Theta radiation, also known as theta-band emissions, is a radiation emitted by contaminated antimatter byproducts. Theta radiation poisoning is extremely dangerous to the health of humanoids. One of the first symptoms is hallucinations. The final stages of theta radiation poisoning involve the degradation of major body organs. Asking if a patient was exposed to theta radiation was thus a very elementary question a doctor would ask his patient. (VOY: "Life Line")
Theta radiation can impede sensor scans. High enough concentrations can disrupt subspace, collapsing the warp fields of nearby starships. Transporter biofilters can stop theta radiation from entering a ship. (VOY: "Night", "Juggernaut")
Captain Jean-Luc Picard had knowledge of theta-band emissions. (TNG: "Chain of Command, Part I")
After the Borg attack on Earth in 2063, Lieutenant Commander Data determined that the damaged warp ship Phoenix was emitting theta radiation, due to a damaged throttle assembly. One of the workers on the warp ship, Lily Sloane, was subsequently diagnosed with theta radiation poisoning. (Star Trek: First Contact)
The Malon civilization produces nine billion isotons of antimatter waste a day. This waste is dumped in open space by the waste export industry, contaminating those regions with theta radiation. The crews of Malon export vessels also suffer from long-term theta radiation exposure.
In the 2370s, Controller Emck began dumping waste in the Void, causing mass theta radiation poisoning in its indigenous inhabitants. His activities were stopped by the USS Voyager in 2375. (VOY: "Night")
As a casual threat during an attempt to steal Voyager's multispatial probe from a gas giant in 2375, Controller Vrelk vented theta radiation gas from his ship. The gas slightly damaged Voyager's shields. (VOY: "Extreme Risk")
Also in 2375, the Kmada attempted to sabotage the life support systems of the biomimetic USS Voyager with low-frequency theta radiation. (VOY: "Course: Oblivion")
- Presumably, theta radiation is closely related to the more general antimatter radiation.