The impact of EMF on the immune system has garnered significant attention in recent years. The immune system comprises intricately structured cells, tissues, and organs. The functions of the thymus, spleen, and macrophage phagocytosis; serum levels of antibodies and cytokines; and the number of immune cells are common indicators for evaluating the immune system's function. Among them, the thymus, spleen, and macrophage phagocytosis indexes are closely associated with non-specific immune function, while the serum levels of antibody IgG can be measured in relation to humoral immune function. Existing reviews have summarized the cellular variations induced by EMF exposure as observed in epidemiological studies and in vitro experiments (Boscolo et al., 2007; Santini et al., 2009). However, to comprehensively evaluate the impact on the immune system's functionality, it remains crucial to consolidate conclusions derived from in vivo experiments. Table 1 shows that exposure to ELF-EMFs can have a positive or negative influence on biological immune function.

For immune organ function, Quaglino et al. (2004) conducted long-term in vivo exposure to 50-Hz ELF-EMF on Sprague-Dawley (SD) rats for 8 months, and the results demonstrated gradual atrophy in the rat thymus caused by collagen deposition and fat substitution. For studies in BALB/c mice, Zhu (2010) found that the low-dose-treatment mice induced an increase in thymus, spleen, and macrophage phagocytosis indexes at 1 mT and 4.5 mT. By comparison, all these indexes and levels of antibody IgG significantly decreased in the high-dose treatment group at 9 mT. This study indicated that low-dose ELF-EMFs were likely to contribute to human health, while a strong magnetic field could weaken the immune system. Zhang H. et al. (2016) denoted that an ELF-EMF of 8 mT at 50 Hz could decrease the concentration of thymus and spleen indexes and weaken the proliferation of T- and B-type lymphocytes, which implied that high-dose magnetic fields would have certain adverse effects on the biological immune system.

Immune cells such as white blood cells, lymphocytes, and natural killer (NK) cells play a vital role in the immune system. de Kleijn et al. (2016) exposed BalB/c mice to the ELF-EMF of 10 μT for 1 week and 15 weeks, respectively and observed a significant increase in leukocyte counts under short-term exposure. Furthermore, Zhang Y. et al. (2016) exerted a 100-μT magnetic field at 50 Hz on SD rats for 12 consecutive weeks, and the outcome showed no significant effects on the peripheral hematopoietic system without influencing the number of white blood cells, the neutrophil ratio, or the lymphocyte ratio. Canseven et al. (2006) applied a 50-Hz 2-mT EMF for 5 days to guinea pigs and found a decrease in NK cell cytotoxic activity in the spleens of the exposed animals.

Recently, researchers have shown increased interest in investigating alterations in cytokines in serum. Mahaki et al. (2020) exposed rats to 50 Hz EMFs at intensities of 1 and 100 μT and observed the reduction of pro-inflammatory cytokines such as IL-9 and TNF-α, alongside the increase of the anti-inflammatory cytokine IL-10, which could potentially trigger an anti-inflammatory response. The results from Wyszkowska et al. (2018) revealed that a single uninterrupted 24-h exposure to 50 Hz, 7 mT ELF-EMF resulted in notable elevations in plasma IL-1β, IL-6, and IL-2 levels, as well as an increase in blood parameters such as white blood cells, lymphocytes, hemoglobin, and hematocrit levels. Conversely, when rats were subjected to repetitive 1-h daily exposures to ELF-EMF over 7 days, there were no discernible alterations.

Based on the results from existing in vitro experiments, the mechanisms underlying the interaction between EMF and the immune system may involve calcium ion regulation, modulation of heat shock proteins, increased production of reactive oxygen species (ROS), cell signal transduction related to free radicals, and DNA damage and repair (Mahaki et al., 2019b). Certainly, some studies have investigated the immunomodulatory mechanisms of EMF and explored its potential therapeutic effects on certain diseases, such as wound healing (Rosado et al., 2018). However, on the whole, these mechanisms interplay, and impacts on the immune system can vary depending on factors such as exposure duration, frequency, and intensity, as well as the specific experiment subject types involved.

On the whole, in assessing immune organ function, in vivo experiments suggest that low-dose EMF exposure may be beneficial to human immune function, while high-dose exposure might have a weakening effect. The effects of EMF on immune cells and possible mechanisms depend on cell types and factors such as the intensity and duration of EMF exposure. The serum level of cytokines exhibited a possible anti-inflammatory effect to promote wound healing, but the impact depended on EMF parameters and might adapt to the changes with a longer exposure duration.

The effects of ELF-EMFs on reproduction can be quite concerning. In daily life, people pay special attention to the effect of electromagnetic radiation on fetal development. However, the adverse effects from epidemiological studies exhibited controversial results regarding both female and male reproductive functions as shown in Table 2. For instance, some studies showed no association between maternal or paternal exposure to ELF-EMFs and harmful reproductive outcomes either at 50 or 60 Hz (Sang-Kon et al., 2016). On the other hand, some cohort studies and case–control studies declared that women exposed to ELF-EMF during pregnancy might have a high risk of miscarriage (Shamsi Mahmoudabadi et al., 2013; Wang et al., 2013).

In vivo experiments offer advantages over epidemiological studies in assessing the effects of EMF on the reproductive system due to controlled exposure and direct mechanistic insights (Karimi et al., 2020). Furthermore, in vivo experiments provide a method for exploring the effects of environmental factors on different compositions of the reproduction system in different periods of pregnancy (Asghari et al., 2016). Considering the weight effects of pregnant rats exposed to electromagnetic radiation, Cao et al. (2006) reported that the weight increase in pregnant mice in the exposed group was significantly lower than that of the control group in the late gestation period after 21 days of exposure to the ELF-EMF of 1.2 mT at 50 Hz. In addition, the delivery rate of pregnant mice in the exposed group was significantly lower than that of the control group, with the occurrence of preterm birth, stillbirth, and teratogenesis. Field exposure to SD rats showed a significant decrease in sex hormones, including luteinizing hormone, progesterone, and estrogen, and ovarian weight, but the progesterone level was partly reversed after removing the exposure (Al-Akhras, 2008). However, Ruan et al. (2020) found through animal experiments that there was no significant difference in the mass or the number of stillbirths among pregnant rats in each group after 30 μT, 100 μT, and 500 μT magnetic field exposure. The evaluation of hormonal changes, including progesterone level and 17-beta estradiol level, also showed no difference between the exposed group and the control group (Aydin et al., 2009). The mentioned studies confirmed the change in hormones due to exposure to EMFs, which might further influence the reproduction and fertility systems of animals, but the mechanism behind this needs to be explored. For male mice, ELF-EMF exposure led to lower sperm counts but did not alter germ cell morphological characteristics (Heredia-Rojas et al., 2018). Earlier rat models exhibited a reduction in reproductive outcomes after exposure to EMF for a short time but recovered in 90 days (Al-Akhras et al., 2001). Furthermore, a reduction in the level of serum testosterone was detected as a result of the reduced functionality of the seminal vesicles and preputial glands (Al-Akhras et al., 2006).

According to Santini et al. (2018) review of the effect of ELF-EMF on reproductive function, in vivo experiments since the early 2000s have exhibited complex results due to different experiment environments or animal species. For the mechanism, an increasing amount of research indicates that EMF exposure could lead to the overproduction of ROS by mitochondria in both male and female mice, which might explain its effect on the reproductive system (Santini et al., 2018), but further specific research is still required to delve into this matter due to the lack of clear mechanisms.

In 2002, the International Agency for Research on Cancer (IARC) classified ELF-EMF as one of the possible carcinogenic factors for humans. In the past several decades, epidemiological studies have been conducted worldwide (Wertheimer and Leeper, 1979; Pedersen et al., 2014), but no concrete conclusion has been drawn regarding the relationship between ELF-EMF and various cancers (Carpenter, 2019). Therefore, to establish the causation between ELF-EMF and tumors, in vivo experiments were conducted to support the causal findings as shown in Table 3.

In vivo experiments have shown mixed results. Galloni and Marino (2000) investigated the influence of a 50-Hz, 2 mT EMF on mammary murine adenocarcinoma-bearing mice, and no association was observed. For a 60-Hz, 2 mT EMF, the same results were observed in the incidence of both benign and malignant tumors (McLean et al., 2003). For rats, Salim et al. (2008) explored the effects of a 1 mT EMF on chemically induced colon cancer and revealed that the influence was neither carcinogenic nor cancer-promoting. The Ramazzini Institute had committed to filling the gap in long-term in vivo experiments with the use of more than 7000 SD rats, and no evidence showing the carcinogenic effects of ELF-EMF was detected (Soffritti and Giuliani, 2019). For breast cancer, Fedrowitz and Löscher (2007) exposed 344 Fischer rats to a 100 μT, 50 Hz EMF for 26 weeks and detected a significant increase in the incidence of adenocarcinomas. In conclusion, the results from in vivo tests can be controversial due to the animal species, cancer model types, and EMF parameters.

The carcinogenic mechanisms of ELF-EMF have been hypothesized to involve various factors, including the induction of DNA damage, cellular repair process impairment (Lai and Levitt, 2023), and altered hormone secretion. In acute experiments, Lai and Singh observed a dose-dependent increase in DNA strand breaks in brain cells in rats exposed to 60 Hz EMF, which could be related to carcinogenesis and cell death (Lai and Singh, 1997). However, in McNamee et al.'s (2002, 2005) experiments, adult rats, adult mice, and immature mice were exposed to 60 Hz EMFs, and no evidence supporting the hypothesis that the exposure might cause DNA damage in the cerebellum or brain was found. In long-term animal experiments, Yokus et al. (2005) measured the level of 8OHdG in DNA, a predominant form of radical-induced DNA lesions, and the results confirmed that an EMF of 0.97 mT at 50 Hz induced oxidative DNA damage. In conclusion, in vivo experiment results showed a difference in the effect of EMF on DNA damage. This could be due to the fact that the systems had diverse genetic characteristics (Santini et al., 2009), and in vitro experiments had already confirmed that the EMF effect was sensitive to cell types (Ivancsits et al., 2005). A hypothesis was put forward that the reduction of the hormone melatonin in the pineal gland might contribute to an increase in breast cancer (Loomis et al., 1994). To verify the hypothesis, Fedrowitz et al. (2002) utilized SD rats to observe the development and growth of mammary gland tumors and found that EMF increased the cell proliferation of mammary epithelium cells but did not influence melatonin levels.

Since some tumor cells go into apoptosis in response to ELF-EMFs (Yuan et al., 2020; Sołek et al., 2022), EMF has also been used for therapeutic purposes. This therapeutic effect on various cancers has long been demonstrated through in vivo experiments (De Seze et al., 2000; Tofani et al., 2002; Novikov et al., 2009; Mansourian et al., 2020; Chen et al., 2022). Several in vitro studies showed cell-type inhibition selectivity of EMF toward specific tumor cells other than normal cells (Koh et al., 2008; Crocetti et al., 2013), but the potential mechanism was still unknown, which necessitated further investigations to uncover the intricate workings at play. Recently, in a study by Barati et al. (2021), BALB/c mice with MC-4L2 tumors were exposed to a 10-Hz 100-mT ELF-EMF for 2 h daily over a period of 28 days. The research revealed a significant augmentation of proinflammatory reactions alongside the inhibition of tumor growth, and further verified through in vitro experiments that the phenomenon was partly caused by ROS accumulation and Ca²⁺ overload-induced necroptosis (Barati et al., 2021). In addition to the in vivo test, an in vitro experiment further proved that the inhibition mechanism was closely linked with the contact between cells and changes in membrane potential (Sun et al., 2023).

The cardiovascular system is directly related to heartbeat and bioelectrical activities, which could potentially be affected by surrounding magnetic fields through inducing electric currents in biological body under the electromagnetic effects. A survey based on power company workers showed that there was a positive correlation between the duration of occupational electromagnetic field exposure and the incidence of arrhythmias and acute myocardial infarction (Savitz et al., 1999). On the whole, current epidemiological study findings identified limited evident cardiovascular risks associated with either short-term or prolonged exposures to ELF-EMFs within exposure limits set by current standards (Jauchem, 1997; McNamee et al., 2009).

Certain metrics, such as heart rate variability, heart rate, and blood pressure, are employed for the assessment and prediction of specific cardiovascular conditions. Since heartbeat and blood pressure are two key factors leading to cardiovascular disease, the effects of ELF-EMFs on heartbeat and blood pressure were mainly included in this review as summarized in Table 4. In short-term exposure, the results indicated that ELF-EMF might influence ventricular repolarization, leading to a suppression in heart rate elevation (Jeong et al., 2005). Other effects, including the reduction of glutathione content in the heart, were also detected in a 2-h exposure (Martínez-Sámano et al., 2010). For rat heart activity, no effect was observed under the exposure of a 50-Hz, 1-μT EMF (Elmas et al., 2012). To explore the mechanisms, Wei et al. (2015) exerted a 2 mT, 15 Hz, 50 Hz, 75 Hz, and 100 Hz EMF on cardiomyocytes isolated from neonatal SD rats and confirmed the modulation of calcium-related activities, including the increase in calcium concentration baseline level and the decrease in the amplitude of calcium transients in the sarcoplasmic reticulum. However, long-term experiments of 24 weeks on rats indicated that 50 Hz EMF had no obvious effects on the cardiovascular system through the detection of blood pressure, pulse rate, heart rate, and cardiac rhythm (Zhou et al., 2016) or based on the results of echocardiography, cardiac catheterization detection, HE staining of cardiomyocytes, and mRNA levels of related genes (Liu et al., 2018; Zhang et al., 2020).

In recent years, EMF therapy has gained significant interest due to its potential protective impact on cardiovascular functions, and various in vivo experiments on humans have been conducted. For instance, a pilot randomized controlled trial on healthy adults showed that a magnetic field of 50 Hz had therapeutic effects to stimulate parasympathetic activity, resulting in enhanced vasodilation and blood flow through a nitric oxide-dependent mechanism (Okano et al., 2021). A double-blind, repeated study explored the effect of ELF-EMFs on heart rate (HR) and HR variability and found a reduction in HR caused by the possible enhancement of parasympathetic predominance by short-term EMF (Binboga et al., 2021). Another study exposed mild-to-moderate hypertension patients to 1 μT ELF-EMF and observed a blood pressure-lowering effect (Nishimura et al., 2011). The phenomenon of EMF-related vasodilation and microcirculation was also detected clinically (Kondo et al., 2019).

Apart from the observed phenomenon, the mechanism behind was further studied. The industry generally believed that despite the lack of a complete understanding of the precise mechanisms, the main mechanisms of action involved the modulation of the autonomic nervous system (ANS), promotion of angiogenesis, enhancement of nitric oxide (NO) synthesis, and exhibition of antioxidant and anti-inflammatory properties (Soltani et al., 2023). Based on that, EMF-related medical devices have been subsequently developed to treat cardiovascular diseases such as hypertension and heart failure non-invasively. Undoubtedly, the emergence of these therapeutic devices has bridged gaps in conventional treatments, yet the substantial benefits of such therapeutic approaches and the specific mechanisms of these modulations still require validation through both in vivo and in vitro experiments.

Many diseases are concerned with the malfunction of the biological neural system, and thus the association between ELF-EMFs and neural function has attracted some interest. In vitro experiments exhibited the potential for cellular behavior modulation by EMF. For instance, immature cerebellar granule neurons (CGNs) of postnatal rats were saved from apoptosis by exposure to the ELF-EMF of 300 mT at 50 Hz, compared with no survival of CGNs without exposure (Oda and Koike, 2004). Ma et al. (2016) found that the proliferation of embryonic neural stem cells (eNSCs) was enhanced under the exposure of 1 mT, 50 Hz ELF-EMF. Furthermore, EMF has the potential to facilitate the differentiation of neural lineages and enhance the maturation of neurons (Ho et al., 2019).

In addition to the cellular-level effects, in vivo experiments were conducted to explore the system-level impacts as summarized in Table 5. Dong et al. (2023) described that it is insufficient to directly activate neuronal action potentials by exposing them to ELF-EMF on the order of milliTesla. However, the ELF-EMFs were able to regulate neural excitability contributing to learning and memory by modulating synaptic plasticity. The effect of ELF-EMF on the spatial memory of mice and rats was complex: Under a short period of exposure, the effect could be ineffective or positive, while EMF could have devastating effects with an increase in frequency, intensity, and exposure duration (Abkhezr et al., 2023). An enhancement of hippocampal neurogenesis in adult mice was also detected after exposure to EMF (Ma et al., 2016). The study conducted by Zheng et al. (2021) explored the impact of EMF stimulation patterns on long-term potentiation (LTP) related to learning and memory at synapses, revealing a consistent decrease in LTP across various waveforms that could be possibly induced by the closure of some calcium channels in the membrane. Rezaei-Tavirani et al. (2018) adopted rat models for the analysis of the proteome. The results indicated that cytoskeletal protein expression increased with the increase in EMF intensity and exposure time, which was possibly induced by the impairment in the short memory of the hippocampus under exposure.

Under conditions of cerebral ischemia, EMF has been found to promote neural regeneration in the hippocampus to possibly repair brain tissue through animal behavior experiments, which could be induced by the modulation of the Notch signaling pathway (Gao et al., 2021). For stroke, Cichoń et al. (2017) denoted that oxidative stress can be modulated by the ELF-EMF, which significantly advanced the functional and mental status of patients with a field of 7 mT at 40 Hz. In a follow-up study on post-stroke patients, they found that exposure to ELF-EMF led to a significant elevation in the levels of growth factors and cytokines associated with neuroplasticity and facilitated a notable improvement in functional recovery (Cichoń et al., 2018). In animal models of transient stroke, the results indicated that exposure to ELF-EMF enhanced the neurological assessment and behavioral outcomes, exhibited a positive impact on neuronal viability, and contributed to a reduction in glial reactivity within the hippocampus, which confirmed the potential of EMF as a clinical therapy (Moya-Gómez et al., 2023).

EMFs have demonstrated promising applications in the regulation of neural activities so as to provide a new means for the treatment of nervous system diseases. Compared with traditional therapeutic methods of electrical stimulation, EMFs in this field have advantages such as indirect contact with tissues to reduce potential damage, induction of focal neural activity, and achievement of selective activation of certain neuron populations. For instance, transcranial magnetic stimulation (TMS) is widely applied in treating neurological disorders, notably depression and anxiety (Fregni and Pascual-Leone, 2007; Klomjai et al., 2015; Cappon et al., 2022). Within intricate systems such as the retina and visual cortex, EMFs were able to offer a less-invasive approach to modulating neuronal behavior. Bonmassar et al.'s (2012) and Lee and Fried's (2015) studies introduced sub-millimeter magnetic coils to activate or suppress neuronal tissue through in vitro experiments, which offered an effective alternative for deeper and sub-cortical targets. Lee and Fried (2017) further utilized the micromagnetic stimulation on layer V pyramidal neurons (PNs) and showed that the coils produced asymmetric fields and activated the corresponding focal region, which was promising for selective stimulation of local neurons. Park et al. (2013) applied this technology in the central nervous systems of hamsters and activated inferior colliculus neurons in the dorsal cochlear nucleus. In vagus nerve stimulation (VNS), Jeong et al. (2022) adopted micro-magnetic stimulation of the rat vagus nerve and found this way of stimulation to be an effective alternative to VNS with fewer heart-related side effects.

Regarding the underlying mechanisms of these effects, the activities of neuronal ion channels were crucial and basic. According to Bertagna et al. (2021), the most frequently observed outcome of EMF exposure appears to be alterations in calcium balance, primarily associated with voltage-gated calcium channels. Under both acute and chronic exposure to ELF-EMF, an elevation of calcium ion concentration was observed intracellularly, which should be the foundation for neural activity modulation. A more intricate network of causal elements could range from interactions at the cell membrane that initiate signaling pathways to the involvement of various mechanisms such as ROS, nitric oxide, growth factors, cryptochromes, and processes related to epigenetic and genetic alterations (Funk and Fähnle, 2021; Dong et al., 2023). This comprehensive interplay within the neural environment underlines the multifaceted nature of the mechanisms that underlie the effects of EMF on the nervous system.