YouTube is raising the bar for videos can take advertising and vowed to add human reviewers to its most popular content, part of an effort to address marketers’ concerns their ads are appearing alongside offensive or controversial content.

The video site said Tuesday that human reviewers would scrutinize every video in Google Preferred, its elite ads program accessible to top creators. New videos uploaded to Google Preferred channels will also have to be manually verified to meet YouTube guidelines before they can run ads.

“There’s no denying 2017 was a difficult year, with several issues affecting our community and our advertising partners,” Paul Muret, a Google vice president, wrote in the blog post Tuesday. “We are passionate about protecting our users, advertisers and creators and making sure YouTube is not a place that can be co-opted by bad actors.”

The changes come in the wake of an advertiser boycott of the Google-owned video site over videos with children that were the target of sexually inappropriate comments. YouTube killed hundreds of accounts, removed more than 150,000 videos from the platform and turned off comments on more than 625,000 videos targeted by alleged child predators.

YouTube is also raising the threshold for channels that want to carry ads. To qualify for the YouTube Partner Program, channels must now have 1,000 subscribers and have accumulated at least 4,000 hours of watch time in the past 12 months. The previous threshold, announced in April, required channels to acquire 10,000 total views to be eligible for the program.

YouTube CEO Susan Wojcicki said in December that changes were coming to the site, including an increase in the number of content moderators and other employees reviewing content and training algorithms. She also said YouTube would take a “new approach” to advertising on the site, deciding which channels and videos should be eligible for advertising.

YouTube said in November it had removed ads from nearly 2 million videos and more than 50,000 channels that tried to pass off as family-friendly but that featured inappropriate content. The company also outlined new rules to make YouTube safer for kids.

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