"High-frequency"or "microwave", or "radio frequency" are electromagnetic radiation of technological origin, that are used for communications systems, both on the outside as the inside of the buildings.

Commercial telecommunications antennas
Cell towers or emergency service
Radio and tv antennas
Radar
Interference from wi-fi networks of neighbors
Wireless phone radiation of neighbors

Wi-fi networks own
Wireless phones available inside the room
Electrical pollution (high frequencies in electricity wiring)
Information displays at a distance (via WiFi)
Microwave devices in kitchens