Google is once again making changes to Youtube's rules to protect advertisers from being associated with videos featuring violence or bad language.

Google said it would remove advertising from a "significant" number of YouTube channels, 99 percent of which generated less than $100 from ads in the last year, in response to advertisers that felt they have been underwriting inappropriate material.

Google plans to have people review every popular video eligible for advertising by the end of March worldwide, and it is modifying the prerequisites for having a commercial channel.

Last year, 250 advertisers cut back on YouTube ad-buying following news reports that found their ads displayed alongside extremist propaganda. However, many brands returned within weeks or months, only to again suspend dealings with YouTube in the autumn because of headlines that linked them to videos in which children appeared endangered or popular cartoons that had been disturbingly altered.

Starting Feb. 20, Youtubers will need to attract users watching a total of at least 4,000 hours of their clips within the preceding 12 months and notch 1,000 followers before being able to host ads and share in revenue from them. The requirement introduced last April had been 10,000 followers with no viewing time standard.

In addition to the new restrictions, YouTube is pledging that its staff of content moderators will screen every single video in Google Preferred, the company's premium offering for marketers. Google said that the manual review process will be set up by the middle of February in the U.S. and by the end of March in other countries.