The study found that filtering software set to an "intermediate" blocking level -- commonly found in schools and libraries -- limited access to about 5 percent of more than 3,000 health sites. But, on average, the software blocked 27 percent of sites specifically about condoms and 20 percent of sites about safe sex -- an impact on Web surfers the study described as "modest."
The study also found that software filters, which can be adjusted in intensity, can be fooled: They commonly block more Internet sites associated with terms like "safe sex" or "condoms" than Web pages a person might find while searching about "birth control" or "herpes." Researchers used results of software from six different companies.
At least-aggressive levels, filtering software prevented people from viewing 1.4 percent of health sites surveyed and 9 percent of sites specifically about sexual health. The study's supporters said that as filtering increased, more health sites were blocked than additional pornography sites.
The study offered a surprising lift for the handful of U.S. companies producing oft-maligned filtering software. Civil libertarians frequently complain about these programs, widely used in the nation's schools and libraries, because they arbitrarily block categories of Internet sites that some find offensive but others may find helpful.
"The message is, a little filtering is probably OK," said Victoria Rideout, a vice president at the Henry J. Kaiser Family Foundation, a philanthropic group that publishes some sexual health information on the Web and sought to find out how much of its work was inaccessible to students and library patrons. Kaiser paid $200,000 for the study.
Congress in 2000 required software filters in schools and libraries that accept federal funding, but a U.S. appeals panel in Philadelphia last year ruled that part of the Children's Internet Protection Act violates the First Amendment because filters also can block sites on politics, health and science.
The Supreme Court has said it will decide the issue by next summer.
At most-restrictive levels, designed to block access to racism, hateful speech or sites about violence or gambling, filtering software blocked 24 percent of health sites and 50 percent of sites specifically about sexual health.
"The problem seems to be, when you get into blocking all these categories, a lot of youthful, mainstream information gets blocked," Rideout said.
Sites blocked by some software filters at their most-restrictive settings included a Web page with facts about genital herpes from the U.S. Centers for Disease Control and Prevention, and a federal guide to birth control from the U.S. Food and Drug Administration. Software from one company, Symantec Corp., blocked a fact sheet about herpes from the National Institutes of Health even at its least-restrictive settings.
One of the study's authors, Dr. Caroline Richardson of the University of Michigan Medical School, predicted that some software filters probably would block access to Web pages with details from this research. The study's results were being published this week in the Journal of the American Medical Association.
Dr. Richardson said filtering software on public computers in schools and libraries can discourage research on health issues by people who need it most.
"People who are least likely to have access to the Internet at home are also at highest risk for some of the medical problems they might be able to get information about on the Internet," she said. "The Internet is a way to disseminate accurate and easy-to-access information in a timely manner; if that access if blocked, it's not clear that everybody will find the information some other way."
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