Retrofit on existing buildings for better energy performance is widely desired due to large share of building energy consumption. Meanwhile, the importance of thermal comfort and air pollution exposure has attracted increasing attention for occupant health, productivity and sustainable development. Heat, ventilation and air conditioning (HVAC) systems are responsible for a substantial proportion of energy use in buildings and are closely related to indoor environment quality. This study examined the energy consumption and indoor environment in a commercial office building with a retrofitted HVAC system. The retrofitting measures included a sensor-based building management system, dehumidification of outdoor air, and a two-stage particle filtration system. Energy data were collected before and after the retrofit. Field measurements were conducted in both winter and summer to evaluate the thermal comfort and indoor exposure to air pollutants in the retrofitted area. An experiment was designed to assess the benefits of upgraded filters on exposure to ambient particles during summer. By combining all of these measures, the retrofitted HVAC system was able to reduce energy use by 50% while maintaining generally acceptable indoor thermal comfort. Most of the time, the indoor particle levels complied with the World Health Organization's guidelines. The upgraded filtration system with a pleated filter reduced outdoor PM ingress by 30% to 60% more than the aluminum filter used before the retrofit. Co-benefit assessment provides insights into sustainability in building development during a retrofit by holistically examining energy use and the environment. (C) 2019 Elsevier B.V. All rights reserved.