Specific Absorption Rate

Specific absorption rate (SAR) quantifies how much radiofrequency energy body tissue absorbs, in watts per kilogram, averaged over a defined mass and time. Averaging convention matters: some regulators specify a peak over one gram of tissue, others over ten, and the figures are not directly comparable.

Non-ionizing is the central distinction. Ionizing radiation — short-wavelength ultraviolet, X-rays, gamma rays — carries enough energy per photon to strip electrons from molecules and damage DNA directly. Radio photons at mobile frequencies carry a tiny fraction of that, and no quantity of them adds up to one ionization, any more than a brighter red lamp turns red light blue. The established mechanism of harm at these frequencies is tissue heating, which is what the limits are written to prevent. Work on non-thermal effects continues and has not produced a consistent, replicated mechanism.

Distance dominates. Radiated power density falls with the square of distance, so a source at arm's length delivers a small fraction of what it delivers against skin. Transmit power also differs by radio type: a cellular handset may reach on the order of a watt at peak and cuts power automatically where coverage is good, while a short-range personal-area radio typically transmits a few milliwatts.

Certification is narrower than it is usually read to be. A device is tested at maximum power in defined positions against a tissue-simulating phantom, and the published figure demonstrates compliance under those conditions. It is not a measure of everyday exposure, not a threshold above which harm begins, and not a safety ranking among compliant devices — reading a lower number as safer is an Exhaustive Claim the test cannot support, and mistaking the certificate for the Ground Truth is a Plausible Mechanism error.