What substance is no longer permissible to add to transformer oil due to environmental concerns?

Polychlorinated biphenyls (PCBs) are no longer permissible to add to transformer oil due to serious environmental and health concerns. PCBs were widely used in electrical equipment because of their excellent insulating and thermal stability properties. However, it was discovered that they are highly toxic, persistent in the environment, and accumulate in the food chain. This led to regulations banning their use in many countries, recognizing the severe risks they posed to human health and the environment.

The concern over PCBs stems from their ability to cause various health issues, including cancer, reproductive disorders, and immune system impairment. Additionally, their long-term presence in soil and water can lead to widespread contamination. As a result, many electrical utilities and manufacturers have transitioned to alternative insulating fluids that are less harmful to both humans and the ecosystem.

The other substances listed, while they may have their own challenges, are not subject to the same level of regulatory scrutiny as PCBs. Organic compounds are a broad category that includes many safe materials, Freon is a refrigerant with its own set of environmental regulations but is not specifically linked to transformer oil, and anti-foaming substances are generally used to enhance performance and are not banned in the same manner as PCBs.