Previous studies performed by the National Aeronautics Space Administration (NASA) indicated that plants and associated soil microorganisms may be used to reduce indoor pollutant levels. This study investigated the ability of plants to improve indoor air quality in schools. A 9-wk intensive monitoring campaign of indoor and outdoor air pollution was carried out in 2011 in a primary school of Aveiro, Portugal. Measurements included temperature, carbon dioxide (CO₂), carbon monoxide (CO), concentrations of volatile organic compounds (VOC), carbonyls, and particulate matter (PM₁₀) without and with plants in a classroom. PM₁₀ samples were analyzed for the water-soluble inorganic ions, as well for carbonaceous fractions. After 6 potted plants were hung from the ceiling, the mean CO₂ concentration decreased from 2004 to 1121 ppm. The total VOC average concentrations in the indoor air during periods of occupancy without and with the presence of potted plants were, respectively, 933 and 249 μg/m³. The daily PM₁₀ levels in the classroom during the occupancy periods were always higher than those outdoors. The presence of potted plants likely favored a decrease of approximately 30% in PM₁₀ concentrations. Our findings corroborate the results of NASA studies suggesting that plants might improve indoor air and make interior breathing spaces healthier.