In some embodiments, there is provided a scrubber system for cleaning return air in an HVAC unit, where the scrubber system attaches directly to an inlet of the return-air side of the HVAC unit, for example, by the mating of a flange on the system with a matching flange on the HVAC unit. The bolt-on scrubber system may comprise one or more sorbent materials, a fan for circulating return air through the sorbent, a damper-controlled inlet and a damper-controlled outlet to the attached return air side of the HVAC unit. Further, an additional air flow channel and a damper may be included in the system to control the flow of outside air into the HVAC unit. In some embodiments, the sorbents may be contained in removable inserts.