Thousands of conferences held all over the world annually are closely related to the theme of environmental sustainability. Various methods and measures have been presented in the effort to reduce environmental impacts. However, the impact of the conference itself receives relatively less attention. This study proposed a mathematical model to identify the optimal conference locations with minimal environmental footprints in travelling to and from the conference by the participants. The historical data of Conference on Process Integration for Energy Saving and Pollution Reduction (PRES) consisting the origin of the participants is applied to demonstrate the location selection. The model is developed to include air transport for short and long distances where the environmental impact is considered in the form of external cost. Participants are geographically distributed into points considering the capital of countries. Socio-economic and geographic data are used to estimate the expected number of participants through a linear regression model. Population, the absolute difference in religion, the distance between countries (with a positive impact on total attendance) and continent conservation (negative impact) were found to be significant. The result suggests the Czech Republic as the optimal location with the external cost of about 242 € / cap. The subsequent locations are Estonia and Romania. Future studies can cover multi-objective decision making, including price and more problematically quantified features as safety, health issues and attractiveness.