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Philadelphia (CNN)A crowd hovered over the man lying on the grass as his skin turned purple. Chera Kowalski crouched next to his limp body, a small syringe in her gloved hand.

Squeeze.

The antidote filled the man's nostril.

The purple faded. Then it came back. Kowalski's heart raced.

"We only gave him one, and he needs another!" she called to a security guard in McPherson Square Park, a tranquil patch of green in one of this city's roughest neighborhoods.

"He's dying," said a bystander, piling on as tension mounted around lunchtime one recent weekday.

"Where is the ambulance?" a woman begged.

Squeeze.

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