Spontaneous Human Combustion

is a worldwide phenomenon that instantaneously bursts humans into flames, turning them into Flame Humans as a result. In some cases, humans can adapt to the flames and awaken the ability to manipulate flames or ignite themselves or objects. The origins and selectivity of the phenomenon are unknown. The Special Fire Force have made it their mission to unravel the secrets behind it.

Overview
Humans who awaken Second or Third Generation abilities without becoming Flame Humans usually become Fire Officers; though not always.

Within the Shinjuku District, Rekka Hoshimiya was creating artificial Flame Humans with use of Bugs.