People were warned to avoid beaches and keep doors and windows shut after an "unknown haze" drifted in off the Sussex coast.
Up to 50 people initially reported irritation to their eyes and throats after the incident on the cliff tops and beach at Birling Gap, near Eastbourne, just before 5pm on Sunday.
This seems to have been caused by an unknown haze coming in from the sea, but the source has not yet been established, said Sussex Police.
The fire service has said that it was extremely unlikely to have been chlorine, as had been suggested on social media, police said.
It appears that the last reports were received some three hours after the first, but long queues built up at Eastbourne District General Hospital well into the evening, with approximately 150 people being treated.
However, the effects were mostly minor and it was not necessary to admit anyone for further treatment.
The first patients were given full decontamination treatment as a precaution but police said it was soon clear that this was unnecessary.
People were advised to wash all over with plenty of soap and water.
East Sussex Fire and Rescue Service said the effects of the haze were also being reported as far as Bexhill and towards Hastings.
Emergency services cleared the area around Birling Gap and told people living along the Eastbourne coast to avoid beaches and shut their windows and doors.
During the afternoon, as the haze came towards the beach, witness Mark Battershill had told Sky News: The haze is pretty bad – it’s coming in.
Eyes are streaming, I’m at home now with my family, we’re all sitting here and the tears keep running, stinging eyes, sore throats.
Mr Battershill was at Eastbourne’s harbour when the haze came in: I was speaking to a colleague of mine and said ‘my eyes are really streaming’.
I looked around and he was the same and we’re all saying ‘what is this?’
He said some people were complaining they could not breathe properly.
Another witness, Jonny Gould, tweeted: I am about four miles along the Sussex coast from #Birling Gap. The smell of plastic is strong in the air and my eyes are irritated.
Sameer Jadhav tweeted: Birling Gap some weird gas leak. People crying. Ambulances called. Don’t go to Birling Gap at the moment.
Many businesses in Eastbourne have also closed, with a sign at the local McDonald’s saying it had shut due to a chlorine gas leak.
The main area affected stretches several miles from Birling Gap to the famous cliffs at Beachy Head, and to the Sovereign Harbour on the eastern fringes of Eastbourne.
Some people vomited as a result of breathing in the haze, said Sky News reporter Katerina Vittozzi.
Vittozzi said just after 11pm that the haze had largely dissipated and that people were being advised not to go to hospital unless they had severe symptoms.