(CNN) Around 150 people have been treated at hospital after being caught in a mysterious "chemical haze" that blanketed the UK's East Sussex coastline Sunday afternoon.
Residents exposed to the cloud reported eye and throat irritations, a spokesperson for the National Health Service told CNN.
In a statement, police warned "hundreds more" people across the region could have been affected.
There are no indications yet as to what caused the haze, though East Sussex Fire and Rescue Service have termed it a "chemical incident."
Authorities said the problems began when the "unknown haze" covered the area after possibly coming in off the sea at about 5 p.m. local time.
It was first reported in Birling Gap, between Eastbourne and Seaford.
"(Given the weather) it is highly likely that any effect will have come from the UK side or, possibly, a vessel running close to shore with the tide bringing material towards the beach," University of Southampton National Oceanography Center's Simon Boxall said in a statement.
"It could also have come from vessels in the Dover Strait."
A police statement Monday confirmed it was "very unlikely" the dangerous gas had come from northern France.
Late on Sunday night, Sussex police said on their social media the haze "seems to be clearing" and by Monday morning they confirmed it had dissipated.
Multiple UK publications were reporting the cloud was chlorine gas -- a toxic chemical weapon that was used to devastating effect during the first World War -- but authorities said this was "extremely unlikely."
"The effects, while uncomfortable, were not serious, and an investigation is now under way by a number of agencies working in partnership to establish the source of the gas," Sussex Police said in a statement.
Some of the first patients to be seen were fully decontaminated, with local media showing photos of hazardous material suits and decontamination tents.
But later arrivals were simply given basic treatment after clinical advice confirmed that decontamination wasn't necessary, according to police.
"If you feel any effects, such as stinging eyes, the South-East Coast Ambulance Service is advising to wash with copious amounts of water and that if you have any serious concerns then you should seek medical advice," East Sussex Fire and Rescue Service said in a statement.
All the patients who arrived for treatment have since been released from hospital.
"The effects were mostly minor and it was not necessary to admit anyone for further treatment," Sussex Police said in a statement.