If your interest is historical, "1x18" may refer to the ratio of firefighters to citizens (there were only for a city of 300,000) during the 1871 disaster. Key research resources for this include: "Chicago Fire" Fireworks (TV Episode 2013) - IMDb
: Tensions between Chief Boden and Benny Severide peak as Peter Mills confronts Benny about the fire that killed his father. Key Themes for Study : Chicago_fire_1x18
If you are looking for an analysis of the television episode, "Fireworks" is a critical point in the first season's narrative arc. While a single episode rarely has its own published academic paper, it is often cited in studies regarding and urban heroism . If your interest is historical, "1x18" may refer