The environmental performance of public transportation services plays a key role in improving air quality in urban areas. Replacing vehicles’ current engines with innovative propulsive systems may be an effective solution, but requires considerable financial resources that are not always available. In this research we aim to assess the impact on the environment of the organizational form of transit systems. We will compare traditional fixed-route services and demand resposive services in terms of their pollutant emission. Research will be conducted with simulation analyses and case studies under a broad range of scenarios with different road networks, service quality levels and demand densities. To ensure a meaningful comparison in each scenario, the same set of customers will be provided with the same level of service. Established emission and level of service models will be used to ensure model validity and compute the results.