As we wrote on Wednesday, the next version of iOS — the operating system that powers iPhones and iPads — will allow developers to make adblockers for Safari. Such a blocker would mean ads don’t load on mobile webpages, which has obvious implications for publishers hoping to pay their bills. Much more about that here.

But how big a deal would this be? How big a factor is iOS Safari for publishers?

The nice people at Parse.ly shared some data on that point, in the form of this chart:

This data is pulled from Parse.ly’s network of publishers, which includes quite a few familiar names: Mashable, Reuters, Conde Nast, Business Insider, Upworthy, and more. In all, it’s a network of over 350 publishers representing about 6 billion pageviews a month.

According to this dataset, pageviews from iOS devices made up 27.6 percent of all publisher pageviews in May. And you can see how that number as climbed over time, up from around 20 percent a year ago. (The equivalent number for Android is 16.7 percent.)

One caveat: This is data for iOS, not iOS Safari specifically. It’s unclear from Apple’s documentation whether these content-blocking extensions will work only in the Safari app or also in in-app webviews (e.g., a link clicked inside the Twitter app) or in other iOS browsers (e.g. Chrome). (In general, iOS extensions can be used or accessed from many different apps, but the documentation only mentions Safari. Meanwhile, nearly all non-Safari browsers on iOS use Safari’s rendering engine.)

If you follow that trend line out to fall, when iOS 9 will be released, it’s fair to guess that around 30 percent of all publisher pageviews — and around 60 or 65 percent of their mobile pageviews — will be coming from iOS.

The next question: How popular will adblocking be on iOS Safari? Tracking ad blocking is hard, and different publications’ audiences will vary substantially. This study — admittedly from a company selling “adblock solutions” — found 27.6 percent of American Internet users said they used adblocking software, including 41 percent of those between ages 18 and 29. Another estimate put usage at 15 to 17 percent in the United States, twice that among younger users.

Will adblocking be more popular on mobile devices than on desktops? I think so. For one, installing an adblocker on your iPhone will be an easier process than it is on your desktop; it’ll involve downloading an app rather that figuring out what an “extension” or a “plugin” is and how to install it. I’d wager that adblocking apps will be at or near the top of the Most Popular lists in the App Store. And the arguments for blocking ads on your phone are simply better than those on desktop — you pay for your data byte by byte, and download speed — often choked by ads — is a real factor. And adblocking rates are doing nothing but climb on desktop.

So let’s imagine that, within a year of iOS 9’s launch, a third of its users are using adblockers. That would take a 20 percent or higher bite out of all mobile ad impressions for publishers — just at a time when “making more money on mobile” is No. 1, 2, and 3 on many publishers’ to-do lists.

Is it the end of the world? No. But I suspect adblocking on iOS will take a real, significant, noticeable chunk out of mobile revenue for publishers.