YouTube is making it tougher for creators to make money from inappropriate and offensive videos.

Real humans, not algorithms, will review the most popular videos on YouTube before ads can run in them, and the platform will tighten the rules for channels that are allowed to feature ads, the company said in a pair of blog posts on Tuesday.

The change comes after a turbulent 2017 when troubling videos slipped into YouTube Kids, and top creators like PewDiePie and Logan Paul drove advertisers away. More than 250 brands pulled their ads from YouTube last spring after they learned that their ads were appearing alongside extremist videos. Major advertisers scaled back after their ads appeared in videos that seemed to exploit children.

YouTube said that, by the end of March, the most popular videos will be manually reviewed to ensure they are safe before ads are allowed to run. These videos are part of the Google Preferred program, which highlights the top 5% of content on YouTube that’s advertiser-friendly. The preferred program isn’t fail safe: Logan Paul Vlogs was part of the program before it posted a video from Japan’s infamous “suicide forest” that showed the body of a teen who had recently died by suicide, and YouTube dropped the channel from the program afterward.

The change won’t prevent the uploading of similar video, it just won’t be eligible for ads. But it could deter creators from posting alarming content because they won’t be able to make money from it.

In addition to human review, starting Feb. 20, channels will need to have at least 1,000 subscribers and 4,000 hours of watch time within the past year to run ads and share in the ad revenue with Google. The new metrics are more restrictive than the previous threshold of 10,000 total views. YouTube said it would consider other flags like spam and strikes, which are issued when someone flags a video for violating YouTube’s community guidelines.

The measures could bar “tens of thousands” of creators from making money through YouTube’s ad share “partner” program, Recode reported, and make it harder for new channels to get started with it. YouTube said the new criteria would affect a “significant number” of channels, but most of them weren’t earning much ad revenue anyway. “99% of those affected were making less than $100 per year in the last year, with 90% earning less than $2.50 in the last month,” said a blog post from chief product officer Neal Mohan and chief business officer Robert Kyncl.

The changes are meant to differentiate established, credible channels from creators that try to game YouTube’s algorithms to make money from offensive or inappropriate videos that rack up thousands of views.