As I was going down the Dupont Circle escalator, I noticed some people ahead of me starting to climb back up the escalator. As I got closer, my eyes started to sting and fumes irritated my nose: I knew I couldn't climb up the escalator faster than it went down, so I started climbing down the escalator as fast as I could, so I could get onto the up escalator. It got pretty unpleasant, but I didn't feel like I was in danger of passing out or anything, it was just unpleasant.
Up top, I thought about walking home or getting a cab. While I was thinking it over, I saw one woman sitting on the sidewalk, obviously having trouble breathing, while a friend tried to help her. He asked her if she wanted an ambulance, and she nodded, so a bystander called 911. A little bit later, she vomited, and I volunteered to wait with her while her friend dashed to CVS and got some wipes. The woman who was on the phone with 911 also went into Panera and got some paper napkins and a cup of water. After a few minutes, she began to feel better, although she still wanted to go to the hospital to get checked out (and her friend did too, although he wasn't affected anywhere near as much).
I went up to the Metro employee who was standing at the top of the escalators, stopping people from going down, and asked if there was going to be a shuttle bus. He said that the station *wasn't* closed, but the police were questioning a subject down below in the station, and I had to use the other entrance. So I guess it wasn't a fire, as I had thought, just some jackass setting off some sort of gas bomb.
I ended up having to blow my nose a few times, but aside from that the fumes didn't have any lasting effect. So not terribly serious, as far as I'm concerned, but at least a couple of people were more seriously affected.
Update: It turns out to have been pepper spray.