Which of the following toxic chemicals are used in electrical transformers?

Concept

A transformer needs a fluid that does two jobs at once: insulate against high voltage and carry heat away from the windings, without itself catching fire. Several chemistries can meet that specification — mineral oil, silicone fluids and synthetic or natural esters are the modern choices — but the historical answer was a heavily chlorinated aromatic (benzene-ring) compound. Substituting chlorine for hydrogen on such a backbone gives a dense, oily liquid that does not conduct, does not burn and does not decompose at operating temperature. The very inertness that made this class an attractive industrial fluid also makes it almost indestructible in nature, which is why it ends up regulated as a persistent organic pollutant (POP).

Applying it to this question

Polychlorinated biphenyls (PCBs) are exactly that molecule: a biphenyl skeleton of two joined benzene rings carrying one to ten chlorine atoms, which gives 209 possible congeners. Sold under trade names such as Aroclor and Askarel from the 1930s, PCB mixtures became the standard dielectric and coolant fluid of electrical transformers and capacitors. Manufacture was banned in the United States in 1979, and the Stockholm Convention of 2001 lists PCBs in Annex A, requiring their elimination from equipment still in service; the US EPA still names transformers as the principal PCB-containing equipment.

Cross-check: what the other compounds are

• Peroxy acetyl nitrate is not emitted directly by any source — it forms in the air itself when nitrogen oxides meet hydrocarbons in sunlight, which makes it a secondary pollutant and a marker of photochemical smog rather than a manufactured fluid.

• Atrazine is a triazine herbicide sprayed on cereal and cane fields; its environmental story is groundwater contamination, not electrical equipment.

• Perfluorooctane sulfonate is a fluorinated surfactant used for stain-repellent coatings, firefighting foams and metal plating; its persistence comes from carbon-fluorine bonds rather than from chlorination.

Hence the toxic chemical used in electrical transformers is polychlorinated biphenyls.