The Human Torch's nervous system generates a field similar to his sister's (the Invisible Woman) that allows her to manipulate the electrostatic fields around her (imitating the effects of solid matter). However, his field is adjusted to cause combustion.
Okay, let's start with fire. What is it? From a chemical point of view, fire is simply rapid oxidation. Oxidation for the chemically challenged is the combination of oxygen and another element or molecule into a compound. Fire is the release of heat energy from oxidation when it occurs very rapidly. Rust is also a form of oxidation (iron oxide usually), but it occurs so slowly that no great amount of heat is released.
If you look closely at fire it is really a mechanical process that releases a lot of energy and that energy propagates the same mechanical process in the surrounding matter. Let's say we have a piece of paper. Paper is essentially organic carbon based fibers. In organic matter, everything but the carbon can burn (and at the right temp even carbon combines with oxygen to form carbon dioxide; by the way, CO2 is formed in your body after the cells "combust" but in such minute levels even though it is very hot at the point of the transfer it doesn't set the body on fire spontaneously). When you set the paper on fire, what you are doing from a molecular level is "infecting" it with a mechanical process that forces the molecules of the paper to combine with oxygen and then emit enough heat to break the surrounding molecular bonds and force them to combine with oxygen as well as the carbon (ash) falls to the floor.
Essentially, the Human Torch's field is an electrostatic process that starts a fire without the initial heat. It performs the first part of the chain reaction by simply breaking the bonds of a combustible material and combining them with oxygen. Then it lets the attendent heat maintain the combustion. Also, it's not too much to assume that the field would also be able to reverse and shut down combustion allowing the Human Torch to control external fires and to prevent himself from burning.
If I had to guess, the reason that the Human Torch is always on fire when he "flames on" despite the fact that there is nothing other than his skin nearby that burns is that he actually uses water (sweat) from the body as the source. The field takes the sweat and breaks it into hydrogen and oxygen, then the hydrogen burns forming water again and that can in turn be converted into fire again by the field; a continuous loop. Of course, the laws of conservation of energy would eventually force this loop to collapse at some point when the Human Torch would need time to "recharge."
The basic point is that the Human Torch's nervous system has been infected by a basically electrical field that both can start and prevent the combustion process. Since it is hooked to his nerves this allows the Human Torch to control it as if it were part of his body.
The Human Torch's nervous system exudes a new element known as "rephosphorous." This is an element that contains altertrons (subatomic particles that are neither electrons, protons nor neutrons) but it has an atomic weight similar to phosphorous. The way rephosphorous works is that, like phosphorous, it bursts into flame upon contact with oxygen and forms rephosphorous oxide. However when rephosphorous oxide encounters a certain temperature, it releases the oxygen atom. This sets up a combustion loop. Each of the Human Torch's pores are output jets for the rephosphorous. It fires the substance away from the body so that he himself will not encounter the great heat as it burns. Also, when he breathes in, the Human Torch reabsorbs non oxidated rephophorous that can be reprocessed inside his body. The Human Torch can also control the pressure of his jet-pores so that he can fire fireballs and long flares of flame or lift himself into the air. There may also be some modifications to his physiology that allow it to adapt to the heat without burning. His flesh may be fireproof, in other words.
- The Human Torch is a character that appeared in the comic book, The Fantastic Four.