The chemical cloud that left beachgoers on the East Sussex coastline with stinging eyes, sore throats and even vomiting on Sunday remains 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.

David Slater of the school of engineering at Cardiff University, said the haze appeared to have 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 that 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.

However, Alastair Hay, an emeritus professor of environmental toxicology at the University of Leeds, said that because the cloud was 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”.

While the cloud has dissipated, quite who is leading the investigation also appears hazy: the Department for Environment, Food and Rural Affairs and the Environment Agency both said they were not involved in the incident, referring the Guardian to Sussex police and fire services – both of which declared they were no longer investigating the cloud.

The police said efforts to investigate further were being led by the coastguard service, and that they thought the cloud had been produced by a vessel out at sea.

However, a spokeswoman from the Maritime and Coastguard Agency said it had not been confirmed whether the cloud had come from a source on shore or off. She added that without knowing what the haze was composed of, it was difficult to investigate further, although generally with incidents of pollution experts look at which vessels might have been in the area. But she warned: “That can take quite a long time to follow up with.”