Spontaneous Human Combustion

was a worldwide phenomenon that appeared shortly after the first Great Cataclysm. It caused humans to instantaneously burst into flames. Spontaneous Human Combustion was caused by doppelgängers encroaching into the bodies of their human world counterparts.

In some cases, humans could adapt to the flames and awaken the ability to manipulate flames or ignite themselves or objects. The Special Fire Force made it their mission to unravel the secrets behind it.

Overview
Spontaneous Human Combustion is when a human becomes a fire elemental. There are three ways that this phenomenon can manifest, referred to as generations. Typically, when a human falls victim to SHC, they transform into creatures known as Infernals. However, as if to naturally combat this condition, humans have also begun to adapt to SHC, becoming second and third generation pyrokinetics. As both groups are partly resistant to being hurt by fire, people in Tokyo often end up becoming Fire Soldiers in the Special Fire Force; though not always.

First Generation
First Generation, more commonly known as Infernals, are humans who have spontaneously combusted and have transformed into misshapen fire monsters. While in this state, the victim is able to create fire at will and is effectively immortal, unless their core, located in the chest, is destroyed.

The change also leaves them in constant pain, which forces the majority of cases to lose their self-awareness and sanity, turning them into rampaging creatures, attacking indiscriminately or driven by simplistic desires from their human selves unless stopped by force.

Some victims are able to remain in control of themselves to a certain degree or, in cases with a notably strong will, remain in complete control of themselves and use their new body and abilities freely.

Second Generation
Second Generation are pyrokinetic individuals who have the ability to manipulate preexisting flames. Though, they cannot create their own fire.

Second Generation pyrokinetic abilities can take many forms; such as amplifying flames, redirecting them, and extinguishing them entirely.

Third Generation
Third Generation are pyrokinetic individuals who have the ability to create their own fire and control it. They cannot control preexisting flames though.

Third Generation pyrokinetic abilities can take many forms; such as blasting fire jets from part of the users body, shaping objects and entities out of fire, and generating other fire based elements, such as plasma and electricity.

Hybrid Generation
There is one known example of someone being both a Second and Third Generation pyrokinetic. This individual has the ability to both control external fire and create and control his own.

Fourth Generation
Fourth Generation, as dubbed by Haumea, are pyrokinetic individuals who possess and have awakened their Adolla Burst and connected with an other worldly being through the Adolla Link. Being a Fourth Generation allows one to achieve abilities that a pyrokinetic would normally not be able achieve, in some cases, abilities that can manipulate the laws of physics. Some known individuals who have achieved Fourth Generation are Inca Kasugatani, Shinra Kusakabe and Shō Kusakabe.

Causes
One of the key mysteries about Spontaneous Combustion is what caused it to appear in the first place and how it spreads. While in some cases it is totally random and sudden, it is also possible to deliberately trigger it using the bite of a certain type of insect. Once bitten, the victim either transforms into an Infernal or awakens their pyrokinetic abilities.

It is confirmed in the series that it is the servants of the Evangelist who are at least partially the cause. Spontaneous Human Combustion occurs due to the encroachment of a persons doppelganger, their counterpart that exists in the world of Adolla, into their human world counterparts body in the physical world. Little else is known about the phenomenon beyond this, such as what exactly determines the difference between someone who gets turned into an Infernal and someone being granted pyrokinetic abilities.

The ultimate goal of the Evangelist is to utilize the Adolla Bursts to force Earth and Adolla to completely overlap, and the Great Cataclysm was an incomplete and failed attempt at this. The world previous to the Cataclysm was so different in nature to that of the world of Fire Force (in that it resembled the real world, visually) that the effects the phenomenon that Spontaneous Human Combustion arises from seem to be much more mysterious than previously assumed.

Drawbacks
Third Generation pyrokinetic abilities are fueled by the oxygen in the users body. If the user goes past their limit, then they will suffer a condition known as Tephrosis, causing their flesh to be carbonized and greatly weakening them, both in terms of physical ability and their fire powers.