About 150 people have been treated in hospital and hundreds more affected by an unknown gas which hit the East Sussex coast.
Birling Gap beach was evacuated on Sunday after people reported breathing difficulties, stinging eyes and vomiting when a “mist” appeared.
Sussex Police said on Monday morning the gas cloud appeared to have cleared.
Agencies are investigating the cause and have not ruled out either on-shore or off-shore locations.
Sussex Police added: “It does appear that it did sweep in from the sea driven by on-shore breezes.
“However, weather models suggest that an onshore source in northern France is very unlikely,” Sussex Police added.
East Sussex Fire and Rescue Service said it was “extremely unlikely” the substance involved was chlorine.
Beaches in the area were open as normal on Monday.
The first reports came in from Birling Gap, between Eastbourne and Seaford, just before 17:00 BST where people reported having irritated eyes.
The plume of gas moved eastwards along the coast as far as Bexhill, police said.
Coastguard rescue teams from Birling Gap, Eastbourne, Bexhill and Newhaven raced to help clear the busy beaches as visitors feared they had been struck by a chlorine leak.
Kyle Crickmore, who was at the Birling Gap beach with his family, said it emptied within 10 minutes after people began to feel unwell.
He described how a “foggy haze rolled in out of nowhere” and there was a “strong smell of chlorine in the air”.
“It was definitely out of the ordinary considering it was a nice, clear, sunny day 10 minutes beforehand,” he said.
“It was stupidly busy and it was a boiling hot day.
“It emptied in about 10 minutes, which was quite staggering considering the amount of people who were there.”
Long queues built up at Eastbourne General District Hospital well into the evening, with approximately 150 people being treated, East Sussex Healthcare NHS Trust said.
“Initially, patients were dealt with as a precaution with a full decontamination treatment, but it became clear that this was not necessary, following clinical advice,” a spokesman said.
Members of the public alerted emergency services, which triggered callouts to the UK Coastguard, Sussex Police, South Eastern Ambulance Service and East Sussex Fire and Rescue Service, who said they were treating it as a “chemical incident”.
The RNLI said that “possibly some kind of gaseous fumes” had drifted over and a significant number of people on cliff tops had been struck down with symptoms including irritation, sore eyes and vomiting.
Dan Sankey wrote on Twitter: “Beautiful afternoon at Birling Gap cut short by some weird mist, burning everyone’s eyes which led to the beach and cliff being evacuated.”
People have been advised to wash their eyes if they are irritated and to wash all over with plenty of soap and water to decontaminate their bodies.
East Sussex Healthcare NHS Trust added: “If you were affected and after taking the precautions recommended you still feel ill, contact 111 for advice and only visit your local A&E if you are still suffering significant effects.”
People living along the coast were advised to remain indoors with windows and doors shut but that advice has now been withdrawn.
The beachfront is a hotspot for tourists and locals on bank holidays.
Thousands of people are expected to head to the coast again on Bank Holiday Monday.
The beaches in East Sussex will be open as normal, police said – and there were no officers at the scene by Monday.
They said it was believed that this was an “isolated incident and is not expected to recur”.
Eastbourne councillor Kathy Ballard said: “We need to find out the cause, where did this come from, and to make sure that steps are taken so that it does not happen again.
“I have not heard of it happening before in this area.”