The chemical cloud that left beachgoers on the East Sussex coastline with stinging eyes, sore throats and even vomiting on Sunday remains something of a mystery, with officials saying it is unclear what caused the haze, or what it was composed of.
Experts have put forward a number of possibilities, ranging from accidental discharges of chemicals at water treatment plants to toxins from algal blooms.
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.
The haze was described by a witness as “immense” as it loomed off the coast of the Seven Sisters chalk cliffs that afternoon.
Birling Gap beach was evacuated by emergency services, and residents along the coastline towards Hastings were advised to keep windows and doors closed.
A statement issued by Eastbourne district general hospital and Sussex police said the first patients to report to hospital received a “full decontamination treatment” that was later found to be unnecessary.
Although long queues amassed at the hospital into the evening, the effects of the unknown substance were mostly minor, with no one admitted for further treatment. The cloud had mostly dissipated by Monday morning, and authorities were treating it as an isolated incident.
David Slater of the School of Engineering at Cardiff University, said the incident appeared to come from a local source. "From my experience in regulating water companies, unplanned discharges are not uncommon," he said.
But a spokeswoman for Southern Water said there was no evidence wastewater works were linked to the haze. “All our sites in the area are constantly monitored and everything is working normally,” she said, adding that chlorine gas was not used at the Eastbourne Wastewater treatment works.
East Sussex fire and rescue service have also said it was "extremely unlikely" the substance involved was chlorine, despite members of the public reporting a smell akin to a swimming pool, although Alastair Hay, an emeritus professor of environmental toxicology at the University of Leeds, said because the cloud being close to the ground it suggested it was composed of a gas heavier than air, noting that chlorine meets the profile and is a known irritant.
Others put forward alternative theories. Dr Simon Boxall, from the University of Southampton, said it was possible that the cloud was the result of an aerosol of toxins from an algal bloom, noting that the environmental conditions were suitable for such an event. "These cause respiratory problems and irritation, particularly in those with asthma," he said. But he admitted the idea was "a long shot".
Several agencies were working together to establish the nature of the gas and its origins, as speculation continued to mount on social media under the hashtag #BirlingGap. The coastguard was understood to be leading the operation.
Authorities dismissed suggestions that the cloud may have crossed the Channel from northern France as "very unlikely". Although chemicals have drifted across the Channel in the past, police said weather models indicated that could not have been the case on Sunday.
“Neither the gas nor its source have been established, but agencies are continuing to investigate and have not ruled out either onshore or offshore locations, although it does appear that it did sweep in from the sea driven by onshore breezes,” the hospital and police statement said.