Chimney schmutz is a term used to describe the SOOT that accumulates in a chimney due to the burning of fuel. The SOOT is composed of tiny particles of ash, tar, and other materials that are released into the air when fuel is burned. The SOOT can cause health problems if it is inhaled, and it can also be a fire hazard if it builds up in the chimney. The term 'SOOT' is a four-letter word that is used to describe this type of chimney schmutz.