edVirtus Reference

What Is Specific Energy Absorption?

How Does SA Describe RF Energy Absorbed per Unit Mass?

Specific energy absorption (SA) is the radio-frequency energy absorbed per unit mass of tissue during a stated exposure interval. It is expressed in joules per kilogram (J kg⁻¹) and is an internal dosimetric quantity rather than a measure of the external field.

SA is the time integral of specific absorption rate (SAR). If SAR remains constant during an interval, SA equals SAR multiplied by time. SAR therefore describes the rate of absorption in watts per kilogram, whereas SA describes accumulated absorbed energy in joules per kilogram.

The quantity is particularly useful for brief localized exposures. A short burst can deposit appreciable energy even when a longer averaging interval would reduce the corresponding time-averaged SAR, so the exposure duration must always accompany an SA value.

In the ICNIRP framework, additional SA restrictions address brief localized exposures above 400 MHz and up to 6 GHz. They complement, rather than replace, the SAR restrictions used for longer exposure intervals in that frequency range.

SA is not a universal RF dose and should not be confused with absorbed dose from ionizing radiation. Both may use joules per kilogram, but they belong to different interaction mechanisms, protection systems, and assessment frameworks.

Specific energy absorption may be determined by experimental or computational RF dosimetry. The reported result should state the frequency, source and body configuration, exposed tissue mass, integration interval, spatial averaging, operating conditions, validation, and uncertainty.

Above 6 GHz, absorption becomes increasingly confined near the body surface, and absorbed power density or absorbed energy density is used instead. SA alone does not specify temperature rise or biological effect, which also depend on heat transfer, blood perfusion, anatomy, and exposure geometry.