Sala, 28, and pilot David Ibbotson, 59, crashed on 21 January while travelling from Nantes in France to Cardiff.
Toxicology tests on Sala's body showed CO levels in his blood were so great it could have caused a seizure, unconsciousness or a heart attack.
The Sala family said there should be a detailed examination of the plane.
Mr Ibbotson, from Crowle, North Lincolnshire, has still not been found.
The Air Accidents Investigation Branch (AAIB) said Sala's blood had a COHb (carboxyhaemoglobin - which forms in red blood cells upon contact with carbon monoxide) level of 58%.
At this level, symptoms would include include seizure, unconsciousness and heart attack.
It added: "A COHb level of more than 50% in an otherwise healthy person is generally considered to be potentially fatal."
It is likely Mr Ibbotson would also have been exposed to carbon monoxide.
Piston engine aircraft such as the Piper Malibu involved in the crash produce high levels of carbon monoxide.
The gas is normally conveyed away from the aircraft through the exhaust system, but poor sealing or leaks into the heating and ventilation system can enable it to enter the cabin.
Several devices are available to alert pilots over the presence of carbon monoxide - they are not mandatory but can "alert pilots or passengers to a potentially deadly threat".
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