SAR and microwave radiation hazards

SAR (Specific Absorption Rate) is the rate at which RF/microwave energy is absorbed per unit mass of body tissue. Unit: W/kg.

SAR = σ|E|²/ρ      (W/kg)
SAR = c·(dT/dt)    (from temperature rise)
σ = tissue conductivity (S/m)
E = rms electric field in tissue (V/m)
ρ = tissue density (kg/m³)
c = specific heat (J/kg·°C)

SAR is averaged over the whole body or over a small mass (1 g or 10 g) for local exposure, and over time (6 min or 30 min).

Hazards based on SAR

• Thermal effects: energy is converted to heat. Whole-body heating causes heat stress; localised heating hurts organs with poor blood flow, such as the eyes (cataract) and testes. The safety limits are set at 1/10 (workers) and 1/50 (public) of the 4 W/kg threshold.
• Frequency dependence: the whole body absorbs most near its resonance (about 70–100 MHz for an adult); at microwave frequencies absorption is concentrated near the surface, so local SAR matters more.
• Local exposure: mobile phones close to the head produce high local SAR. Limits: 2 W/kg over 10 g (ICNIRP/IEEE, head and trunk); 1.6 W/kg over 1 g (FCC, USA).
• Non-thermal effects: possible effects on the nervous system, sleep and cell function at low SAR are still being studied; IARC classes RF as Group 2B (possibly carcinogenic).

Standard SAR limits (ICNIRP 2020 / IEEE C95.1-2019)

SAR is measured with a phantom head/body filled with tissue-simulating liquid and an isotropic E-field probe, and it is the basis for certifying mobile phones.