Interesting day. We were at Beloit College, the football stadium, and the woodchips caught on fire while we were chipping. We were doing some extremely dry trees. After about 15 minutes of chipping, the dust was really bad. Boss's son and I played it off as the trees being really dead and dry and our new chipper blowing all the dust out of the box back at us, because it blows harder than our other one.
About 15 minutes later, we knew something was right, the dust cloud was behaving more like smoke and it smelled like burning wood. Boss's son yells up to the boss in the bucket, who is in fresh air, and says "it's like standing in a fire." We stop chipping and the boss comes down. Boss's son says he thinks the chips are on fire, boss doesn't believe so and tells him to just move the chute to stop the airflow into the box. Smoke continues billowing out of the box, including the side vents. Boss's son gets a fire extinguisher and starts spraying down the chips. Doesn't do anything. Boss calls the landscaping crew and asks if there is a hose around that we can use. There is, but on the opposite end of the stadium. We unhook and boss's son takes the truck through town to the other side, smoking like hell going down the road.
Boss and I walk up to the other end and meet one of the grounds crew guys who gets the hose out. We take turns spraying down the chips because the smoke was so thick we could only do it for a couple minutes at a time. After about 10 minutes of no progress, boss's son climbs into the box and shuffles around the chips with a small fork, while the grounds crew guy sprays it down. The flameless fire was way down towards the bottom of the chips. Finally after about another 15 minutes, the smoke dwindles and the fire is out.
So we were inhaling smoke for a while and had no idea. We don't wear dust masks because dust blowing back at us is never a problem, even with dry/dead trees. With this new chipper though, we might consider it.