In fact, the FCC has a chart for permissible field strengths for public
exposure, with a separate curve for trained worker exposure. The trained
worker exposure is 5 times higher across the spectrum as it is assumed the
worker will take steps to manage the risk and minimize exposure, while the
public may sit in the field and have a picnic. The chart is fairly simple,
with only a few bends in the "curve" to take into account the wavelength in
comparison to the size of body parts. Again, the only recognized risk is
from heating, and the power levels are balanced against the body's ability
to regulate its blood temperature. In addition, the exposure can be averaged
over a 30 minute period for public settings, so the total energy from a
keyfob averaged over half an hour is miniscule indeed.

More info is available at
http://www.arrl.org/news/rfsafety/exposure_regs.html - one of the best
non-subscription online sources.

My father was a radar tech in WW II (he had a Marine with a .45 assigned to
him to protect our country's secrets in the event of imminent capture). He
told me of warming himself in front of radar dishes, and that the Army told
him he would be sterile as a result. My four brothers and I were born after
that, so I guess he wasn't all that sterile.

Mike (Sr. Communication Field Engineer for an electric utility)