Around 150 people have been treated at hospital after being caught in a mysterious "chemical haze" that blanketed the UK's East Sussex coastline on Aug. 27.
Residents exposed to the cloud reported eye and throat irritations, a spokesperson for UK's 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.
A satellite-imaging agency may have located a possible source of the chemical plume to a boat close to the East Sussex shore. Neodaas, a facility of the UK Natural Environment Research Council that works with the Plymouth Marine Laboratory and the University of Dundee, tweeted images appearing to show a plume coming off a boat on Sunday morning.
One image showed a plume of around 3.5 km long. Neodaas is urging the Met Office to help determine whether wind speeds at the time could have drawn the plume onshore.
Two pollution-monitoring sites on the south coast picked up a quadrupling in ozone levels in a 30-minute period on Sunday afternoon, suggesting the toxic cloud came from a nearby ship.
One site at Devonshire Park in Eastbourne and the national monitoring site at Lullington Heath, East Sussex, are both within five miles of Birling Gap.
"(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."
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