“What’s the etiquette around vaping in public spaces (especially enclosed ones)?”

Photo by PoPville flickr user Victoria Pickering

“Dear PoPville,

I’m (currently) riding the red-line metro during rush hour, sitting near a group of teenagers. The metro is fairly full- but one of the teenagers is using his vape pen on the train. I’m not sure if this is illegal, or just plain stupid- but what’s the etiquette around vaping in public spaces (especially enclosed ones)?

Thanks-
Your resident DC Asthmatic”