Some 150 people required treatment for stinging eyes, sore throats and vomiting after the noxious haze rolled in from the sea, reaching the shore 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, while people were warned to keep their doors and windows closed.
The Maritime and Coastguard Agency (MCA) said it was examining whether the emissions came from a vessel, previously unreported lost cargo or known shipwrecks, as it continues to investigate.
"As part of our investigations we are considering a number of possibilities, such as discharges from a vessel, previously unreported lost cargo, and emissions from known shipwrecks," a spokesperson for the MCA said.
"We have identified approximately 180 vessels that passed through the English Channel off the coast of Eastbourne on Sunday August 27.
"We are working with all relevant environmental and public health regulators to conclude these investigations. We have no further information at this stage."
Sussex Police said those who required treatment experienced "mostly minor" effects from the toxic cloud.