The cause of the gas-like smell in Manhattan today remains mysterious, though it’s said that it’s not dangerous. Cf. Don DeLillo, White Noise, pp. 116-17:
"It doesn’t cause nausea, vomiting, shortness of breath, like they said before."
"What does it cause?"
"Heart palpitations and a sense of déjà vu."
"It affects the false part of the human memory or whatever. That’s not all. They’re not calling it the black billowing cloud anymore."
"What are they calling it?"
He looked at me carefully.
"The airborne toxic event."