Advertising fraud, falling digital ad rates and Google’s shifting algorithms. What’s another headache to online publishers? Ad-blocking software.

Ad-blocking browser extensions were used by 198 million people worldwide in June, according to a study published today by software maker Adobe and ad filtering startup Page Fair of Dublin, up 41 percent over last year. The report said the use of ad-blocking software could cost the advertising industry as much as $21.8 billion in revenue this year.

Ad blockers are typically free-to-download software extensions installed into Web browsers. These programs make money by asking for donations, taking payments from publishers to have their ads unblocked or by upselling premium user features.

Their growing popularity has ad-reliant publishers worried that they are beginning to undercut their business model.

“It’s a huge issue for publishers and therefore for users, consumers who value free content,” said Jason Fairchild, chief revenue officer of OpenX of Pasadena, an automated digital exchange where publisher advertising inventory is bought and sold. “These ad blocking systems threaten (the advertising business model), therefore it’s a huge issue to the whole industry.”

Fairchild faulted poor user experience – ads that serve irrelevant information, popup unexpectedly or auto play sound – for driving the adoption of ad blockers. He said publishers are increasingly aware of the importance of user experience and are starting to keep an eye out for poorly served advertisements.

“If a bad ad slips through, our phone rings,” he said. “They pay attention to it because it’s their lifeblood. There can definitely be a correlation between adding too many advertisements and seeing your user retention go down or your page views go down, and publishers are attune to those metrics.”

It may take a while for publishers to turn public opinion back in favor of the free, advertising-supported model. Until then, publishers and advertisers are grasping for solutions.

“It’s one of those issues where the answer or the solution itself is not very clear,” said Brian Fitzgerald, president of Evolve Media, an online publisher with more than 40 websites and a reach of about 95 million unique visitors a month.

“Of all the hurdles that are thrown in the path of publishing, this is a significant one.”

Many have suggested developing software to neutralize advertising blocking, but Fitzgerald said those solutions are lacking.

“The anti-ad blocking products out there they aren’t great,” he said. “They basically block the customer.”

Blocking the customer isn’t favored because those users are often part of a valuable demographic that websites don’t want to alienate: tech savvy, male millennials.

“There are gaming sites we work with or own that are 50 percent being blocked,” said Fitzgerald. “I’m getting a haircut with (advertising) inventory and there isn’t really much I can do about it.”

Technology reporter Garrett Reim can be reached at [email protected]. Follow him on Twitter @garrettreim for the latest in L.A. tech news.