Toxin used to be an arrogant environmental terrorist named Albert Ferns who with delusions of grandeur
plus time in the military made him a candidate to destroy several toxic waste dumps and buildings, injuring many in the process.
One mission had him attacking a chemical plant that was trying to recycle waste, he was attacked by the hero Amphibios and was accidentally kicked over a railing and into a vat of the poisonous chemical toxins. He managed to climb out of it, but soon discovered that he couldn't breath regular air and had to fashion himself a suit that could pump the toxic fumes into his body. His warped mind changed due to the chemicals and now he believes that instead of helping mankind, he would turn them into horrid toxic breathers such as himself.
Toxic bombs that can melt steel, he also has some that if splashed upon an animal or plant, turns them into a hideous mutation of what they had been.