Bringslimark, Hartig, and Pati conducted a literature review of empirical studies investigating the psychological benefits of indoor plants, all of which adopted an experimental design and were written in English. However, the researchers did not follow the rigorous Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) protocol. Additionally, the reviewed data were obtained at least a decade ago, and thus require an update. Therefore, the present article reports a systemic review of the quantitative empirical research (in English and Chinese) exploring the effects of indoor plants on subjective psychological perceptions. The information sources were mainly Web of Science (1900–) and WANFANG DATA (1980–), in which only empirical studies using quantitative methods were selected. The eligibility criteria were studies (1) involving any participants, (2) regarding indoor plants (real or surrogates) as an intervention, excluding those equipped with a power-driven biofilter, (3) involving comparators to facilitate within-subject and between-subject comparisons, (4) presenting psychological perceptions as outcome measures, and (5) of all study designs. The two authors collected 45 journal papers (with 50 studies in total). The (1) authors and publication years, (2) participants’ nationalities, sex, and age, (3) number of interventions, (4) duration of plant exposure, (5) participant–plant distance, (6) experiment room size, (7) room climate (i.e., lighting, temperature, and humidity), (8) study designs (i.e., experimental, field experimental, quasi-experimental, field quasi-experimental, or survey designs), and (9) self-reported perceptions (i.e., emotion, cognition, health, restoration, thermal comfort, productivity, and satisfaction) were identified and reviewed. These 50 studies were also analyzed for their quality. Most had moderate research quality. The synthesized results of the reviewed studies showed that the most noticeable effects of indoor plants on participants were increasing positive emotions and reducing negative feelings, followed by reducing physical discomfort. Because members of the general public today spend most of their time indoors and are faced with increasingly severe problems associated with physical–mental health and wellbeing, the application of indoor plants to improving people’s physical–mental health is worth exploring.

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The influences of indoor plants on people have been examined by only three systematic reviews and no meta-analyses. The objective of this study was therefore to investigate the effects of indoor plants on individuals’ physiological, cognitive, health-related, and behavioral functions by conducting a systematic review with meta-analyses to fill the research gap. The eligibility criteria of this study were (1) any type of participants, (2) any type of indoor plants, (3) comparators without any plants or with other elements, (4) any type of objective human function outcomes, (5) any type of study design, and (6) publications in either English or Chinese. Records were extracted from the Web of Science (1990–), Scopus (1970–), WANFANG DATA (1980–), and Taiwan Periodical Literature (1970–). Therefore, at least two databases were searched in English and in Chinese—two of the most common languages in the world. The last search date of all four databases was on 18 February 2021. We used a quality appraisal system to evaluate the included records. A total of 42 records was included for the systematic review, which concluded that indoor plants affect participants’ functions positively, particularly those of relaxed physiology and enhanced cognition. Separate meta-analyses were then conducted for the effects of the absence or presence of indoor plants on human functions. The meta-analyses comprised only 16 records. The evidence synthesis showed that indoor plants can significantly benefit participants’ diastolic blood pressure (−2.526, 95% CI −4.142, −0.909) and academic achievement (0.534, 95% CI 0.167, 0.901), whereas indoor plants also affected participants’ electroencephalography (EEG) α and β waves, attention, and response time, though not significantly. The major limitations of this study were that we did not include the grey literature and used only two or three records for the meta-analysis of each function. In brief, to achieve the healthy city for people’s health and effective functioning, not only are green spaces needed in cities, but also plants are needed in buildings.