The coastguard, police, ambulances and fire and rescue crews are all at Birling Gap near Beachy Head - a notorious spot for suicides.
People were warned to avoid beaches and keep doors and windows shut after an "unknown haze" drifted in off the Sussex coast.
About 150 people required treatment for stinging eyes, sore throats and vomiting in the hours after the haze was first reported to have arrived from the sea at Birling Gap, between Eastbourne and Seaford, just before 5pm on Sunday.
"It is believed that it came in from the sea, but this has not been confirmed and work continues to locate the cause", the statement continued.
"Beautiful afternoon at Birling Gap cut short by some weird mist, burning everyone's eyes which led to the beach and cliff being evacuated", tweeted Mr Dan Sankey.
East Sussex Fire and Rescue Service said it was "extremely unlikely" the substance involved was chlorine, added BBC News. There was a really strong smell of Chlorine in the air too.
The local hospital says it has treated 133 people.
Shortly after other areas were affected, with the coast between Birling Gap across to Bexhill affected, there have also been reports of irritation in Brighton.
People complained of sore eyes and throats after a haze was seen coming in from the sea.
Police were searching today for the source of a toxic gas cloud that forced evacuations along the East Sussex coast over the bank holiday weekend.
An investigation has been launched to establish the source of the gas cloud. However, the vast majority of the gas appeared to have dissipated by later yesterday evening. The RNLI said "gaseous fumes" possibly afflicted people with the symptoms that required medical care.
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