While a low oxygen environment may not cause discomfort or alarm, the body will detect any associated build-up in carbon dioxide (the normal product of respiration). Rising levels of carbon dioxide cause distress, alarm and the sensation of ‘air hunger’. If one places a plastic bag containing normal air over their head, they will breathe easily and use up the oxygen. However, there will also be a corresponding rise in the level of carbon dioxide within the bag. When the body detects this increase of carbon dioxide, a warning message from the brain alerts us. The person will be roused and will react by gasping, feeling like they cannot breathe. This is the sensation known as ‘air hunger’. They will quickly pull the bag from their head. This reaction is known as a Hypercapnic (high carbon dioxide) Alarm Response.
To ensure that carbon dioxide does not accumulate in a bag, there must be a gas flow into the bag even when the bag is pulled down over the head. Any exhaled carbon dioxide can be flushed away as the inert gas exits the bag from around the neck.
For a peaceful, hypoxic death - a so called ‘happy hypoxia’- one needs a low oxygen (hypoxic), low carbon dioxide (hypocapnic) environment.
Note: It was once suggested that a plastic bag could be used in combination with strong sedatives to end life (no gas would be required). The hope was that the drugs would induce sleep as the oxygen in the bag was used up, and that the sedation would be so profound that the alarm response from the rising carbon dioxide would not re-awake the deeply sleeping subject. This approach is now considered risky with an uncertain outcome and is not recommended.