A poisons expert says people are frequently exposed to carbon monoxide but the public can be assured the George family tragedy was a highly unusual accident.
Ashburton woman Cindy George died alongside her three children, 5-year-old Pio Scarlet Jetejura Raukete, 3-year-old Teuruaa Junior George, and 2-year-old Telyzshaun Gordon Ricardo Nelder Kruz George.
Their bodies were found on Thursday and a car was believed to have been left running in an internal garage.
National Poisons Centre Toxicologist Dr Leo Schep said carbon monoxide could be an "insidious" killer, leaving its victims little or no time to act.
Apart from symptoms of drowsiness or headaches, people may not know they were being poisoned.
When air was not rapidly replenished with enough oxygen, carbon monoxide could kill.
Dr Schep said any investigation into the Ashburton deaths would need to determine how long the car was left on for and how much fuel was in the car's tank before more definitive statements could be made.
But he said it was immediately clear "an unusual situation" was behind the deaths.
Dr Schep said people frequently exposed themselves to carbon monoxide but in most cases the environment only had low doses of the gas.
"When we're walking down the street we're exposed to it. When you smoke cigarettes you're exposed to carbon monoxide and your levels will be higher."
Dr Schep said carbon monoxide poisoning deprived the brain of oxygen as the gas binded to haemoglobin.
Haemoglobin is a iron-containing protein molecule in red blood cells. It carries oxygen from the lungs to the body's tissues and returns carbon dioxide back to the lungs.