Traces of toxic polychlorinated biphenyl (PCB) were also found in blood samples of some people, tested under an ongoing project of the Department of Environment (DoE). It has been assumed that unscrupulous businessmen have been engaged in mixing the coolant with edible oil for a long time.

However, tests of edible oil to find out traces of PCB are yet to be conducted.

A study conducted by experts of DoE under the project recently found presence of the poisonous chemical in cooked and packed food items.

A draft report of the study titled 'survey and research on DDT and PCB in food items' mentioned that presence of PCB was found in piaju, jilapi, puri, and potato chops sold in road side cafés and in packed chanachur sold in stores.

PCB has hazardous effects on humans, the experts said. Different symptoms of the effect include nausea, eye and respiratory tract irritation, dehydration, weight loss, cyanosis, and altered function of the neuromuscular system, liver, kidneys and pancreas, the experts added.

PCB can remain in fat cells of a human body up to 120 years with the possibility of altering the DNA structure of an affected person and traces of it can also be transferred to offspring, according to the experts.

The experts confirmed that the lone source of PCB in the country is a kind of industrial oil which is used in electric transformers and capacitors as a cooling agent.

An organised group of unscrupulous edible oil traders have been buying used transformer oil and mixing it with edible oil, a field investigation revealed.

The main sources of used transformer oil are the storage of Dhaka Electric Supply Authority (Desa), Power Development Board (PDB), Rural Electrification Board (REB), and Dhaka Electric Supply Company (Desco), sources concerned said.

"We change the oil of transformers when performance of a transformer declines. Then we sell it in auctions for recycling purposes or for other uses," a high official of Desa said requesting anonymity.

"We don't dump it due to lack of disposal facilities. So we prefer further use of it and sometimes people misuse the used oil as it has no colour or smell," he said.

The study also reveals that there are 2,481 power transformers, 3,74,132 distribution transformers, and 77 circuit breakers in the country which contain no less than 1,07,473 metric tons of PCB mixed oil containing at least 56.1 metric tons of PCB.

Besides, PCB is also available on board of old ships as it works as flame retardant also, sources from DoE said.

Sources working with Desa said a small 10 kilo volt-ampere (kVA) transformer can contain as much as 30 litres of oil. A big one can contain around 200 barrels of oil.

"The price of used transformer oil is really low. May be only six to seven taka per litre," said an engineer working with Desa, requesting anonymity.

There are some plants in Savar recycling used transformer oil in a limited quantity and selling it to private transformer owners, the Desa official said.

However, besides being mixed with edible oil, PCB is also being used as surface coating for carbonless copy paper, and as plasticizer in sealants, rubber, paint, wax and asphalt.

"It is very hard to control misuse of used oil. Everywhere you will find dishonest people," another engineer said who also requested anonymity.

But people can also buy used transformer oil from the employees of the Tongi store of Desa and PDB, sources said.

Dr Md Iqbal Rauf Mamun, a teacher of the chemistry department of Dhaka University, who conducted the PCB test, however said the level of PCB traces in human blood samples was very low.

"The range of PCB traces in human blood samples was from only 7 to 52.2 nanograms (ng) per gram of blood. But the safety level of PCB in human blood is yet to be fixed," Dr Iqbal said.