Doctors have warned that skin problems could be caused by the foam, which forms every monsoon season but has been particularly bad this year.
Word has not got through to the hundreds of families who throng India's longest urban beach, letting children happily skip in the toxin-filled froth, according to AFP.
The Tamil Nadu Pollution Control Board said it is analysing samples from the foam which has spread several kilometres along the beach.
"It is definitely not good for people to go into the foam but they just do not understand the risks," said Pravakar Mishra, a scientist at the National Centre for Coastal Research in Chennai who has seen the clouds of foam grow in recent years, AFP added.
