# Progress in the Knowledge, Application and Influence of Extremely Low Frequency Signals
## 3 Results and Discussion
### Community Analysis
It is stated that biological effects occur when a change in the environment causes some notable or detectable physiological change in a living system, and that these changes do  
Figure 7: Representation of the ELF community in health: (**a**) isolated distribution of publications. (**b**) keywords of the community. not necessarily have to be harmful to health. The body has mechanisms to adjust to various average influences, but when changes are accentuated for a long time it can happen that compensation does not occur, and unwanted effects appear. Therefore, determining the exposure times to these fields significantly marks the production of harmful effects on human health. In this book, a table is shown with the maximum permitted exposure values for ELF fields, as well as recommendations on the evaluation of the risks caused. From [28] it is extracted that for the exposure to these ELF fields to be evaluated, the fact that the exposure is imperceptible, has multiple sources, and may vary according to time and distance should be considered. In addition, the extraction of low-frequency signals does not occur naturally. There are numerous approaches to generating or collecting power from multiple low-frequency sources in the literature. These ecological generators convert the low-frequency mechanical shaking of ocean waves [60] or human motion [61] into alternating low-frequency electrical signals for energy collectors. One such energy collector, developed at [62], allows efficient energy collection from low-frequency voltage pulses, such as those typically generated by triboelectric or piezoelectric generators. The exposure period, which has an unknown duration and induction period, with no known appropriate exposure metrics and there are no biological data to rely on, should also be studied. [PERSON] (1992) presents a review of the knowledge about the effects of non-thermal levels of ELF electromagnetic fields on the biochemistry and activity of immune cells [63]. The examination of the results suggests that the calcium molecule (Ca 2\({}^{+}\)) plays an important role in the induction of these effects within the cellular field of the immune system. In another investigation, different cells associated with leukemia were exposed for EMF-ELF for 24 to 72 h, observing a 30% increase in cell proliferation after exposure [30]. The ability of these fields to induce DNA damage was also determined, measuring chain breaks and detecting an increase in DNA damage in all dose-dependent cell lines. Despite these findings, the precise mechanisms through which ELF fields increase cell production remain unknown and depend at least partially on experimental factors such as intensity, duration of exposure, and cell types used in the study. Therefore, a recurring theme in all the works is the urgent need to demonstrate a unique and unequivocal response induced by ELF-EMF, which is consistently reproducible in independent laboratories. Until this problem is resolved, the issue of biological responses to ELF-EMF will continue to be considered with great skepticism by the scientific community and extensively researched. The community for lightning ELF phenomena is the second in size, with a percentage of 19% of the works studied and its isolated distribution is shown in Figure 8a. The documents of this community develop themes that encompass the propagation and characteristics of ELF electromagnetic resonance phenomena occurring in the cavity between the Earth's surface and the lower layer of the ionosphere, mainly due to the presence of lightning and storms. The references in Scopus of the publications represented by the most important nodes sorted by size in this community are:  
1. \"Schumann Resonance for Tyros: Essentials Of Global Electromagnetic Resonance In The Earth-Ionosphere Cavity\" with 41 references [64]. 2. \"Sprites, ELF Transients, And Positive Ground Strokes\" with 286 references [65]. 3. \"Criteria for Sprites and Elves Based on Schumann Resonance Observations\" with 182 references [16].