahoo and Tacoda.

NAI executive director Charles Curran says the new add-on is one component of the industry's attempt to beef up its efforts to ensure that consumers who want to avoid behavioral targeting can do so. "We think this is an important step along the path of our program for self-regulation" he says.

Earlier this year, Federal Trade Commission member Pamela Jones Harbour criticized the opt-out cookie as "fundamentally flawed," in part because it's subject to deletion. "It is unrealistic to rely on an assumption that the opt-out cookie will remain on a user's computer indefinitely. Cookies can be and are deleted (intentionally or unintentionally) by individual users, automated software (e.g., anti-virus and anti-spyware tools), or chance," she said in February, when the FTC issued self-regulatory principles for online behavioral advertising.

But the NAI's browser add-on isn't a cure-all, says Lee Tien, an attorney at the Electronic Frontier Foundation -- which is urging Congress to limit companies' ability to collect online data. He warns that companies have started using new tracking technology, such as Flash cookies -- which are harder for users to delete or control than traditional HTML cookies.

"Standing alone, this tool is better than not having it," he says. "But many of the benefits are going to be washed away by what is happening in the rest of the tracking world."

He adds, "There are many roads to the Rome of tracking."

Users who download the NAI tool and then change their minds about behavioral targeting can always change their opt-out preferences, or can uninstall the tool altogether.

The NAI isn't the only industry player to come up with a more permanent opt-out method. In March, when Google launched a behavioral targeting program, the company also offered a browser plug-in that preserves users' opt-outs.

BlueKai helped design the new tool. The NAI plans to make a comparable tool available for other browsers, including Internet Explorer.