Tidally locked to the star Zelene, Nepyma has the expected "hot pole" and "cold pole." Along the terminator is a thin band of nearly-habitable terrain. Unfortunately, the local biosphere is based on a chlorinated oxygen atmosphere. It is not sophisticated, but it has proven highly dangerous.
The asari surveyor Verallas landed on Nepyma in 1684 to study the local ecology. Unbeknownst to the crew, and a handful of native chlorine-fixing microbes passed through biohazard screening and entered the ship. The Verallas returned to the port of Nos Parnalo on Illium, where the Nepyman microbes escaped into a temperate environment with plentiful unused chlorine.
The microbes devoured the chlorides in the earth; as metabolic byproducts, they produced toxic polychlorinated biphenyls (PCBs). By the time the infestation was contained, an area of nearly 30 square kilometers had effectively turned into a toxic waste dump. Nos Parnalo had to be abandoned, accelerating the development of Nos Astra.