The impact of plants on indoor air quality and the wellbeing of building occupants

Indoor plants are introduced into buildings to benefit the occupants’ wellbeing or comfort, through enhancing the visual design of the interior or improving the indoor air quality (IAQ). However, their room-scale impact on the IAQ and effect on people’s responses is poorly understood. Indoor humidity is an important aspect of IAQ. Plants can humidify the air through evapotranspiration (ET) but few studies have quantified this in office environments. Air temperature (Ta) and relative humidity (RH) were measured over multiple days and seasons within naturally-ventilated, unoccupied offices (28 m3), with and without 12 Ficus, or 6 Epipremnum plants. Through ET, and depending on the season, the plants released between 35 g and 65 g of moisture (plant-1 day-1), resulting in a small but significant increase in the offices’ indoor absolute humidity (AH). Most of the moisture emitted from the plants was removed through room air exchange and moisture sorption. In controlled chamber experiments, Ta and RH conditions significantly impacted the ET rate of five plant species, with different inherent transpiration rates. All ET rates were higher at 30% RH compared to 50/70% RH. Leafy Epipremnum had the highest ET rate under all conditions and would have the greatest humidification potential for indoor environments. Succulent Sansevieria had the lowest ET rate and would be ideal where a low moisture contribution is required. Whilst the office-scale impact of potted plants on IAQ/AH was small, the plants’ physical appearance had a significant impact on the responses of 520 participants, measured through a photo-questionnaire. Participants associated perceived wellbeing benefits with plant interest and beauty, and IAQ benefits with canopy density and healthiness of appearance. Participants perceived the plants would have a greater benefit for their wellbeing than for IAQ.