The number of smartphone owners using ad blocking software has risen by 90 percent over the past year, according to a new report, reaching nearly 420 million people worldwide. That represents about one out of every five smartphone users across the world, according to figures released this week by PageFair, a startup that helps publishers get around ad blocking software, and Priori Data, which tracks mobile apps.

The report shows that mobile ad blockers have grown especially popular in emerging markets like China, India, and Pakistan. Thirty-six percent of smartphone owners in the Asia-Pacific region now block ads on the mobile web, including nearly two-thirds of all smartphone owners in India and Indonesia. Mobile ad blockers are comparatively less popular in Europe and North America, where 14 million people regularly use them. Just 4.3 million Americans — around 2 percent of the smartphone owning population — use mobile ad blockers, compared with 159 million people in China.

"We found the results surprising because in the West we don’t often consider what’s going on in developing countries," PageFair CEO Sean Blanchfield tells The New York Times. "It’s only a matter of time until mobile ad blocking comes to the West."

The report suggests that ad blockers are popular in emerging countries because they help reduce load times and bandwidth use, thereby cutting spending on data plans. Ad blocking browsers have become the dominant form on smartphones, PageFair says, with 408 million global users as of March 2016.

PageFair's findings add to the ongoing debate surrounding ad blockers, which intensified last year after Apple announced that users could block ads in Safari on iOS 9. Browsers like Opera and Mozilla have since released their own products to block ads on Apple's operating system. Supporters of the software say users should have the choice to opt-out of seeing aggressive advertisements, while publishers and other opponents say it threatens to undermine their ad-based business model. In a previous report, PageFair estimated that ad blocking resulted in global revenue losses totaling $21.8 billion in 2015.

"Mobile adblocking is a serious threat to the future of media and journalism in emerging markets, where people are coming online for the first time via relatively expensive or slow mobile connections," the report released this week says. "Usage in western economies is likely to grow as more manufacturers and browsers start to include adblocking as a feature."