1. For nuclei with low neutron to proton ratios, 2 common radioactive decay processes occur :positron emission and electron capture
depends on the underlying cause of instability. Above the band of decay have too many neutrons to be stable. Below have too many protons to be stable.
2. Both the number of neutrons and the number of protons must be reduced in order to make these radioisotopes stable
3. Captured electron combines with a proton to form a neutron
Decay first manifested on five-year-old Tenko as he played with his dog, Mon. Tenko felt a shock through his left arm moments before getting physically punished by his father. While embracing Mon, Decay's effects materialized for the first time, resulting in Tenko accidentally killing his dog and then, later, his sister. Eventually, Tenko, who was resentful about his family and in a manic state, destroyed the rest of them as well as their household with his newfound Quirk.
Decay allows Tomura to destroy anything his hands touch, making it crumble to dust. Tomura's disintegration spreading through a crowd of individuals. Whether the target is organic or not, Decay will work without fail. As such, this Quirk is extremely lethal against living beings, having their flesh rot away if hit by it.
Due to the fact that the user's Quirk factor lies within their hands, more specifically their fingers, Tomura would lose the use of Decay if he lost them, similar to Overhaul .
Because the Quirk requires Tomura to touch a solid object in order to be activated, it is possible to counter Decay through Quirks that allow for the manipulation of fluids or particles, which Tomura cannot properly touch with his fingers.
As his past started sorting itself through his mind due to damage caused to his family's hands, Tomura started relearning and showcasing how truly powerful Decay is while battling against the Meta Liberation Army.
Tomura is often careful with his hands when touching objects he doesn't want to destroy, letting at least one of his fingers free from use while holding things. He employs a similar method when holding people hostage, threatening to disintegrate them if they don't comply or cooperate with him. If Tomura gets angered or frustrated, he is likely to drop his carefulness.
Through vigorous training via several weeks of combat against Gigantomachia, Tomura has greatly improved the strength of his Quirk. Decay works almost instantaneously now rather than taking several seconds before a target crumbles to dust and continues to evolve under high-stress environments. Using his hands everything he touches is affected by Decay's spread; the function of which working in a solipsistic fashion where anything or anyone connected to what he's touching suffers its effects.