In this paper, starting from previous works, further analyses on district heating cogeneration systems are proposed and particular attention is given to the NOx pollutant. In addition to the mean yearly concentrations, the hourly limit values for the protection of human health are also taken into account: the 18th maximum hourly concentration is considered. The main steps necessary to characterize a district heating plant are shown and two case-studies are discussed. As far as the energetic aspects are concerned, these new plants always offer a primary energy saving, and the global environmental effects are improved due to a reduction in CO2. Local NOx emissions of a future plant do not decrease with respect to the existing emissions, and this aspect is closely connected to the type of CHP unit that is adopted (in this paper, natural gas fuelled engines). The local environmental effects due to NOx are estimated using a dispersion model, and the NOx concentrations (annual mean value and maximum hourly value) introduced by the new plant and those avoided through the non-use of existing domestic boilers are compared. For one of the case-studies emerges that some zones do not satisfy ambient air quality limits. For these reasons, a higher stack has been assumed for this critical case and the new results show that a reduction in the frequency of the overcomings, which is in agreement with expectations, can thus be obtained. The analysed case-studies have shown that even a small CHP district heating plant can produce critical aspects and a pollutant concentration analysis is a useful tool to check local environmental aspects. (c) 2008 Elsevier Ltd. All rights reserved.