# Radio Frequency Electromagnetic Fields Exposure Assessment in Indoor Environments: A Review
## 1 Introduction  
The pervasive use of wireless communication devices in each aspect of everyone daily life, emphasized the need for assessing the level of radio-frequency electromagnetic field (RF-EMF) exposure. Several studies aimed to estimate the exposure level for RF-EMF source in the 10 MHz-6 GHz frequency range, related to radio and television broadcasting, base stations for telecommunications and mobile phones, cordless (DECT) phones, RF identification tagging systems, and wireless communications applications, such as WiFi, wireless local area network (WLAN) and mobile telecommunications networks--Global System for Mobile Communications (GSM), Universal Mobile Telecommunications System (UMIS) and Long Term Evolution (LTE) [1]. Different approaches with different levels of accuracy have been used to estimate RF-EMF exposure, e.g., spot and long term measurements, personal measurements with portable exposimeter, simulations based on computational electromagnetics techniques.  
Surveys with different aims have been carried out on: population surveys, i.e., studies aimed at investigating the differences between the levels of exposure of different groups of individuals (see, e.g., [2; 3]), micro-environmental surveys, i.e., studies aimed at investigating the differencesbetween the exposure in relevant micro-environments, both indoor and outdoor [4], and temporal studies, aimed at investigating temporal variability of the exposure, by measuring along the same itinerary at the same time over a period of several months or years [5; 6].  
Nowadays the rapid evolution of technology and network infrastructures towards the fifth-generation (5G) systems requires a similar evolution in the methods of assessment of the exposure, in order to face the increasing complexity of the spatio-temporal indoor and outdoor exposure. This is particularly needed in indoor environments, in which it was estimated that people spend about 70% of their daily time [7]. In an indoor environment, the RF exposure depends on outdoor sources such as radio, television and/or mobile phone antennas, as well as on indoor sources such as e.g., mobile phone handsets, DECT base stations, Wireless Local Area Network (WLAN) net, etc. While radio and television transmitters have a large coverage area and therefore operate at relatively high power levels, the power level inside a building can be up to 100 times lower than that outside the building, depending on the number of windows and the structure of the walls, and the exposure may vary from floor to floor. As to the indoor sources, their number and their locations, as well as the structure of the rooms and their furniture, are affected by a lack of information especially for private environments. All these aspects cause the assessment of the exposure to RF-EMF to be affected by uncertainties and represents a challenging task.  
In this document, we aimed to review research efforts done in the last ten years in assessing RF-EMF exposure in indoor environments, presenting our findings in terms of those RF-EMF sources that were found to cause exposure in indoor environments, in terms of the different indoor environments that have been investigated and in terms of the different approaches used to assess the exposure. A summary of the levels of exposure for each type of indoor environment will be presented, highlighting limitations and gaps.