# Progress in the Knowledge, Application and Influence of Extremely Low Frequency Signals
## 3 Results and Discussion
### Community Analysis
The works of this community were developed between 2007 and 2017 and 80% of them were published in Chinese agriculture or engineering journals. The publication with the most important node is [103] with 34 Scopus citations, and studies the influence of the pulsating ELF electric field on the growth of soybean seedlings. In this publication the growth of plants is tested depending on the frequency of the fields to which they are exposed. It also allows establishment of future research where the measurement systems are improved, trying to apply them to other biological species in order to build a reproducible model globally. The geomagnetic pulsations community was formed by publications developed between 1969 and 1983, being the decade of the early 1980s where 70% of the publications were carried out. It should be noted that this community does not have keywords due to the age of the documents and the magazines where they have been published. All works are based on the analysis and measurement of geomagnetic pulsations or variations of the Earth's magnetic field. The article of [104] represents the most important node of this community: it is the oldest and most referenced work, with 271 references. This publication is considered the basis of further research--it is a review article that establishes a classification of the pulsations according to their periods and the mechanisms of the sources that generate them. A detailed analysis of the characteristics and models of these pulsations had already been established at the end of the 1980s, which is why there are no further works. Figure 12: Representation of other communities: (**a**) isolated distribution of publications, (**b**) keywords of ELF in crop effects community, (**c**) keywords of ELF in power lines community, (**d**) keywords of under extremely low frequency (UELF) waves community. The ELF in power lines community is one of the smallest, together with the UELF waves community, both formed by five works. All investigations have a theme based on the evaluation of the electromagnetic fields of transmission lines and very-high-voltage power lines. Figure 12c shows the 10 most-represented keywords in the documents, and due to the small number of publications that make up this community the number of repetitions of these words is small. The word that is repeated the most is \"high voltage power lines\" with 2 repeats. The rest only appear 1 time. The most-important publication by its node is [105] and this has no reference in Scopus so far. This work is published in 2011, which establishes the evaluation of health security areas for exposure to overhead power lines. One of the rising trends in recent decades involves reducing exposure to artificial electromagnetic fields, with special emphasis on exposure to ELF electromagnetic fields, in order to discover if they have adverse health consequences. This document contains and identifies the safety zones of the magnetic and electric field around the high voltage power lines (HVPL), for the two types of HVPLs most used in Portugal (60 kV and 220 kV), developing a 3D simulation tool for these lines. The limit values established by the International Commission Non-Ionizing Radiation Protection (ICNIRP) of 100 uT and 0.4 uT for the magnetic field, and 5 kV/m and 100 V/m for the electric field are also considered. Several research studies have been published studying the control of childhood cancer, commonly leukemia, and exposure to fields derived from lines at 60 Hz [106]. The evaluation of the effect of fields derived from these lines showed significant elevations in the risk of high exposure to them [107]. So, the determination of these safety zones helps companies and health authorities to reach agreements on this sensitive issue and is of interest to the general population  
The UELF waves community is constituted by publications whose theme is the study of the electromagnetic spectrum in the range below the ELF band, the UELF band (under extremely low frequency).