# Progress in the Knowledge, Application and Influence of Extremely Low Frequency Signals
## 4 Conclusions  
In this work, research with the main theme of \"ELF\" signals, in any of its variants from 1956 to 2019, has been considered. After establishing the search for the keywords, 12,436 documents were obtained with a total of 42,813 relations between authors. After the use of bibliometric techniques, 5028 documents remained with 30,897 relations. These publications are carried out in different sub-themes, which are associated with 12 research communities, of which 3 of them had to be discarded after the analysis process despite having the same keywords, because they meant different concepts to the ones considered in this work. So, the documents studied are concentrated in nine communities or categories: ELF in health, lightning ELF phenomena, electrophysiological effects, ELF wave generation, natural ELF phenomena, ELF in crop effects, geomagnetic pulsations, ELF in power lines, and UELF waves. The ELF in health community has the highest concentration of documents (58.50%), followed by the lightning ELF phenomena (20.06%), electrophysiological effects (8.00%), ELF wave generation (6.97%), and natural ELF phenomena (5.77%) communities. The rest of the communities have a much smaller representation (0.71%), so they were not studied in detail in isolation. They do not have a main nucleus like the others, but they are constituted by small nuclei with ties of relationships between them and with other communities. In the spatial distribution of communities in Figure 2, a distribution in two regions is observed, one of them forms communities whose content is related to health (ELF in health and electrophysiological effects) and the rest of the communities are observed in the other region. The community for ELF in health as well as being the largest, is the most-researched at present and the one with the most work developed in 2019. The trend of these investigations is aimed at determining the most accurate way for possible effects of ELF fields on the body, either at the level cellular [8], on hematological parameters [112], or the immunity system [35]. A topic of special interest today is the relationship between ELF exposure and the development of relatively modern diseases such as sclerosis [113], stress, or depression. Research on ELF fields and cancer is numerous [114]--due to the impact of this disease today, the research is trying to control cancer cells with a specific ELF irradiation and lay the foundations for a possible personalized cancer medicine [115]. In the other community related to health, electrophysiological effect, five papers have been published in 2019. They are aimed at determining the effects of ELF-EMF on cognitive abilities such as learning or memory [116], as well as the modeling of the induced-ELF electric field in the human body and understanding its effects [117]. In the lightning ELF phenomena community, the investigations in 2019 focus on the modeling of the ionospheric cavity through various computational techniques with the aim of establishing a model as real as possible [118]. SR measures are still used for the reconstruction of global lightning activity [24]or for the analysis of the effects of geomagnetic storms [119], coinciding most of the works in which the number of SR measurement stations should increase in the near future to correlate the records. The relationship between SR perturbations and seismic activity is also currently being investigated [21] in order to carry out an increasingly effective prediction of the presence of seismic phenomena and alert the population of their effects. In the last year, the ELF natural phenomena community has also been investigating, developing numerical models of plasma spherical emissions [120], as well as plasma nonlinearity [121]. In the wave generation community, research continues the generation of these waves through increasingly complex mechanisms [122]. In the rest of the communities, no work has been published during the year 2019, although most of the documents were developed in the last decade.