Emergency services were called to a beach after bathers were reportedly 'struggling to breathe'.
Police, the ambulance service and the fire service went to Frinton seafront in Essex after several reports of people coughing.
The reports came from the seafront, off Fourth Avenue, shortly after 2pm on Sunday. Some children were said to have been taken to hospital.
Families were out soaking up the sun at the beach today until they suddenly found their children were returning from the water with severe coughs, according to the Clacton and Frinton Gazette.
One parent told the paper: "After a short while my daughter started coughing.
"We thought she had swallowed some sea water but then we noticed a few people around us with kids coughing."
Beachgoers said they had been left struggling to breathe but the cause is unknown and under investigation.
Speculation online of a fuel spill was not confirmed by police or the ambulance service.
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.
What happens when you call 999 for an ambulance?
Another said: "We have just left Frinton and have seen lots of fire engines on the way out. Has there been an incident? We were on the beach and all developed a cough and were struggling to breathe."
A spokeswoman for East of England Ambulance Service said: "We are aware of an incident on Sunday 25 August with reports of a number of people suffering from coughing on the seafront off Fourth Avenue, Frinton.
"We are assisting the police and fire services with this incident. The cause is currently unknown."
A police spokesman added: "We were contacted at 2.15pm on Sunday 25 August with reports of a number of people suffering from coughing on the seafront off Fourth Avenue, Frinton.
"The cause is currently unknown and emergency services are in attendance."