Fire has played a leading role in the development of civilizations and fire and the energy produced by fire have been effective in every turning point throughout history. The main energy of the cannons used by Fatih Sultan Mehmet in the conquest of Istanbul in 1453 is the combustion reaction. On the basis of the industrial revolution and mechanization, the main energy source is the combustion reaction. Today, the combustion reaction is the basis of the operation of automobiles, heating, feeding and many other processes. Cooking, heating, lighting and pottery making, which are also the most basic needs, have always been thanks to fire. In this way, fire has become so important for human beings that it is the subject of myths. However, especially in the last century, with the rapid development of technology, the variety of building materials, items and devices used in the construction of living spaces has increased and the use of synthetic-based products as raw materials has become widespread in these areas. This situation can cause today's fires to be much more destructive. In today's fires, many different fire gases occur. Likewise, the heat value produced by fires can be much higher. This not only facilitates the growth and spread of fires, but also makes extinguishing efforts more difficult. It absolutely necessitates the use of personal protection equipment so that the heat and fire gases generated do not affect the extinguishing personnel. In recent years, many firefighters who entered the fire environment without using personal protective equipment or wearing them incorrectly have been injured or lost their lives. In this paper, the heat transfer patterns and fire stages that cause the rapid growth of indoor fires are examined, and the indicators that may be the harbinger of possible dangers in these stages are emphasized.