(p. 58) One thing that did not escape notice in medieval times was that nearly all the space above head height was unusable because it was so generally filled with smoke. An open hearth had certain clear advantages–it radiated heat in all directions and allowed people to sit around it on all four sides–but it was also like having a permanent bonfire in the middle of one’s living room. Smoke went wherever passing drafts directed it–and with many people coming and going, and all the windows glassless, every passing gust must have brought somebody a faceful of smoke–or otherwise rose up to the ceiling and hung thickly until it leaked out a hole in the roof.
Source:
Bryson, Bill. At Home: A Short History of Private Life. New York: Doubleday, 2010.