Anyone who is responsible for operation of a radio transmitter should be aware of the effects that electromagnetic radiation may have on the human body. Exposure to high levels of radio energy can have dramatic consequences on water-based and carbon-based materials, as experience with your microwave oven has probably demonstrated. Thus, it is very important to be sure that no one is in any danger of overexposure to your transmssions.
The FCC and the ARRL have both produced detailed guidelines for RF exposure limits. Unfortunately, they tend to be very wordy and difficult to understand. If you want to get the official word, the ARRL's site is here.
Ths FCC has issued a publication, called Bulletin 65, which specifically addresses RF exposure considerations for radio transmitters. It is a large publication, containing many parts, covering all types of RF emitting devices, including cell phones and broadcast stations. The part pertaining to Ham Radio systems is Section B. It is recommended that you read Section B, even though it is a bit difficult. For the complete Bulletin 65, in ZIP format, click here. Just be warned - it's a long read!
Bulletin 65-B also provides formulas to estimate power density in the main lobe of an antenna. These formulas have been incorporated into a program written by Wayne Overbeck - N6NB, which has been converted to web format. This program allows easy estimation of the potential RF exposure levels. The RF Exposure program may be accessed here. Note: If you are estimating the RF exposure from a Ventenna or an HFp, use 2.2 as the Antenna Gain figure in the program.
The RF Exposure program also includes this caveat, -
"This program is intended for approximate far-field calculations. It may overestimate the actual field strength of high-gain antennas in the near field (within several wavelengths of the antenna.) However, it may also underestimate the strength of fields that may be encountered in hot spots in the near field. No computer program can predict where wiring or reflective objects may create hot spots in your particular installation."
In plain language, this means that you should not just depend on the RF exposure program, but also apply some judgement to your situation. If you are going to make an error, make it on the side of safety!