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

How did ex-spy Sergei Skripal and his daughter survive nerve-agent poisoning?  Washington Post Sergei Skripal Is 'Improving Rapidly,' But His Cat And Guinea Pigs Are Dead  NPR Former Russian Spy No Longer in Critical Condition After Poisoning  Wall Street Journal Why Russia keeps laughing at the world  CNN Skripal Cousin Says She Doesn't Trust Britain on Chemical Attack  New York TimesFull coverage