Sussex Police said an unknown “haze” coming from the sea had left up to 50 people along the coast from Eastbourne to Birling Gap with irritation to their eyes and throats.
East Sussex Fire and Rescure Service said the “chemical incident” was ongoing.
The first calls were made to the emergency services just before 5pm.
The cause of the haze is currently unknown.
The RNLI said that “possibly some kind of gaseous fumes” had drifted over the scene and a significant number of people on cliff tops had been struck down with symptoms including irritation, sore eyes and vomiting.
Fears from the coastguard that people could be trapped on the beach saw the RNLI launch all-weather lifeboats from Eastbourne and Newhaven to the Birling Gap area.
There was a doctor on board the Eastbourne lifeboat.
A spokesman said: “We have been checking along the shoreline to try and ensure that everyone is safe.”
He said a “plume” had drifted across the area bringing “some sort of substance” with it which seemed to affect a number of people.
Police said the source of the haze has not yet been established.
A spokesman said: “This seems to have been caused by an unknown haze coming in from the sea.
“Emergency services are treating people at Birling Gap and clearing the area.
“People living along the coast in the area have been advised to keep doors and windows shut and to move away from the Birling Gap area in particular.”