An unfortuitous downdraft could carry those chemical fumes back into the house... also I wonder what the heck his creosote situation is like? There must be some wonderful stuff condensing out of that smoke on the upper, cooler part of his fireplace. Could create one marvel of a chimney fire if he isn’t super diligent about keeping it clean.
I doubt he ever had his chimney checked or cleaned in the thirty years he's been here. When I moved in over twenty-five years ago the first thing I did is to call a chimney sweep to inspect the fireplaces. He found problems that could have burned the house down. Most people around here ignore their chimneys, at their own peril. My bad neighbor burns so much stinky crap I'm hoping his place burns down; I'm not warning him about creosote build-up.