Ricky Bess spends eight hours a day in front of a computer near Orlando, Fla., viewing some of the worst depravities harbored on the Internet. He has seen photographs of graphic gang killings, animal abuse and twisted forms of pornography. One recent sighting was a photo of two teenage boys gleefully pointing guns at another boy, who is crying.

An Internet content reviewer, Mr. Bess sifts through photographs that people upload to a big social networking site and keeps the illicit material  and there is plenty of it  from being posted. His is an obscure job that is repeated thousands of times over, from office parks in suburban Florida to outsourcing hubs like the Philippines.

With the rise of Web sites built around material submitted by users, screeners have never been in greater demand. Some Internet firms have tried to get by with software that scans photos for, say, a large area of flesh tones, but nothing is a substitute for a discerning human eye.

The surge in Internet screening services has brought a growing awareness that the jobs can have mental health consequences for the reviewers, some of whom are drawn to the low-paying work by the simple prospect of making money while looking at pornography.