California Department of Public Health (CDPH):

"Although the science is still evolving, there are concerns among some public health professionals and members of the public regarding long-term, high use exposure to the energy emitted by cell phones," said CDPH Director and State Public Health Officer Dr. Karen Smith. "We know that simple steps, such as not keeping your phone in your pocket and moving it away from your bed at night, can help reduce exposure for both children and adults."

The CDPH has issued guidelines to reduce the risk of cell phone radiation exposure, and it includes avoiding products claiming to block the radio frequency energy:

• Keep your cell phone away from your bed at night.
• Reduce cell use when the signal is weak.
• Remove your headset when you're not on a call.
• Reduce use of cell phones for streaming audio or video.
• Avoid products that claim to block radio frequency energy. The health department says those products may actually increase exposure.
  (via ABC7, "Health department to you: Step away from your smartphone")

High use of cellphones may be linked to brain cancer, headaches, and sleep troubles.

View the CDPH Cell Phone Guidelines document here.