The threat is maximum near the Nazafgarh area of West Delhi, where PCB concentration rises as high as 782 parts per billion (ppb) during the monsoon, said A K Bhattacharyya, a professor at the School of Environmental Science in JNU.

The maximum permissible limit of PCB in water, as fixed by the US Federal Department of Agriculture, is 600 ppb. PCB has also been found in the portion of the Yamuna adjacent to Okhla Industrial area in South-East Delhi, Nizamuddin in South Delhi and Wazirabad in North Delhi.

Bhattacharyya and his colleague P K Seth carried out the study along with the National Institute of Oceanography in 1994-96.

``Though the study was carried out early, it is likely that the concentration has increased in the last few years with the growth of small- scaleindustries,'' he said, adding the situation needed constant monitoring.

The sources of PCB, suggests Bhattacharyya, may be the batteries and electrical components like capacitors and transformers as well as plastic bags and containers which are used excessively in most small-scale industries.

According to Bhattacharyya, PCB concentration started increasing in the pre-monsoon period, maximised during the monsoon and then gradually decreased.

``The threat may not be very great at present, but if preventive measures are not taken immediately, it can cause public health disorders after a few years,'' he observed. The scientists have also found a high concentration of toxic chemicals like Dichloro Diphenyl Trichloroethane (DDT), Hexachloro Hydrocarbons (HCH) and Cyclodiene, all of which are known to have mutagenic properties.

The concentration of DDT is five times more than the World Health Organisation standard, he said.

In fact, the Biological Oxygen Demand (BOD) value -- the standard scale ofmeasuring water contamination -- was found to be 60, but should be within 10, Bhattacharyya pointed out.

All these chlorinated hydrocarbons, being fat soluble, can reside in body fat for a long time and get biomagnified through the food chain in course of time.

Other sources of Yamuna's pollutants include municipal sewage and agricultural discharges from the nearby localities, he added.