A woman and three children aged under 10 have been treated in hospital for the effects of carbon monoxide poisoning.
Firefighters were called to a chimney fire at a house on the Bushmills Road in Coleraine on Tuesday afternoon.
Carbon monoxide was detected at the house, but no-one there was suffering from its effects.
However, the woman and children next door all had symptoms. They have all since been discharged from hospital.
It is thought the gas had leaked through the walls into the house.
The fire service said it believes the chimney flue may be detective.
It appealed to householders with open fire stoves or boilers in their houses to fit carbon monoxide detectors and to have chimneys and flues swept twice a year.