What is the safe current level for electrical stimulation to the head,
that is, the level that will not cause any permanent damage? (A
psychiatrist or neurologist could probably answer this question.) The
answer should be in mA's (milliamperes) and a source should be listed
for double-checking.

Greetings, and thank you for turning to Google Answers with your
query.

In November 2001, the Department of Health of South Africa published a
document “Limits for Human Exposure to Time-Varying Electric, Magnetic
and Electromagnetic Fields”
(http://www.who.int/peh-emf/EMFStandards/who-0102/Africa/South_Africa_files/table_datoteke/South%20Africa_Dept%20of%20Health%202001.pdf),
based on 1998 guidelines of the International Commission on
Non-Ionizing Radiation Protection.  The maximum permitted current
density is given in mA per square meter.  Maximum occupational
exposures for head and trunk are given as 40 for frequencies up to
1Hz, 40/f for 1-4 Hz (where f is the frequency in Hertz), 10 for
4Hz-1kHz, f/100 for 1kHz-10MHz, and no permitted exposure above 10MHz.
 Maximum exposures of the general public are 8 for frequencies up to
1Hz, 8/f for 1-4 Hz (where f is the frequency in Hertz), 2 for
4Hz-1kHz, f/500 for 1kHz-10MHz, and no permitted exposure above 10MHz.
 Above 100 kHz, there are also restrictions on the SAR (specific
absorption rate).

Incidentally, low-level electrical stimulation to the head can be
beneficial to health and useful for pain relief, as discussed in a
1998 document by the American Academy of Pain Management
(http://www.elemaya2.com/Cesinfo1.htm ).  These currents usually do
not exceed 1 mA.

I hope this provides a suitable answer to your query.