Developed by UK company Thruvision, the body scanners are non-invasive, passively screening individuals for suspicious objects. They work by detecting the absence of the naturally occurring, extremely high-frequency ‘terahertz’ waves, emitted by anything that is warm – including the human body. The terahertz waves can penetrate clothing and thin plastics or ceramics but not metal or liquid water. However, the Thruvision system can also detect plastics guns and ceramic knives.