Biological responses to 30 mT static magnetic field in young and 36-month-old rats.

This study aimed to investigate the effects of subchronic exposure to a 30 mT static magnetic field (SMF) on hematological parameters, spleen and tibia cellularity in 36-month-old and young rats. A total of 27 rats were divided into four groups (Young, Young SMF, Old, Old SMF) and two groups were exposed to SMF for 10 weeks. After exposure period, blood counts, neutrophil-to-lymphocyte ratio (NLR, an index of systemic inflammation), platelet-to-lymphocyte ratio (PLR, a platelet-based inflammatory marker) and cellularity of immune-related organs were analyzed. SMF exposure reduced lymphocyte counts and increased NLR in both age groups, while PLR increased only in young rats. In 36-month-old rats, SMF significantly reduced platelet counts, whereas this effect was not observed in young animals. SMF exposure also enhanced tibial and splenic cellularity in both groups but exerted opposite effects on the proportions of lymphocytes and erythrocytes depending on age. These findings suggest age-dependent immune modulation by SMF. In young animals, SMF likely promoted a proinflammatory shift, reflected by elevated NLR and PLR. In contrast, in 36-month-old rats, SMF may act as a nonspecific physiological stressor, potentially triggering the General Adaptation Syndrome (three-stage stress response), leading to corticosterone-mediated immunosuppression and cell redistribution. To our knowledge, this is the first study demonstrating age-dependent differential modulation of NLR and PLR by subchronic SMF exposure, linking proinflammatory shifts in youth with stress-related immunosuppression in aging. Overall, age appears to be a critical factor in determining the biological responses to SMF, underscoring the need for age-specific evaluation of SMF exposure.