A woman and her infant son died of carbon monoxide poisoning at a construction site in Alberton on the East Rand on Tuesday, paramedics said.
The family was apparently using a charcoal fire for warmth at an informal settlement on the construction site in Randhart, Netcare 911 spokesman Jeffrey Wicks said.
"Either falling asleep or overwhelmed by the noxious gas, the couple failed to extinguish the fire, which is thought to have burnt through the night in an adjacent room."
A passer-by found their bodies in the morning and called paramedics.
The woman's husband was in a critical condition.
"Her husband is fighting for his life in hospital... Carbon monoxide poisoning is particularly prevalent in the winter months, as many resort to fires for warmth," said Wicks.