Four young children were treated for carbon monoxide poisoning after their grandmother brought a barbecue indoors to dry her washing.
The woman, who has not been named, lit the device which she set up in the family kitchen before leaving the house.
Her three-year-old granddaughter collapsed when she was overcome by the deadly gas later that afternoon and was treated in hospital with five of her relatives.
They included two boys aged two and 10 months, a four-year-old girl and the woman's two daughters-in-law, aged 26 and 29.
Firefighters were called to the home on Hockley Avenue, in East Ham, east London, on Wednesday afternoon, soon after the grandmother set up her makeshift launderette.
A spokesman for London Fire Brigade said the elderly woman carried the barbecue in from her garden at around 3pm and placed it near the back door to speed up the drying of clothes which she hung out around the kitchen.
She then walked out, leaving her daughters-in-law in the house with her grandchildren, two of whom were asleep upstairs.
One adult began to feel unwell before the little girl collapsed.
All six have now been discharged from hospital.Fire chiefs branded the grandmother's behaviour dangerous and campaigners warned of the hazards of bringing barbecues indoors.
The Department of Health believes 50 people are killed by the gas each year, while at least 4,000 are treated in hospital.
But this figure is likely to be much higher because of difficulties surrounding the diagnosis of CO poisoning.Symptoms are often similar to common illnesses like flu and food poisoning.