Tainted Chi is a form of Chi that was corrupted through the vile influence of the Yama Kings, tainting and poisoning everyone who tries to use it.
Overview
This form of Chi can be used in order to fuel Demon Shintai and Bile Shintai, but those who do so continually without proper rites are inevitably drawn to the Scorpion Eaters and their ilk, until they surrender to the will of the Yama Kings and become Akuma.
The massive accumulation of tainted chi during these days, from repeated nuclear detonations, waste spills, air pollution, and outbreaks of virulent diseases, taints the Dragon Lines in a way that they are not able to cleanse themselves properly. In some locales -- like nuclear test sites and toxic waste zones, as well as the worst disease-ridden slums and settlements -- the system is overwhelmed and the poison enter the flow of Yin and Yang energies.
Fortunately, Chi carrying this taint can't easily flow along Dragon Lines. Corrupt Dragon Nests have a kind of "gravity," which attracts and isolates toxins and taints within them. Clean Chi is repelled by tainted Chi, so the system acts to quarantine such pockets of foulness. Unfortunately, these unclean sites can't be easily severed from the remaining "circulatory system." As taint accumulates, it overflows the capacity of the dragon nests, and seeps into adjoining sites like a back-flow of sewage. This slow, but steady spread of poisoned Chi thus far resists all efforts to staunch it through Tapestry, Feng Shui, and a host of other related Disciplines. The ancestors and bodhisattvas have no idea where the flow will eventually stop.
Kuei-jin who ingest tainted Chi gain no sustenance from it and quickly grow violently ill while their systems purge the foul energies. The Kuei-jin's body expels its various humors, which must then be replenished from a clean source. Taking tainted Chi into one's system isn't pleasant and can lead to uncontrolled vomiting, weeping and sweating when the body purges itself.
Mortals who accelerate tainted chi usually don't live long and their deaths are often very painful. Tainted Chi is accumulated with certain deadly illnesses such as Ebola.