Most Americans now use them, but a new study is fueling concerns about the effects of cell phones on our health.

The study, from the National Institutes of Health, showed changes in the brain activity of cell phone users when the phone was turned on, and held against the head.

However, researchers do not know if that activity was harmful.

There's been a long debate on the potential impact of a cell phone on health.

Most previous studies have not shown an increase in brain tumors or cancer.

However, experts recommend limiting childrens' access to cell phones as a precaution.