brain sciences Review Extremely Low-Frequency Magnetic Field as a Stress Factor—Really Detrimental?—Insight into Literature from the Last Decade Angelika Klimek and Justyna Rogalska * Department of Animal Physiology and Neurobiology, Faculty of Biological and Veterinary Sciences, Nicolaus Copernicus University, 87-100 Torun, Poland; [email protected] * Correspondence: [email protected]; Tel.: +48-56-611-26-31 Abstract: Biological effects of extremely low-frequency magnetic field (ELF-MF) and its consequences on human health have become the subject of important and recurrent public debate. ELF-MF evokes cell/organism responses that are characteristic to a general stress reaction, thus it can be regarded as a stress factor. Exposure to ELF-MF “turns on” different intracellular mechanisms into both directions: compensatory or deleterious ones. ELF-MF can provoke morphological and physiological changes in stress-related systems, mainly nervous, hormonal, and immunological ones. This review summarizes the ELF-MF-mediated changes at various levels of the organism organization. Special attention is placed on the review of literature from the last decade. Most studies on ELF-MF effects concentrate on its negative influence, e.g., impairment of behavior towards depressive and anxiety disorders; however, in the last decade there was an increase in the number of research studies showing stimulating impact of ELF-MF on neuroplasticity and neurorehabilitation. In the face of numerous studies on the ELF-MF action, it is necessary to systematize the knowledge for a better understanding of the phenomenon, in order to reduce the risk associated with the exposure to this Citation: Klimek, A.; Rogalska, J. factor and to recognize the possibility of using it as a therapeutic agent. Extremely Low-Frequency Magnetic Field as a Stress Factor—Really Keywords: magnetic field; stress; HPA axis; catecholamines; cytokines; hormones; behavior; anxiety; Detrimental?—Insight into Literature neuroplasticity; cell survival from the Last Decade. Brain Sci. 2021, 11, 174. https://doi.org/10.3390/ brainsci11020174 1. Introduction Academic Editor: Ulrich Palm Many studies have suggested an association between extremely low-frequency mag- Received: 22 December 2020 netic field (ELF-MF) exposure and anxiety and/or depression. On the other hand, the ELF- Accepted: 27 January 2021 MF-induced improvement of brain function has also been found. The mechanism of Published: 31 January 2021 these effects is assumed to be a stress response induced by ELF-MF exposure. Extremely low-frequency MF is natural physical phenomenon in our environment. The rapid de- Publisher’s Note: MDPI stays neutral velopment of science and technology resulted in the introduction of many new devices with regard to jurisdictional claims in and technologies in industry, agriculture, and everyday life. We are continuously exposed published maps and institutional affil- in our environment to ELF-MF (range of 0–300 Hz) [1]. MFs are either of natural ori- iations. gin (geomagnetic field, intense solar activity, thunderstorms) or human-made (factories, transmission lines, electric appliances at work and home, magnetic resonance imaging, medical treatment, etc.) [2]. Common used frequencies of electric and magnetic fields of the electric power supply and of electric and magnetic fields generated by electricity power Copyright: © 2021 by the authors. lines and electric/electronic devices are 50 Hz in Europe and 60 Hz in North America [2]. Licensee MDPI, Basel, Switzerland. Biological effects of ELF-MF and their consequences on human health have become the This article is an open access article subject of important and recurrent public debate. Until now the reported studies are distributed under the terms and largely contradictory with regard to epidemiologic studies (some of the research studies conditions of the Creative Commons found a relationship with development of diseases while the others failed to find any [3–8] Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ (Table S1). Whether or not ELF-MF exposure is related to increased health risks, it has led 4.0/). many scientists to examine the potential mechanisms by which ELF-MF might affect human Brain Sci. 2021, 11, 174. https://doi.org/10.3390/brainsci11020174 https://www.mdpi.com/journal/brainsci Brain Sci. 2021, 11, x FOR PEER REVIEW 2 of 21 Whether or not ELF-MF exposure is related to increased health risks, it has led many sci- entists to examine the potential mechanisms by which ELF-MF might affect human health. Special attention is paid to the adverse impact of both low- and high-frequency MF (radio waves) due to many possible pathological effects and numerous reports on MF-induced Brain Sci. 2021, 11, 174 carcinogenicity [9]. ELF-MF was proved to be a stress factor and as a consequence, it2 ofcan 20 provoke morphological and physiological changes in stress-related systems [10]. Some authors argue that ELF-MF evokes cell/organism responses that are characteristic to gen- eral stress reaction. ELF-MF exposure “turns on” different intracellular—compensatory health. Special attention is paid to the adverse impact of both low- and high-frequency or deleterious—mechanisms and modifies stress-related function of nervous, hormonal MF (radio waves) due to many possible pathological effects and numerous reports on MF- and immunological systems (Figure 1). ELF-MF influence on living matter can cause a induced carcinogenicity [9]. ELF-MF was proved to be a stress factor and as a consequence, detrimental increase in free radicals levels and radical-evoked damages in macromole- it can provoke morphological and physiological changes in stress-related systems [10]. cules [11]. Most studies on ELF-MF effects concentrate on its negative influence; however, Some authors argue that ELF-MF evokes cell/organism responses that are characteristic to ingeneral the last stress decade reaction. there ELF-MFwas an increase exposure in “turns the number on” different of research intracellular—compensatory studies showing stim- ulatingor deleterious—mechanisms impact of ELF-MF on brain and modifies plasticity stress-related processes (the function production of nervous, of protective hormonal pro- teinsandimmunological (e.g., Hsp70 or systems BDNF)(Figure or an 1increase). ELF-MF in influencethe activity on of living antioxidant matter can enzymes) cause a detri- [12]. Furthermore,mental increase long-term in free radicals exposure levels to andELF-MF radical-evoked can cause permanent damages in changes macromolecules in behavior [11]. (towardsMost studies depressive on ELF-MF and effectsanxiety concentrate disorders) that on its are negative relatedinfluence; to exposure however, to chronic in the stress last [10,13,14].decade there was an increase in the number of research studies showing stimulating impact of ELF-MFIn the face on brain of numerous plasticity studies processes on(the the productioneffects of the of ELF-MF, protective it proteinsis necessary (e.g., to Hsp70 sys- tematizeor BDNF) the or knowledge an increase for in a the better activity understanding of antioxidant of this enzymes) phenomenon, [12]. Furthermore, in order to reduce long- theterm risk exposure associated to ELF-MF with exposure can cause to permanent this factor, changes but also in to behavior recognize (towards the possibility depressive of usingand anxiety it as a therapeutic disorders) that agent. are related to exposure to chronic stress [10,13,14]. FigureFigure 1. 1. EffectsEffects of of extremely extremely low-frequency low-frequency magnetic magnetic field field (ELF-MF) action in the organism organism.. 2. Stress—AIn the face Factor of numerous Determining studies the on Function the effects of Organism of the ELF-MF, at All it isLevels necessary of Organiza- to system- tionatize the knowledge for a better understanding of this phenomenon, in order to reduce the risk associated with exposure to this factor, but also to recognize the possibility of using it It is accepted that ELF-MF exposure may count as a mild stress situation [10,15–17] as a therapeutic agent. and it could activate a wide spectrum of interacting neuronal, molecular, and neurochem- ical2. Stress—A systems that Factor underpin Determining behavioral the Function and physiological of Organism responses. at All Levels Chronic stress can promoteof Organization and exacerbate pathophysiology leading to allostatic overload in human body [18]. TheIt is acceptedbrain developed that ELF-MF some exposureadaptive maymechanisms count as in a mildthe face stress of situationchanging [ 10environ-,15–17] mentsand it and could stress activate factors a wide imposed spectrum on the of nervou interactings system. neuronal, Integrated molecular, response and neurochem-to stressful stimuliical systems is an essential that underpin component behavioral of adaptive and physiological processes critical responses. for survival Chronic of the stress organ- can ism.promote Failure and of exacerbate this stress pathophysiologyadaptation is considered leading toas allostaticone of the overload primary in neuropathological human body [18]. The brain developed some adaptive mechanisms in the face of changing environments and stress factors imposed on the nervous system. Integrated response to stressful stimuli is an essential component of adaptive processes critical for survival of the organism. Failure of this stress adaptation is considered as one of the primary neuropathological causes of stress-related disorders. A healthy