Emergency services have been called to the seafront in Essex after receiving several reports of people coughing and struggling to breathe.
Police, the ambulance service and the fire service are at the seafront off Fourth Avenue in Frinton, after receiving calls shortly after 2pm on Sunday.
Reports also came in from Clacton and Walton, with Tendring District Council warning sunbathers to avoid the water.
Beachgoers said they had been left struggling to breathe but the cause is unknown and is being investigated.
Speculation online of a fuel spill was not confirmed by police or the ambulance Service.
A mother who was on a family day out at the busy beach told how her one of her twin daughters was left "gasping" for breath.
The 45-year-old mental health worker from Derbyshire, who was visiting her parents in Essex for the bank holiday weekend, said she had also had difficulty breathing as she lay on the sand drying off after a dip in the water.
She said they all began to breathe easier when they moved further away from the beach, but took the 10-year-old girls to a walk-in clinic to be checked over by medical staff.
Ms Lansdell said her father had been told by someone in a speedboat, who he assumed to be associated with the coastguard, that there may have been a fuel spill.
One person tweeted that there were "lots of people coughing heavily", while a mother said her son began coughing after swimming and had to be given his inhaler.
A spokeswoman for East of England Ambulance Service said they are aware of the incident and are helping police and fire with the response.