Octachloronaphthalene is a polychlorinated naphthalene primarily used historically as a heat transfer fluid, dielectric fluid, and insulator in electrical equipment such as capacitors and transformers due to its thermal stability and electrical insulating properties. It has also been employed as a flame retardant, plasticizer, and additive in high-temperature industrial applications, including lubricants and wood preservatives. However, its use has significantly declined and is largely phased out due to high toxicity, environmental persistence, bioaccumulation potential, and classification as a persistent organic pollutant (POP) under the Stockholm Convention, leading to strict regulatory restrictions or bans in many countries.