Why is duty cycle one of the factors used to determine safe RF radiation exposure levels?

Duty cycle refers to the ratio of time a transmitter is actively emitting RF energy compared to its total operating time. Since RF safety limits for human exposure are based on tissue heating over time, a lower duty cycle means the body has time to dissipate heat between transmissions, which lowers the overall average exposure. Different operating modes inherently have different duty cycles. For example, CW (Morse code) and SSB (voice) have lower duty cycles because the transmitter is only producing full power during key-downs or voice peaks. Continuous modes like FM or RTTY transmit continuously, resulting in a 100% duty cycle while the push-to-talk is engaged. Because the hazard comes from heat accumulation over time, evaluating the average exposure using the duty cycle is essential for determining if a station complies with safety guidelines.