Residents and workers exposure to high carbon dioxide (CO2) levels in buildings may cause headache, dyspnea, fatigue, or drowsiness. However, the effect of plants on in-building CO2 reduction and adverse effect relief is largely unknown. We recruited 36 healthy participants from an office room with plants and 32 healthy participants from another office room without plant in the same office building in Taipei. The participants in the office room with plants during 2020 would move to the office room without plant in 2021. The twelve repeated measurements per year of CO2, fine particles (PM2.5), total volatile organic compounds (TVOCs), blood pressure (BP), serum CO2 (TCO2), and four rating questions of headache, dyspnea, fatigue, and drowsiness were obtained for each participant. The statistical results showed that levels of drowsiness and systolic BP were significantly lower among participants in the office room with plants compared to those in the office room without plants by t -test and paired t -test. The associations between increased indoor CO2 and increased serum CO2 were observed in the office room with plants (1.32%) and without plant (4.52%) by mixed-effects models. Also, the associations between indoor CO2 and drowsiness were observed in office rooms (with plants: 14.57%; without plant: 3.82%). The conclusion of the present study is that plants in office environment can reduce CO2 levels and may lower CO2-related health effects.