Diphenyls are potential threat to the environment. How are these produced from aryl halides?

In environment, diphenyl is formed during the incomplete combustion of mineral oil and coal. It is present in the exhaust gases of vehicles and in exhaust air from residential and industrial heating devices. Acute exposure to high levels of biphenyl has been observed to cause eye and skin irritation and toxic effect on the liver, kidneys and central/peripheral nervous system. Kidneys of animals are also affected due to the ingestion of biphenyls.

Aryl halides, when treated with sodium in the presence of dry ether gives diphenyl. This reaction is called Fittig reaction.