A 62-year-old woman was pronounced dead after two people were found unconscious in a house in Ystrad Mynach.
A 63-year-old man was taken to Prince Charles Hospital in Merthyr Tydfil after the pair suffered suspected carbon monoxide poisoning.
Police were called to the property on Davies Street at around 8am this morning, November 4.
A 63-year-old-woman was pronounced dead at the scene.
The man is now in a stable condition in hospital.
A Gwent Police spokeswoman said: “Officers are supporting the family and enquiries are on-going.
“We suspect carbon monoxide poisoning but we are awaiting further tests to be carried out to confirm this.”
• A national campaign to raise awareness of carbon monoxide poisoning was launched last week.
The Carbon Monoxide: Be Alarmed campaign claim two thirds of people in Britain do not have a carbon monoxide alarm.
The campaign can be contacted on 02077 472941.