Parents and policymakers need to take action to protect children from being harmed by TV, the Internet and other types of media, a new report says.

Researchers have performed individual studies for years to learn how media affect children. A review released Tuesday, which analyzed 173 of the strongest papers over 28 years, finds that 80 percent agree that heavy media exposure increases the risk of harm, including obesity, smoking, sex, drug and alcohol use, attention problems and poor grades.

Some of the links are particularly strong. For example, 93 percent of studies found that children with greater media exposure have sex earlier. Authors say the soundest studies are those linking media use with obesity, while the evidence linking media exposure to hyperactivity is weaker.

The study provides overwhelming evidence of the importance of limiting children's use of media and teaching them to critically evaluate the ever-growing volume of text, images and sounds with which they are bombarded, says co-author Ezekiel Emanuel of the National Institutes of Health. He says the report also urges Hollywood and technology makers to create entertainment that is less toxic and more family-friendly.

"The idea that this is having a really measurable adverse impact on health makes it important to take this seriously," Emanuel says. "Every year, we have 4 million new kids. How long are we going to wait?"

The average child spends nearly 45 hours a week immersed in media — almost three times the amount of time they spend with their parents, according to the report, commissioned by Commonsense Media, a nonpartisan watchdog group. In comparison, kids spend an average of 30 hours in school.

Keeping an eye on kids' media use is tougher today, says Jane Brown, a journalism and mass communication professor at the University of North Carolina-Chapel Hill who was not involved in the report. In the past, families often watched TV together, and parents could easily change the channel or voice their disapproval. Today's technology often isolates kids, who may tune out their families to concentrate on a cell phone screen that only they can see.

Even pediatricians struggle to stay connected to their kids. Victor Strasburger, a pediatrics professor at the University of New Mexico who was not involved in the study, says he took away his 15-year-old daughter's phone when he caught her text messaging at Thanksgiving dinner.

Michael Brody, a child psychiatrist at the University of Maryland who was also not involved in the study, says the country needs to address the onslaught of negative images. He says children today have greater exposure to online pornography and Internet "hate sites" that attack minorities and gays.

Authors say that policymakers also need to establish "clear limits" on marketing products such as junk food to kids.

Ignoring these issues, Brody says, will only lead to even higher rates of childhood obesity, type 2 diabetes, violence and teen pregnancy.

"At some point," Brody says, "we are all going to be paying for this."