Investigations are continuing into how beachgoers were left struggling to breathe on a packed beach during the hottest late August bank holiday weekend on record.
Several families on the beach at Frinton, Essex, reported having trouble breathing on Sunday.
Police have advised people not to enter the sea on Monday.
The Maritime and Coastguard Agency said received reports of swimmers suffering from a possible respiratory irritant.
It said there was no evidence of any fuel spillage from vessels.
The ambulance and fire service attended the scene shortly after 14:00 BST on Sunday.
One mother described how one of her twin daughters was left gasping for breath.
Miriam Lansdell said: “My daughter started coughing. She said, ‘I don’t feel good. It hurts to breathe in’.
“My other daughter was gasping and couldn’t form words because she couldn’t breathe well enough.”
Keith Canning, 67, went into the sea in the early afternoon. As he was drying himself he felt “an ache at the back of my throat”.
Mr Canning said: “I noticed several people around me were coughing. My daughter and teenage granddaughter were also coughing and chesty.
“I thought it may have been the sea air as it was so hot on the beach and colder in the water.
“We decided to go home but on the way called 111 for advice. They took the matter seriously and checked that none of us were showing signs of a heart attack.
“We were advised to get home change our clothes and wash them. We were all told to have showers, and we even bathed our Jack Russell dog.”
While laboratory test results of samples are awaited, Tendring District Council advised beachgoers to avoid going into the water until a cause could be established.
Beach patrols were almost doubled but no further reports of illness have been received.
Will Lodge, council communications manager, said: “We will continue to monitor the situation.”