Most of the reporting on ad blocking focuses on the people who deliberately block ads either because they don’t want to see any ads or want to eliminate elements that slow down the web. But there’s another group of people who didn’t necessarily set out to block ads but are doing other things that have the side effect of blocking them. Call them the accidental ad blockers.

Wired, which has been in the ad-blocking trenches for a while, says it sees this happen two ways. There are those who are access the internet through an institutional, governmental, or corporate internet connection and their IT department has put web filtering on their firewall that acts as an ad blocker by blocking JavaScript or entire domains.

There are also those who are using a feature in Firefox’s private browsing mode called “tracking protection” that effectively blocks out ads by using Disconnect’s tracking blocking list.

Another publisher, The Atlantic, says visitors appear as ad blockers when they’re using script blockers, that come in the form of privacy tools like Ghostery; custom host files; and Javascript blockers like NoScript.

Of course, people who have the sophistication to use those tools are likely to be aware of and using ad blocking software already. GlobalWebIndex research found that 23 percent of internet users aged 16-64 in the U.S. were using anti-tracking software like Ghostery. A large share of them (79 percent) said they are also blocking ads in some way, the research found.

Meanwhile, many people seem to use ad blockers as a means to avoid ad tracking, when tracker blockers would be a better solution, said Sean Blanchfield, CEO of PageFair, which helps publishers unblock ads.

People who follow the issue say they believe the number of accidental ad blockers is small now; the top three privacy tools, Ghostery, Disconnect and Privacy Badger have 3.5 million users in the Chrome store, Blanchfield noted. But it could grow if more browsers and wireless carriers make ad blocking the default, as Alibaba’s UC Browser in India and China and mobile carriers Digicel and Three have done. This will extend default ad blocking to mainstream users who are less likely to know how to change their settings to unblock ads.

“For publishers that ban users’ access to sites for unknowingly using ad blockers as a part of their browser for example, this is a terrible experience for the consumer and terrible brand building by the publisher, because it may mean you that you are actually punishing your readers for something they didn’t even consciously do,” said Karol Severin, analyst at Midia Research.

On the bright side, publishers have gotten more nuanced with their messaging, asking people to whitelist in exchange for an ad-light experience or support the site financially in other ways — options that also take into account the inadvertent ad blocker, said Brian Kane, COO of Sourcepoint, a startup that provides anti-ad blocking tech to publishers.

The Atlantic is one publisher that’s thinking about how to communicate with these visitors, some of whom have complained when served the message, “We noticed that you have an ad blocker enabled.” The problem is, the Atlantic can’t track these people so it can’t quantify them or tailor its messages to them, said Kim Lau, svp of digital and head of business development there.

“Our current message uses the word ‘ad blocker’ and makes certain assumptions about their intent,” she said. “I think in future iterations we will be more likely to reference the result rather than the utility we ‘think’ they’re using, or we’ll be more inclusive when referencing those utilities.”

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