According the best available data—a PageFair/Adobe report issued last month—only 6% of Internet users around the world are actively blocking ads.

But the phenomenon is growing quickly. Adblock use last quarter grew 41% worldwide year over year and 48% in the U.S. As of Q2, 45 million Americans were using ad blockers.

Who are these people?

• They are mostly men. In a 2014 survey, men were 48% more likely than women to use adblock plug-ins when browsing.
• They are largely millennials. Peak usage is with 18-29 year olds, 41% of whom claim to use adblock software.
• They tend to live in the Northeast and Pacific coasts. Oregon has the highest ad blocking rate at 16.4%. Washington DC, at 8.2%, has the lowest.
• They’re not on mobile devices. Smartphones and tables represented 2% of adblock use in Q2 2015, desktops and laptops represented 98%.

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Ironically, given that online advertising accounts for 90% of Google’s revenue, it’s Google Chrome, not Apple’s Safari, that’s been driving most of the adblock growth. Ad block use on Google’s browser increased 51% from Q2 2014 to Q2 2015, reaching 126 million average monthly active users.

According to PageFair and Adobe, the economic cost of ad blocking—in terms of lost ad revenue—will top $21 billion in 2015 and nearly double again next year.

“This has the potential,” the report concludes, “to challenge the viability of the web as a platform for the distribution of free ad-supported content.”

Follow Philip Elmer-DeWitt on Twitter at @philiped. Read his Apple (AAPL) coverage at fortune.com/ped or subscribe via his RSS feed.