Narrator: This is Science Today. A class of PCBs that was not considered particularly hazardous in the past has been found to cause developmental abnormalities in rat pups. Study leader Dr. Michael Merzenich of the University of California , San Francisco simulated the levels that had been recorded in human babies that were breast fed by mothers exposed to high levels of the toxicant. The consequence in the rat study was significant.

Merzenich: It basically frustrated the normal plastic process of the brain so that developmental progressions were very abnormal and the brain remained into adulthood in a very primitive, underdeveloped state.

Narrator: PCBs were mostly used as coolants and lubricants and were banned in the late 1970s, but they degrade very slowly and accumulate in the fat tissues of those exposed to contaminated fish, soil, water and air in high risk environments.

Merzenich: Really what these studies strongly indicate is that they raise a caution – a flag of warning and a concern that we hope will be aggressively addressed by other scientists.

Narrator: For Science Today, I'm Larissa Branin.