People spend around 90 percent of their time indoors (home, oce, transportation) in developed countries [1]. This means a long exposure to indoor conditions that have higher concentrations of pollutants compared to outdoors. This exposure can aect the well-being, performance and health of the occupants, making indoor environments even more important. On the other hand, the design and operation of buildings need energy. Following the oil crisis in 1974, building envelopes were aimed to be more air tight to decrease the energy consumption for heating and cooling. In addition to the economic costs, according to International Energy Agency (IEA), building sector is responsible for almost forty percent of energy-related CO2 emissions worldwide [2]. The Intergovernmental Panel on Climate Change (IPCC) report by UN body stresses the importance of evolving to greater sustainability in land, energy, buildings and cities to reach the reduced CO2 levels in order to limit global warming by 1.5°C [3]. With increasing concerns about environment and global warming, decreasing the energy intensity of the building operations became a goal. As a result, creating comfortable indoor environments with less energy demand became a hot topic and green building certications were promoted. While environmental concerns led to focusing on the energy and water conservation, the indoor environmental quality and health aspects of green buildings were less recognized [4]. The green building certications had their sections related to occupant health and well-being, but recently there has been a prioritization of human health relative to the energy consumption. The companies understand that the cost of workers is higher than the cost of energy. In a company, 90% of operating costs is salary/benets of sta, 9% is rental costs and only 1% is energy [5]. A study done in Harvard School of Public Health shows that elevated CO2 levels result in a 50% decrease in cognitive function [6]. In addition, a lack of fresh air is shown to cause 35% of total absenteeism in a company [7]. In relation to these aspects, WELL certication is a new rating system that focuses on occupant health and wellness through design and operation of buildings. However, no studies were done so far to see its eectiveness. This study was designed to address this knowledge gap. It is the rst cohort study done in WELL buildings. The project aims to compare the satisfaction levels, productivity and SBS symptoms with an online indoor environmental quality (IEQ) survey done in non-WELL and WELL buildings.