Publisher Summary Flammable gases and volatile liquids are hazardous because of the relative ease with which they produce mixtures with air within the flammable range. An increase in the surface area of any liquid facilitates vaporization. For each substance, there is a minimum concentration of gas or vapor below which flame propagation will not occur (that is, the mixture is too lean). There is also a concentration above which the mixture is too rich to ignite. The limits of flammability are influenced by temperature and pressure (for example, the flammable range expands with increased temperature). Generally, the wider the flammable range, the greater the fire risk.