During September we had about ten days of hazardous air quality from fires in California and Oregon. I learned all about the Air Quality Index. It goes up to 500 (extremely hazardous), and we were between 450 and 500 for five days or so. The other five days we were merely very unhealthy or unhealthy. During the worst times, you really couldn't be outside for longer than half an hour or so. My eyes burned and teared up, and my asthma (normally not even an issue) flared up badly. Your clothes and hair would reek of smoke.
Of course, there's nothing to do for the horses except keep an eye on them. You certainly can't work them when the air's that bad. Dakota is always a bit of a cougher when you first start lungeing/riding him, so I was especially worried about him. But they both seemed to weather the miasma just fine. There are many recommendations on how long to wait before bringing your horses back into work post-smoke-inhalation. The two I saw repeated the most often were a) two weeks or b) the same amount of days as the horses were exposed. I went the conservative route and waited two weeks, and then I brought them back very slowly: handwalking and tack walking, slowly adding a bit of trot, then canter, and keeping exercise periods short. Thank goodness we didn't have, on top of the smoke, actual wildfires nearby.
I've picked up a maybe-client with a young OTTB, so we'll see how that progresses. The gelding could do with twelve weeks on the lunge, just learning to relax and pay attention. He's smart and sweet, but also tense and easily distracted. He has great gaits and lots of potential!