RF exposure is the radio-frequency energy your body absorbs from a transmitting antenna. At amateur frequencies and powers, the effect that matters is heating. The FCC sets exposure limits, and the licensee is responsible for keeping everyone, including family and neighbors, within them.
How RF affects the body
Radio waves have far too little energy per photon to ionize atoms. Their risk is heating.
Radio waves carry far too little energy per photon to damage molecules the way X-rays do. What they can do is warm tissue, much as a microwave oven warms food, but at levels far lower. That is why the limits are about heating, and why "non-ionizing" does not mean "harmless": enough RF can still overheat tissue.
The body does not absorb every frequency equally. Absorption is strongest around the VHF range, so the limits are strictest there. Limits are also tighter for the general public, who may be exposed without knowing it, than for people who know about the hazard and can control it.
Touching an antenna while it transmits is a separate hazard: an RF burn at the point of contact.
What sets your exposure
Relative and far-field only. Very close to an antenna, the distance rule fails. See near field and far field.
In the far field, power density is roughly power times antenna gain, divided by the square of the distance:
Far-field estimateS = P × G ÷ (4π × R²)
So distance is your strongest lever: doubling it quarters the exposure. Close to an antenna the simple formula is unreliable, because the fields are complicated; see Near field and far field.
Limits apply to the average over a set time, so duty cycle matters: the share of time you actually transmit. FM and most digital modes put out constant power whenever they are keyed; SSB speech and CW average much less. Pattern matters because a beam concentrates power in one direction. Behind or beside it you usually see far less than in front.
Schematic and relative. Frequency also matters: the body absorbs some bands better than others.
Keeping it under control
Stations are expected to be evaluated for compliance, and the rules include exemptions for lower-power operation that depend on band and power. Do not rely on memory or rules of thumb: check the current FCC rules and OET Bulletin 65, which covers amateur stations. Even an exempt station must still meet the limits. You can evaluate by calculation, computer modeling, or measurement with calibrated equipment, and you should re-evaluate whenever the transmitter or antenna system changes.
The practical controls are simple:
Put antennas high and away from where people sit, sleep, work or play. Indoor and attic antennas deserve extra care.
Use the lowest power that makes the contact, and transmit only when you need to.
Never point a directional antenna at people. High-gain dishes and long beams can put a lot of power in a narrow beam.
With a handheld, keep the antenna away from your face.
Turn the transmitter off before touching or adjusting an antenna.