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How did ex-spy Sergei Skripal and his daughter survive nerve-agent poisoning?

Police officers stand outside the house of former Russian double agent Sergei Skripal on March 6 in Salisbury, England. (Frank Augstein/AP)

LONDON — It is an indelicate question, but why aren’t the Skripals dead?

The former Russian double agent attacked by a military-grade nerve agent a month ago is no longer in critical condition, his doctors said Friday, having joined his daughter in a recovery that could lead to the pair helping investigators solve the mystery of exactly how, where and by whom they were poisoned.

Sergei Skripal, 66, is “responding well to treatment, improving rapidly and is no longer in a critical condition,” said Christine Blanshard, medical director at Salisbury District Hospital.

The ex-spy’s 33-year-old daughter, Yulia Skripal, was also poisoned, but her condition had improved to stable last week.

In 1992, two Russian scientists approached The Post’s Will Englund, then the Moscow correspondent for the Baltimore Sun, with news of a secret nerve agent. (Video: Joyce Lee, Will Englund/The Washington Post)

Yulia Skripal is speaking, issuing statements and allegedly talking to relatives back home in Russia on the telephone.

“As Yulia herself says, her strength is growing daily and she can look forward to the day when she is well enough to leave the hospital,” her doctor said.

How can this be? As the Russian government has asked, in its campaign to discredit British assertions that Russia was behind the attack, why didn’t the Skripals collapse immediately if they were poisoned by a powerful nerve agent? If they came into contact with the agent at their doorstep, how did they have the ability to first go to a pub and a restaurant before they succumbed? 

State-owned Channel One Russia aired audio April 5 of an alleged phone call between Yulia Skripal, who was poisoned with her father, and her cousin Viktoria. (Video: Channel One Russia)

On March 4, the Skripals were found on a park bench. She was comatose; he was awake but disoriented. Medical experts attributed their recovery to the readily available treatments and antidotes they probably received at the hospital.

“Poisoning does not necessarily result in death,” said Michelle Carlin, a senior lecturer in forensic and analytical chemistry at Northumbria University. “If treated quickly and promptly enough, it is possible to recover from poisoning of many compounds.”

Asked if it was surprising that at least one of the Skripals regained consciousness after several weeks in a coma, Carlin said: “We don’t really know what normal is with Novichok agents, but what you’ve got to remember is their body has been poisoned with a compound, and although giving somebody an antidote will block the signal, the body still needs time to recover, and it needs to recover naturally, and obviously that takes time.”

She said it was difficult to discern why Yulia Skripal recovered faster than her father. “We don’t know if that’s because she had less administered, or it took longer to have an effect or because she’s younger or healthier — there’s too many factors to take into account,” she said.

Toxicologists and medical forensic specialists said nerve agents are most dangerous in the first days after an attack, but given that the Skripals pulled through that, it wasn’t surprising that their condition was improving.

Malcolm Sperrin, a medical physicist, said that a long-term prognosis for the Skripals remained somewhat uncertain. Referring to Yulia, he said: “She might talk and converse, but what we don’t know is if she will go back to where she was before the event, not just about cognition, but there’s other aspects that could be affected, like mobility, problems with mental state, anxiety, depression. There are so many unknowns.”

After the doctor’s statement Friday that the elder Skripal’s health had improved, the Russian Embassy in Britain tweeted, “Good news!”

Russia has denied having anything to do with the attack, with top Russian diplomats insisting that Britain either rushed or bungled the investigation — or that British agents themselves may have poisoned the father and daughter to smear Russia, justify increases in military spending or distract residents from the failures of Brexit negotiations.

A director of a British military laboratory said this week that the nerve agent was definitively from the Novichok class, a type designed and manufactured by the former Soviet Union and Russia. The lab, however, could not definitely say where this batch was manufactured.

British police investigators say the highest residual concentration of the nerve agent was found at Sergei Skripal’s front door in the quiet town of Salisbury, down the road from the Stonehenge ruins.

Prime Minister Theresa May has condemned the poisoning as a reckless and hostile attack by Russia on British soil. She and her government say that in addition to forensic evidence, Russia is suspect because it has assassinated critics abroad before.

Skripal was a double agent who sold secrets to Britain. He was convicted and jailed in Russia but released in a swap of intelligence agents in 2010.

Britain’s Foreign Ministry issued a statement Friday saying, “Let us be clear, this was attempted murder using an illegal chemical weapon that we know Russia possesses.” 

Britain said the improving health of the Skripals is “a tribute to the hard-working and talented National Health Service (NHS) staff in Salisbury who have provided outstanding care. The NHS will continue to provide ongoing care for the Skripals, both of whom are likely to have ongoing medical needs.”

May’s government, the United States and more than two dozen other countries have expelled about 150 Russian diplomats and intelligence agents from their embassies abroad. Russia has retaliated with its own expulsions.