Radiation hazards and microwave oven zones

How microwaves are hazardous to humans

Thermal effects (main, well-established):

• Water molecules in tissue rotate with the field; the energy becomes heat (dielectric heating).
• Organs with poor blood flow cannot remove heat: eyes (cataract) and testes (temporary sterility) are most sensitive.
• Whole-body heating raises body temperature, causing heat stress, fatigue and burns at high levels.
• Penetration depth falls with frequency: at 2.45 GHz a few cm, above 10 GHz mostly skin heating.
• Human body resonance (about 70–100 MHz for an adult) gives the highest whole-body absorption.

Non-thermal effects (studied, less certain): headaches, sleep disturbance, changes in nervous and cardiovascular response, and possible genetic effects. IARC (WHO) classes RF fields as "possibly carcinogenic to humans" (Group 2B).

Other hazards: sparks and ignition of fuel (HERF), triggering of electro-explosive devices (HERO), and interference with pacemakers and medical implants.

Radiation zones of a microwave oven

A domestic oven works at 2.45 GHz (λ = c/f = 3×10⁸/2.45×10⁹ = 12.24 cm). The magnetron energy is confined in a metal cavity; only small leakage escapes through the door gaps and mesh. Around the oven three zones are defined:

 [OVEN]|<-- ~2 cm -->|<-- few λ -->|
 cavity| reactive    | radiating   | far field
       | near field  | near field  | (> ~0.5 m)
       | (door/mesh) |             |

1. Inside the cavity: very high fields (standing waves, kW level). Hazardous; interlocks switch off the magnetron when the door opens.
2. Reactive near field (within about λ/2π ≈ 2 cm of the door): leakage fields are stored around gaps; E and H are measured separately. Leakage is measured at 5 cm from the surface.
3. Radiating near field (few cm to several λ): leakage power spreads and varies with position.
4. Far field (beyond about 0.5–1 m): power density falls as 1/r² and is far below limits.

Safety limit (US FDA 21 CFR 1030.10): leakage must be under 1 mW/cm² at 5 cm for a new oven and 5 mW/cm² during its life. Damaged doors, hinges or seals can increase leakage, so they must be repaired.