Three radical person been recovered dormant and 4 radical taken to infirmary aft an incidental astatine a attraction home.
Emergency services were called to Gainsborough Care Home successful Ulwell Road successful Swanage, Dorset, astatine 05:20 BST.
Residents person been evacuated from the gathering to a adjacent religion hall.
Dorset Police said the deaths were being treated arsenic "unexplained".
The unit added: "The families of those who person died person been informed and the coroner has been notified.
"At this clip determination is thing to bespeak immoderate hazard to the wider public.
"We recognize this incidental volition person a important interaction connected the Swanage assemblage and our thoughts are precise overmuch with the families of those who person sadly died astatine this highly hard time."
The home, tally by Agincare, has 48 residents, each aged and immoderate with dementia.
Residents were seen being transferred, immoderate successful wheelchairs, to All Saints Church a abbreviated region away.
Emergency beds person besides been delivered to the hall.
In a connection Agincare said it has allocated "extra staffing resources" to supply enactment aft an incidental astatine the site, and that residents moved to a colony hallway and religion are "safe and well".
They said: "We admit that this is simply a stressful and worrying clip for those involved, and person allocated other staffing resources to supply enactment locally. We are contacting and updating residents' families.
"We volition proceed to enactment with the pb section authorization services to instrumentality residents into their location astatine the earliest opportunity, and volition supply further updates successful owed course."
Gas organisation institution SGN said it was called to Gainsborough Care Home earlier connected Wednesday.
It said engineers were sent to implicit information checks successful the building.
"Our thoughts are with everyone who has been affected by this tragic incident," a connection said.
BBC newsman connected the country Tristan Pascoe said: "At the infinitesimal it's unclear what precisely happened here, but you tin archer from the standard of the exigency services' effect that it's intelligibly a superior incident."
He said helium was told by a comparative who had visited a nonmigratory that "gas oregon c monoxide" was involved.
The section ward subordinate connected Dorset Council, Gary Suttle, suggested determination was a suspicion the deaths "could beryllium attributable to c monoxide, but it has not been proven".
South West Ambulance NHS Trust said a hazardous country effect squad situation unit was sent to the country on with 10 double-crewed ambulances.
Fire crews from crossed Dorset were sent to the scene. They person been assisted by a unit from Winchester successful Hampshire.
Dorset South MP Lloyd Hatton said the deaths astatine Gainsborough Care Home would "certainly beryllium hitting the full assemblage hard".
He told BBC 5Live it was "really important" to springiness the exigency services abstraction to transportation retired their inquiries".
The location received a Good standing erstwhile it was past inspected by the Care Quality Commission (CQC) successful 2021.
Additional reporting by Emily Ford