As mentioned in the previous section, electric fields present in the atmosphere are typically in the range of 120–150 V/m, although under specific conditions, they can reach values up to 10 kV/m, for example, near thunderstorms. This kind of fields does not vary with time and hence they are defined as static; they are generated between the positively charged ionosphere and the Earth’s negatively charged surface. Static electric fields are not able to penetrate the human body because of its high conductivity, and for the same reason, their displacement is always perpendicular to the body surface. It is assumed that a static electric field within the body is attenuated by a factor of about 10−12 from an external source. These fields induce a surface electric charge and when the strength of the field is sufficiently high, the induced charge may be perceived through its interaction with body hair. If the charge reaches a sufficiently high level, a corona discharge may occur (Repacholi and Greenebaum 1999). Such a perception threshold depends on different factors and can vary between 10 and 45 kV/m. Some evidence is present regarding the capability of both humans and animals to detect and respond to static stimulation (Petri et al. 2017). In particular, experiments on the perception of static electric fields showed evidence that detection thresholds are lower for whole-body exposure than that for limb exposure. Moreover, it has been suggested that hair movements due to electrostatic forces play an important role in this kind of perception. However, apart from this type of interaction, no other direct action of these fields on living systems is known. These outcomes have also been standardized by the World Health Organization (WHO) in the “Static Fields Environmental Health Criteria Monograph No.232,” where an international group of experts reported that the only adverse acute effects of static electric fields are associated with direct perception and discomfort from micro shocks (World Health Organization and others 2006).
Different is the case of the high-intensity electric fields generated during lightning. The role of lightning and atmospheric discharge in the evolution of life has been greatly suggestive, with a direct link to the natural horizontal gene transfer in bacteria (Kotnik 2013). The molecular mechanism, in this case, is related to electroporation: an electric pulse with amplitude and duration sufficient to induce membrane electroporation has been shown to allow nucleic acid penetration in the cells exposed (as presented in Section 3). However, to prove the hypothesis of a gene transfer mediated by atmospheric lightning, controlled exposure conditions are necessary (Liberti et al. 2013), as well as a combined approach integrating reproducible experimental data with numerical modeling aiming at understanding and interpreting biological results (Denzi et al. 2017). In particular, the readapting version of the Kitano cycle proposed for system biology (Kitano 2002). The basic concept is the possibility to perform “Model-Driven” and “Experiment-Driven” results, outlining the importance of continuous exchange between the biological results and the numerical modeling. This can be convincingly performed on a molecular level, where it is possible to perform modeling with atomistic details. For example, it was recently predicted via molecular dynamics simulations that a tubulin protein conformation can be affected by an intense electric field at the nanosecond timescale (Marracino et al. 2019). Consequently, it was demonstrated experimentally that indeed tubulin conformation in vitro can be modulated by intense electric pulses, and depending on the parameters of the treatment, the protein self-assembles to different structures (Chafai et al. 2019).
Another natural electromagnetic phenomenon associated with atmospheric nEMF is the electrical discharge and the related electromagnetic waves that are generated by lightning during a thunderstorm. As discussed in Section 1, these natural fields, consisting of short-duration and damped oscillating electromagnetic impulses, are able to affect the Earth’s environment and the living organisms (Panagopoulos and Balmori 2017). Despite one recent in vitro result of the influence of electromagnetic fields at the Schumann resonance in rat cardiomyocyte cultures (Elhalel et al. 2019) and some evidence that this kind of fields can be sensed by humans through different types of indications, such as headache and fatigue (Panagopoulos and Balmori 2017), up to now, a clear explanation for this association has not been provided.
Regarding the static magnetic field, the natural geomagnetic field of the Earth is around 50 μT and varies between values of about 35 and 70 μT, depending on the geographic location. Static magnetic fields have been mainly involved in the migratory behavior of certain animal species. According to the World Health Organization and others (2006) the assessed experimental evidence has made possible the classification of the physical mechanism of interactions of static magnetic fields with biological systems in three classes: (i) interaction with ionic conduction current; (ii) magneto-mechanical effects; (iii) effects on electronic spin states of reaction intermediates. Exposure to static magnetic fields will affect electrically charged particles and cells in the blood when moving through the field; for example, the field can reduce the velocity of blood cells flowing through blood vessels. However, only for magnetic induction fields exceeding 8 T, which are orders of magnitude larger than the nEMF, acute effects are likely to occur, ranging from minor changes in a heartbeat to an increase heart rhythm (arrhythmia) (Repacholi and Greenebaum 1999). For non-acute effects, outcomes related to human psychological, neurological, cardiovascular, immunological, and behavioral variations are controversial even if the rationale behind this association has been rigorously investigated (Close 2012). One possible hypothesis is related to the influence of cryptochrome on circadian rhythm in response to magnetic exposure. This kind of role could be related to radical-pair mechanism where the yield of a biochemical reaction might be sensitive to the orientation of an external magnetic field. Somewhat surprisingly, while magnetodetection in humans is not widely accepted, there is increasing evidence suggesting that such a sense may exist. In recent studies, it has been demonstrated that weak magnetic fields as the geomagnetic ones can provoke evoked potentials in humans (Carrubba et al. 2007) or can influence the visual sensitivity of man (Thoss and Bartsch 2007), supporting the evidence for the radical-pair retinal model in humans. Further experiments on magnetodetection have demonstrated, as the first thing, that cryptochrome is necessary for a light-dependent magnetic sense in Drosophila (Gegear et al. 2008) and successively that the human cryptochrome CRY2 has the molecular capability to function as a light-sensitive magnetosensor (Foley et al. 2011). In these last experiments, using a transgenic approach, it has been demonstrated that human cryptochrome can function as a magnetosensor in the magnetoreception system of the Drosophila in a light-dependent manner, opening the way to a renewed interest for human magnetoreception. As a whole, the conclusive recommendation of the WHO group of experts on static magnetic fields is that more research should be accomplished with in vitro studies, animal and volunteer experimental studies, and epidemiological studies (World Health Organization and others 2006).
As a final consideration, the only way to prove a possible effect of electromagnetic fields on human beings is to link experimental and modeling aspects. As a first step, a systemic approach should be adopted combining together all the positive studies to identify specific plausible targets and related pathways. Successively, a multiscale methodology should be used as reported (Apollonio et al. 2013). In fact, a biological effect can be considered as the ultimate step of a chain of events starting with the field interacting with a biological system at the level of a single molecule or structure, through the modification of its charge distribution, its chemical state, or its energy. The change provoked by the field at the molecular level can be sensed and reinforced across the complexity of the biological scale to produce a response of the whole organism. Multiscale approach, extending from the most basic of amino acid sequence that constitute protein function to concerted multicellular signaling cascades, thus stratifying different levels of biological organization, each with its own complexity, structure, and function, is the unique way to provide not only a scientific support to experimental evidence but also a useful interpretation of the results obtained. Figure 2 and Table 1 summarize the mentioned biological effects.