A chroniton is a subatomic particle with temporal properties. Although not toxic to humanoids, they can have unpredictable effects, and are a oftentimes associated with time travel. They also have the ability to affect ship systems with prolonged exposure.
Romulan cloaking devices generated chroniton particles as a side effect of their operation. In 2368, Lieutenant Commander Geordi La Forge and Ensign Ro Laren became "phased" due to exposure to chroniton particles being emitted from the cloaking device of a Romulan science vessel which the Enterprise was assisting. As the Romulans had attempted to produce an interphase generator by combining their cloaking device with a molecular phase inverter, this had the effect of altering La Forge and Ro's molecular structure, making it impossible for them to interact with anything but each other. (TNG: "The Next Phase")
In 2371, the chronitons produced by the USS Defiant's cloaking device, which had lodged in the ship's ablative armor matrix, shifted into a high state of temporal polarization due to a temporal surge from an exploding microscopic singularity passing through the Sol system. This caused a transporter beam passing through the polarized particles carrying Captain Benjamin Sisko, Lieutenant Jadzia Dax, and Doctor Julian Bashir, to be redirected through time, resulting in their materializing in San Francisco, the year 2024. Chief O'Brien and Major Kira were later able to use the remaining polarized chronitons to mount a rescue mission for their stranded crewmates. (DS9: "Past Tense, Part I", "Past Tense, Part II")
Chroniton particles have a great effect on the Prophets and Pah-wraiths, entities which exist out of time. In 2373, a Pah-wraith inhabiting Keiko O'Brien had Miles O'Brien prepare a chroniton beam to fire at the Bajoran wormhole and kill the Prophets. O'Brien was later able to fire the beam at a runabout he and his wife were in, driving out or killing the Pah-wraith. (DS9: "The Assignment")
In 2374, the crew of Deep Space 9 planned to drive out two of these entities from Kira Nerys and Jake Sisko. Benjamin Sisko wanted the Reckoning to go ahead, but Winn Adami activated the chroniton pulse, driving out the Prophet and the Pah-wraith. (DS9: "The Reckoning")
Krenim weapons were chroniton-based, allowing them to pass through enemy shields by being in a state of temporal flux. (VOY: "Year of Hell")
The Borg use chroniton fields throughout their vessels, projected through specially designed conduits. These fields are essential when Borg vessels travel through the Borg transwarp network, to keep the various parts of the cubes in temporal sync and minimize the temporal stresses on the vessels. The warp core of a Federation starship and the bio-neural circuitry can be reconfigured to create a similar effect, although temporarily. (VOY: "Shattered")