Seawater samples collected from the sites where people were treated by the ambulance service has thrown up no concerns.
On 25 August, paramedics were called to Walton, Frinton and Clacton seafronts. A large number of people who had been in the water suffered coughing, irritation of the eyes and some minor difficulty breathing.
Patients all made a full recovery after being treated with fresh water and rinsing off their clothing.
Water samples were taken at the sites and analysed by experts at theEnvironment Agency on behalf of Tendring District Council.
“We are pleased that the analysis revealed nothing untoward, and the absence of any further reports of similar problems since 25 August suggests whatever caused the incident has now passed.”
Testing by Centre for Environment, Fisheries and Aquaculture Science (CEFAS) also revealed no algal blooms in the water.
Tendring District Council said the cause of the incident would remain a mystery.
“We are grateful to our colleagues from a number of organisations, including the Environment Agency, CEFAS and Public Health England who supported us with testing, analysis and advice. We are satisfied having discussed the issue with them that there are no further avenues of testing to explore."