Due to concerns about potential airborne chemical and biological (chembio) releases in or near buildings, building owners and managers and other decision makers are considering retrofitting buildings to provide some degree of protection against such events. A wide range of technologies and approaches are being proposed with varying levels of efficacy and cost, as well as varying degrees of applicability to particular buildings and ventilation systems. This document presents the results of an effort to evaluate chembio retrofit options for buildings. A number of retrofit options are identified, and their potential to protect building occupants from a number of generic contaminant releases is evaluated using building airflow and contaminant transport modeling. In addition, a case study is presented in which specific retrofit options were considered for two actual buildings and pre-installation designs and cost estimates were developed. Based on the analyses performed, the results of the case study and other available information, guidance on the application and effectiveness of various retrofits are presented. An economic analysis software tool employing life cycle cost analysis techniques was developed as part of this project, and its use is described in an appendix to this report. The retrofit options considered fall into two categories, the first being stand-alone technologies or devices such as enhanced particulate filtration that are installed and implemented as purchased. The second category includes retrofit approaches that employ operational strategies or building modifications to increase building protection, such as outdoor air purging or building envelope tightening. The guidance section describes each retrofit technology and approach in some detail, presenting relevant performance data and the level of protection that might be expected from the retrofit. Potential disadvantages and knowledge gaps are also discussed for each technology. The retrofit technologies considered include enhanced particle filtration, sorbent based gaseous air cleaning, ultraviolet germicidal irradiation, photocatalytic oxidative air cleaning, and work area air capture and filtration equipment such as mail handling tables. The approaches include ventilation system recommissioning, building envelope tightening, building pressurization, relocation of outdoor air intakes, shelter-in-place (SIP), isolation of vulnerable spaces such as lobbies, system shutdown and purge cycles, and automated HVAC operational changes in response to contaminant sensing. The filtration and air cleaning options are noted to have an advantage of always being operational, which is an advantage as long as the systems are properly designed, installed and maintained. However, the lack of standard test methods for sorbent-based gaseous air cleaning and other air cleaning approaches is identified as a critical impediment to the application of these technologies. Building envelope air sealing and pressurization can be quite effective in protecting against outdoor releases as long as effective filtration against the contaminant of concern is also in place. The protection provided by operational changes such as system shutdown and purging are shown to be very dependent on the timing of their implementation, with the possibility of increasing occupant exposure if the timing is inappropriate. Isolating vulnerable zones and other system related modifications are highly dependent on the building layout and system design, and their implementation must be well conceived to be effective under the range of conditions that exist in buildings. Finally, many retrofits are noted as also providing additional benefits of increased energy efficiency and improved indoor air quality, which need to be included in the life-cycle cost comparison of different options to the degree possible.