Nonachlor is what remains after the organochlorine insecticide chlordane breaks down in the environment, and it was itself a component of technical chlordane mixtures. Fat-soluble and stubbornly resistant to metabolism, it builds up in animal tissues and in milk, where it can linger for years, biomagnifying up food chains into fatty fish, meat, and dairy products that were historically screened for it. Chlordane is now banned or severely restricted in most countries, so modern detections reflect legacy contamination of soils and sediments. Maximum residue limits and ongoing surveillance programs guard against its reappearance in the food supply.