edVirtus Reference

What Is Specific Absorption Rate (SAR)?

How Is Absorbed RF Power Expressed per Kilogram of Tissue?

Specific absorption rate (SAR) is the rate at which RF electromagnetic energy is absorbed per unit mass of biological tissue. Its unit is the watt per kilogram (W/kg). A SAR of one watt per kilogram means that one watt of RF power is being absorbed in each kilogram of the specified tissue mass.

SAR is a rate of power absorption, not an accumulated ionizing-radiation dose. It describes energy absorption while exposure is occurring. Specific absorption, expressed in joules per kilogram, is the corresponding energy per unit mass over a stated exposure interval.

Whole-body average SAR represents total absorbed RF power divided by total body mass. Local SAR describes absorption within a smaller region. Exposure standards may specify a spatially averaged local SAR over a defined tissue mass so that small regions of concentrated absorption are assessed consistently.

SAR depends on both the external field and the exposed body. Frequency, field strength, polarization, body dimensions, posture, tissue conductivity and permittivity, distance from the source, and antenna position can all change the value and its distribution. The same transmitter power does not therefore imply the same SAR in every exposure situation.

Because SAR is an internal quantity, it is not usually measured directly in a living person. Compliance assessments use standardized laboratory phantoms, electric-field probes, temperature-rise methods, or computational models to estimate the absorbed power within tissue.

SAR is especially useful where RF energy penetrates far enough for absorption within the body to be important, including many devices operated close to the head or body. At higher frequencies, where absorption is increasingly confined to superficial tissue, standards may use absorbed power density instead.

SAR and temperature are related but are not identical. Temperature rise also depends on exposure duration, blood flow, thermal conduction, tissue heat capacity, and environmental conditions. SAR provides a dosimetric input from which heating can be assessed; it is not itself a direct temperature measurement.

A quoted SAR value is meaningful only when its averaging mass, averaging time, frequency, and exposure configuration are known. Regulatory limits and test procedures can differ by jurisdiction, so compliance should always be assessed against the applicable standard rather than by comparing isolated SAR numbers.