Mobile phones have been around for more than 20 years, and are now used by more than 3 billion people. Yet questions linger over whether mobile phones can contribute to health problems, including cancer. Cell phones have only been mainstream products for 10 years or so, and it may take much longer than that for adverse effects to show up.

New studies on health and cell phones are tough to get going. In the United States, most research on the topic was discontinued at the beginning of the decade, largely because industry groups and government considered the questions resolved and haven’t been willing to finance new studies.

But scientists are concerned that cutting off studies could be a mistake. “It was 15, 20 years after people began smoking that we saw concerns associated with it,” says Michael Kelsh, principle scientist and epidemiologist for Exponent, a scientific consulting firm. “Down the road, the same could happen with phones.”

The latency period for brain tumors can be 10 to 15 years.

Children whose mothers used cell phones frequently during pregnancy are more likely to have behavior problems, especially if they themselves are cell phone users.

A team of scientists looked at a group of more than 13,000 children. When the children reached age 7, mothers were asked to complete a questionnaire about their own cell phone use in pregnancy and their child’s use of cell phones, as well as their children’s behavior and health.

Children with both prenatal and postnatal cell phone exposure were 80 percent more likely to have emotional problems, conduct problems, hyperactivity, or problems with peers. Children who were only exposed prenatally also had a higher likelihood of behavior problems, but not as high as those who were also cell phone users.

Despite myriad studies showing no increased cancer risk from up to 20 years of cell phone use, some scientists continue to probe — as they should, given the omnipresence of cell phones. One alternate theory is that heat generated by cell phones can cook brain cells.

The director of a Pittsburgh cancer institute has issued a warning to employees on avoiding cell phone usage, especially for children, due to possible cancer risks. In a memo sent to faculty and staff of the University of Pittsburgh Cancer Institute, Dr. Ronald B. Herberman listed ways to limit exposure to electromagnetic radiation emitted by cell phones by using various alternatives such as texting, shortening conversations and using headset or speaker phone options, the Pittsburgh Post-Gazette

Toronto’s department of public health has advised teenagers and young children to limit their use of cell phones, in order to avoid potential health risks. The advisory is the first of its kind in Canada.

Officials have warned that because of possible side effects from radio frequency radiation, children under eight should only use a cell phone in emergencies, and teenagers should limit calls to less than 10 minutes.

For many years, most government health agencies were dismissive of any risk. But with more studies, a pattern is emerging that suggests people who have used their cell phones for a long period of time are at greater risk of certain kinds of brain tumors.