Large amounts of fuels, burned in various types of furnaces are one of the main sources of pollution in the environment. Flue gases pollute the atmosphere and the associated ashes pollute soil and water. To compare the influence of various fuels burned in different installations on the environment, one universal index would be helpful. Such a coefficient, which represents the harmfulness of combustion processes of a particular fuel in a particular installation, is defined in this paper. This coefficient takes into account the composition of the fuel, the thermal efficiency of the installation, the type and process efficiency of creation of harmful compounds in the combustion chamber, efficiency of cleaning devices, ability of emitter to propagate pollutants into the atmosphere, as well as the relative toxicity of various components. Finally methods of calculations of propagation of pollutants in the atmosphere are introduced. All these factors are essential for a reliable assessment and comparison of fuel and installations. An individual coefficient of harmfulness could be calculated for each compound present in the flue gases as well as a total coefficient for all compounds. Each compound created in the combustion processes is the source of many types of environmental impact. It could have significant influences on human health, plants and animals as well as for example on the corrosion process taking place in many different installations. We have to take into consideration that the result of a combustion process could appear in many different places, sometimes distant from the place of the process itself. All these results should be analyzed and introduced into the universal coefficient of "harmfulness" of specific compound. Economic parameters and methods are important and therefore are also introduced into the assessment method presented in the paper. In this way we are able to compare the values of extremely different phenomena such as a human health and corrosion. (C) 2003 Elsevier Science Ltd. All rights reserved.