What are the things that make fire dangerous? This looks like a simple question, as everyone knows that getting a large burn injury can be deadly, as can the toxic gaseous products of combustion, which act either by simply displacing oxygen from the air or from red blood cells (like carbon dioxide and monoxide), or by causing irritation and damage in the airways (like hydrogen chloride and other nasties that can form when PVC plastics used as electrical insulators decompose at high temperatures).

Recently, our research group at Aalto University has participated in a study of the effects of pressure rise in fires that take place in apartments or other closed compartments. Dangerous pressure rise has usually been seen as a thing that is relevant in explosions, where the release of a large amount of energy and gaseous products raises the air pressure so much that it can cause mechanical damage to structures. However, in the latest study, performed in collaboration with the local emergency services, it was found out that even in normal non-explosive combustion, there can be a rise of pressure that makes it impossible to open an inward-opening apartment door, slowing down evacuation.

In the experiments, the combustibles used were heptane (a volatile flammably hydrocarbon) and polyurethane plastic. Graphs of the air pressure during these fire experiments are shown below.


Figure 1. Pressure development in a heptane fire inside closed apartment.


Figure 2. The combustion of some plastics can be surprisingly vigorous, exemplified by the large overpressure created in this polyurethane fire experiment.

In the heptane combustion graphs one can see how the period of overpressure is followed by brief underpressure when the burning ceases and the gases, rarefied by thermal expansion, cool down again. In the polyurethane test, it was found out that the pressure can get high enough to damage structures (dislodge window frames from the walls, etc.), which is a significant finding.

A YouTube video about these experiments can be found here.

This experiment can be practically very important, as newer apartments in cold climates like here in Finland, are often intentionally made as airtight as possible to prevent heat energy loss and to lessen apartment heating costs. A less airtight apartment couldn’t confine as large an overpressure inside it, and fortunately lack of airtightness is not necessarily mutually exclusive with good heat insulation.