Loss of consciousness was due to cerebral hypoxia due to cardiac arrest resulting from myocardial hypoxia. Factors of temperature, pressure and environmental concentrations of carbon monoxide, carbon dioxide, oxygen and pulmonary irritants were changing extremely rapidly. It is impossible to integrate these variables on the basis of available information with the dynamic physiological and metabolic conditions they produced, in order to arrive at a precise statement of time when consciousness was lost and when death supervened. The combined effect of these environmental factors dramatically increased the lethal effect of any factor by itself. It is estimated that consciousness was lost between 15 and 30 seconds after the first suit failed. Chances of resuscitation decreased rapidly thereafter and were irrevocably lost within 4 minutes.”
More likely: suffocated (as with most fire deaths) https://history.nasa.gov/Apollo204/invest.html:
”d. MEDICAL ANALYSIS
Loss of consciousness was due to cerebral hypoxia due to cardiac arrest resulting from myocardial hypoxia. Factors of temperature, pressure and environmental concentrations of carbon monoxide, carbon dioxide, oxygen and pulmonary irritants were changing extremely rapidly. It is impossible to integrate these variables on the basis of available information with the dynamic physiological and metabolic conditions they produced, in order to arrive at a precise statement of time when consciousness was lost and when death supervened. The combined effect of these environmental factors dramatically increased the lethal effect of any factor by itself. It is estimated that consciousness was lost between 15 and 30 seconds after the first suit failed. Chances of resuscitation decreased rapidly thereafter and were irrevocably lost within 4 minutes.”