Jaki, literally "evil air/breath/atomosphere", is the manifestation of negative emotions. In the modern world of higher stress and pressure, more jaki gets generated than the simpler times.
Typically jaki is harmless. In fact, its existence is necessary for the balance of emotions of the world. However, in high concentrations it can be used for malicious purposes.
The high concentrations of jaki leaking out from the Crystal Palace, slowly overtime corruptes people into Yn.