It is important to know whether a flammable material will rise or fall in the atmosphere.
Knowledge of where a flammable material will collect ensures that gas detectors can be located at the correct level, and that ventilation is directed accordingly.
The materials are compared with air and allocated a number to denote their relative density, given by the Greek letter ‘p’, (rho). Air has p = 1. A material with p more than 1 will fall in the atmosphere. Materials with p less than 1 will rise in the atmosphere. Typical values of density (p) of some gases are given in the table:
| Material | Relative vapour Density ‘p’ |
| Air | 1 |
| Propane | 1.56 |
| Methane | 0.55 |
| Ethylene | 0.97 |
| Hydrogen | 0.07 |
| Acetylene | 0.90 |
| Diethyl Ether | 2.55 |
| Hydrogen Sulphide | 1.19 |
| Carbon Disulphide | 2.64 |
Materials which rise in the atmosphere can collect in roof spaces.
Those which fall, such as butane or propane, can drift along at ground level and possibly into a non-hazadous location, or may collect in locations lower than ground level without ever dispersing.
Such locations should be well ventilated in order to avoid ignition due to a stray spark etc.
