Accelerated development of spontaneous and benzopyrene‐induced skin cancer in mice exposed to 2450‐MHz microwave radiation

C 3 H/HeA mice with high incidence of spontaneous breast cancer and Balb/c mice treated with 3,4‐benzopyrene (BP) (by painting of the skin resulting in the development of skin cancer) were irradiated with 2,450‐MHz microwaves (MW) in an anechoic chamber at 5 or 15 mW/cm 2 (2 h daily, 6 sessions per week). C 3 H/HeA mice were irradiated from the 6th week of life, up to the 12th month of life. Balb/c mice treated with BP were irradiated either prior to (over 1 or 3 months) or simultaneously with BP treatment (over 5 months). The appearance of palpable tumors in C 3 H/HeA mice and of skin cancer in BP‐treated Balb/c mice was checked every 2 weeks for 12 months. Two additional groups of mice were exposed to chronic stress caused by confinement or to sham‐irradiation in an anechoic chamber; these served as controls. Irradiation with MWs at either 5 or 15 mW/cm 2 for 3 months resulted in a significant lowering of natural antineoplastic resistance (mean number of lung neoplastic colonies was 2.8 ± 1.6 (SD) in controls, 6.1 ± 1.8 in mice exposed at 5 mW/cm 2 and 10.8 ± 2.1 in those irradiated at 15 mW/cm 2 ) and acceleration of development of BP‐induced skin cancer (285 days in controls, 230 days for 5 mW/cm 2 and 160 days for 15 mW/cm 2 ). Microwave‐exposed C 3 H/HeA mice developed breast tumors earlier than controls (322 days in controls, 261 days for 5 mW/cm 2 and 219 days for 15 mW/cm 2 ). A similar acceleration was observed in the development of BP‐induced skin cancer in mice exposed simultaneously to BP and MWs (285 days in controls, 220 day for 5 mW/cm 2 and 121 days for 15 mW/cm 2 ). The acceleration of cancer development in all tested systems and lowering of natural antineoplastic resistance was similar in mice exposed to MW at 5 mW/cm 2 or to chronic stress caused by confinement but differed significantly from the data obtained on animals exposed at 15 mW/cm 2 , where local thermal effects (“hot” spots) were possible.