By Tom Geoghegan
BBC News Magazine
The United States is one of 55 countries that practise the death penalty as the ultimate sanction on convicted criminals. But in the majority of US states the procedure is frozen while the Supreme Court decides whether lethal injection is a "cruel and unusual" form of punishment that violates the constitution.
But Michael Portillo, who has voted for and against the death penalty in his time as an MP, says he changed his position because of miscarriages of justice.
For BBC Two's Horizon, he set out to investigate whether one method was any more humane than another, but concluded they were all imperfect:
- lethal injection is a combination of three chemical injections - one which makes the inmate unconscious, another that paralyses all muscles except the heart, and a final drug that stops the heart, causing death. But opponents say that if one of the chemicals fails or is badly administered, the prisoner suffers excruciating pain
- hanging causes a fracture between the second and third cervical vertebrae, fracturing the joint, tugging the spinal cord, damaging the brain stem and causing the heart to stop. Still common in many parts of the world, it's nevertheless an exact science - if the rope is too short, the prisoner may not die instantly; too long and he may be decapitated. The latter seems to have been the case last year in the botched hanging of Saddam Hussein's half-brother
- electrocution provides 15 seconds of about 2,450 volts applied to the human body at three points, the head and two calves. A sponge soaked in brine, which is a good conductor, is used on the head. But the voltage may not be sufficient to stop the heart and prisoners sometimes require more than one blast, despite severe damage to internal organs
- gas is used in five US states. Cyanide interferes with the human cells' ability to carry oxygen. Scientist Christopher Cooper described the pain as a combination of brain seizure, heart attack and asphyxiation. Prisoners can minimise the pain by breathing deeply, but in practice this is very difficult.
Having ruled out all four methods, mainly because there was a risk of pain, Mr Portillo looked at an alternative, the deprivation of oxygen - hypoxia. It's commonly used in the killing of lab animals because it preserves their body tissue.
He discovered that nitrogen could do the job in about 15 seconds, and the prisoner would not feel pain - on the contrary he would feel euphoric, like being drunk.
"No method of execution can prevent the knowledge that you are going to die by the state in the future," says Jon Yorke, a law lecturer who has done extensive death penalty research in the US. "That will have a psychological impact, it can never be humane."
The so-called "death row phenomenon" affects an inmate in two ways, says Mr Yorke. One concerns the mind. In 1986 in Florida, Alvin Ford escaped the death penalty because he had become insane on death row.
The other is the physical impact of the structure in which an inmate is being held. In Oklahoma, where cells on death row are deprived of sunlight, a prisoner may endure 25 years without Vitamin D.
Lawyer Clive Stafford-Smith, who has represented death row inmates, concurs and says the mental torture of being on death row is far more horrific than what awaits them at the end.
But Tom Sorell of the Global Ethics Centre in Birmingham says philosophically speaking, the death penalty can be humane, if it's restricted to very serious crimes and due legal process is followed.
However he admits some of the current methods are unreliable and painful, and suggests that methods used in euthanasia would be more appropriate.
When asked if nitrogen would be a more humane way for the state to kill, the leading voice of the American pro-death penalty movement, Professor Robert Blecker, strongly disagrees.
What makes any method perfect is completely subjective, says Mr Portillo. "For the pro-death penalty lobby, using a painless method of execution is inhumane to the victim of the crime.
"I set out to discover whether science could offer a painless way of killing people and it does.
"And I think that is the right thing to be looking at, because for as long as the state is going to kill people I think it has the obligation to do it in the way that least resembles murder."
Horizon: The Science of Killing is on BBC Two at 2100GMT on Tuesday 15 January