We've recently been making and smoking mostly bho; we have been smoking more oil so our harvest has more time to properly cure! But I do notice a definite increase in coughing since we've been smoking more bho. It's actually making me think about quitting smoking cigarettes! lol....I think it has a lot to do with how strong concentrates are. I don't know about anyone else, but we used to only smoke bho on top of bud, and we used to cough like crazy just from adding it to the bud, so it makes sense that it would be more harsh all on its own. But, you should also always keep in mind that bho extractions may still contain trace amounts of butane, no matter how much you purge it. So that could be it too.
You mentioned you let your bho sit out in the heat to dry....have you ever tried warming it slightly on a coffee mug warmer and stirring? It gets a LOT of the butane out, and if you do that 2 or 3 times and THEN let it sit you might notice a difference!