Radiation was the transmission of energy in the form of small, fast-moving particles. Excess exposure to ionizing radiation is the cause of radiation poisoning. Various types of radiation poisoning can be avoided via inoculations.
The planet Terra Nova suffered from surface radiation following the impact of an asteroid in 2083. As of 2151, the surface radiation was only little less than eight hundred millirads which posed no threat for lifeforms when exposed a few hours. (ENT: "Terra Nova")
The Kataan probe left behind a traceable radioactive trail, caused by its main fuel, crystalline emiristol. Even after a thousand years a Federation probe could be programmed to follow this trail and thereby trace the origin of the probe. (TNG: "The Inner Light")