Quantitative risk assessment is the main stage of a risk-based approach to industrial safety management of a hazardous production facility. The most important problems of this assessment: presence of uncertainty peculiar for risk indicators; quantitative assessment of this uncertainty and the current practice of ignoring both when making quantitative risk assessment. It is common practice to express quantitative risk assessments by point (scalar) numbers, however, due to the presence of uncertainty, their values are determined by intervals.

Among the parameters characterizing explosion-fire hazard of gas-vapor mixtures, the lower and upper concentration limits of flame propagation take the important place. Their values were published long ago for a vast majority of combustible substances, which as a result of an accident at a hazardous production facility can form explosive gas-vapor mixtures. In the reference literature, these quantities are traditionally given in the form of scalar numbers. However, the analysis shows that this is a very rough assessment: the values of lower and upper concentration limits of the combustible gases and vapors flame propagation vary in certain intervals or are not exactly known. The main sources of this uncertainty are specified:

• uncertainty of measurement (if the value of lower concentration limit of flame propagation is obtained by the laboratory method);
• type of ignition source (causes significant uncertainty);
• air-fuel mixture parameters (temperature, pressure, oxygen concentration) — it was experimentally proved long time ago that the value of the lower concentration limit of flame propagation substantially depends on them. The thermodynamic conditions of emergencies at a hazardous production facility can vary significantly, so the value of the lower concentration limit of flame propagation will also be different.

In the experiments on a simple laboratory setup, different ignition sources were used — an electric spark, a heated nichrome wire. The influence was studied concerning the type of ignition source on the value of the lower concentration limit of air-vapor mixtures flame propagation — for flammable liquids: heptane, octane, ethyl acetate and ethanol. The difference (depending on the ignition source) was 1.5–2 times, and the boundaries of the interval values did not intersect.