Mr. Skirpal, a former Russian double agent, and his daughter collapsed on a bench in Salisbury, a city in southern England, several hours after they were exposed to novichok, a nerve agent that Soviet scientists developed for battlefield use against Western troops.

Investigators have said that the substance was applied to the door of Mr. Skripal’s home, and that it likely seeped through their skin over the course of several hours, rendering them unconscious after they left a restaurant in central Salisbury.

Russian officials, who deny any involvement in the attack, have suggested that Western laboratories may have synthesized the poison used. Mr. Uzumcu said that, if Western laboratories had produced novichok for research purposes, or to develop an antidote, they would have created a smaller quantity than what was used in the March 4 attack.


Ahmet Uzumcu, director general of the Organization for the Prohibition of Chemical Weapons, in March.

Peter Dejong/Associated Press

“For research activities or protection you would need, for instance, five to 10 grams or so, but even in Salisbury it looks like they may have used more than that, without knowing the exact quantity, I am told it may be 50, 100 grams or so, which goes beyond research activities for protection,” he said.

He said the novichok used in Salisbury was in liquid form, and that O.P.C.W. experts had collected samples from the door handle of the Skripal home, the park where the two collapsed and “a few other places where the Skripals were present.”

“One thing, perhaps, which is important to note is that the nerve agent seems to be very persistent,” he said. “It’s not affected by weather conditions. That explains, actually, that they were able to identify it after a considerable time lapse. We understand it was also of high purity.”

He said the agent could be applied with an aerosol spray or, “if you take the necessary measures, you could use it as a liquid.”

Health officials have begun a meticulous decontamination process, warning citizens of Salisbury that there might still be toxic “hot spots” in some areas, and that a thorough cleanup may take months.

Though citizens have been reassured that there is little threat to their health, the decontamination promises to be a vast undertaking, requiring backup from 190 army and air force specialists as well as input from the Defense Science and Technology Laboratory, the Home Office and the Ministry of Defense, the authorities said.

“The chemical does not degrade quickly,” Ian Boyd, the chief scientific adviser at the Department for Environment, Food and Rural Affairs, told a town meeting. “You can assume it is not much different now from the day it was distributed.”

The novichok strain, developed in the Soviet Union with the goal of evading NATO chemical weapons detection, was not included in the list of banned chemicals compiled in 1987, so Russia was under no obligation to regularly report on its stockpiles or their destruction.

That may change in the coming months, said Mr. Uzumcu, who said he has already solicited a report from the group’s scientific advisory board. He said the organization’s executive council will make a decision about adding novichok to the schedule at one of its next meetings, in July or October.

“This nerve agent should clearly be part of our scheduled chemicals, and it should be included in the database which is being used for our laboratories,” he said.

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