A stretch of coastline was evacuated after a chemical 'haze' left dozens of holidaymakers with streaming eyes, sore throats and vomiting.
Emergency services warned people along the East Sussex coast to keep doors and windows closed after people were affected along the shoreline from Eastbourne to Birling Gap, near Beachy Head.
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.
A spokesman for Eastbourne District General Hospital told the BBC that 133 people had received treatment on Sunday.
Kyle Crickmore, who had been enjoying the Bank Holiday weekend sunshine at the beach at Birling Gap with his family, tweeted: "Some sort of chemical incident at Birling Gap, eyes are streaming and there's a strong smell of chlorine in the air".
He had finished swimming when his eyes started stinging. He looked around and saw everyone on the beach also rubbing their eyes and coughing in what he described as "all sorts of strange behaviour".
He told BBC News: "Then looking out towards the water, there was just this crazy foggy haze rolling in off the sea out of nowhere. It was definitely out of the ordinary considering it was a nice clear sunny day 10 minutes beforehand.
"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."
Members of the public alerted emergency services just before 5pm.
It triggered call outs to the UK Coastguard, Sussex Police, the South Eastern Ambulance Service and the East Sussex Fire and Rescue Service who said they were treating it as a "chemical incident".
The beachfront is popular with dog walkers and is a busy spot for tourists and locals enjoying the bank holiday sunshine.
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."
Sameer Jadhav tweeted: "Birling Gap some weird gas leak. People crying. Ambulances called. Don't go to Birling Gap at the moment".
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.
The RNLI launched all-weather lifeboats from Eastbourne and Newhaven to the Birling Gap area after fears from the coastguard that people could be trapped on the beach. There was a doctor on board the Eastbourne lifeboat.
UK Coastguard commander Steve Carson said: "This seems to have been caused by an unknown haze coming in from the sea, but the source has not yet been established. People living along the coast in the area have been advised by emergency services to keep doors and windows shut and to move away from the Birling Gap area in particular."
Eastbourne councillor Kathy Ballard described it as "a matter of definite concern," adding: "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."