Miasma

Miasma is a natural accruing vaporous substance, from the Dark World. Normally in lesser concentrations invisible, it can have an effect on the cells of living beings of the Human World, leading to Demonization.

Creatures like cats can become tiger sized and normal people can become demons. However, the change tends to be mostly minor physical for alot of people. Miasma has no strong effect on people that already went through some Demonization. Taimanin are also immune either because of their demonic heritage or ability to use Taima Particles that counter Demonization.

Miasma can bind together with strong radioactivity sources of the Human World, which makes both far more dangerous. Killing both humans and demons through prolonged exposure.

Trivia

 * Miasma exists around the streets of Amidahara and in Yomihara, which both have a large Demon Gate.