| PLANTS encourages fungal and mould growth, and contributes to the deterioration of building materials. Low RH can cause dryness of the eyes, skin and mucus membrane, enhance indoor ozone, increase the likelihood of inuenza transmission, and exacerbate problems of static electricity. Our review aims to improve the current understanding of which indoor pollutants and at what concentrations are harmful to health. A systematic review of the literature was conducted to determine the indoor pollutants measured in home environments, up to and including 2018. Logue et al compared indoor pollutant concentrations with relevant health guidelines produced by the Environmental Protection Agency (EPA) and California Ofce of Environmental Health Hazard Assessment (OEHHA) for 67 home environments between 1998 and 2010. They identied nine priority indoor pollutants (see Figure 1, which does not include butadiene) considered to be harmful. All were chosen on the basis of the measured concentration data exceeding health guidelines and the number of homes affected. Since 2010, an assessment of Logues priority pollutants and their mean concentrations in indoor environments has not been carried out. So we have used data from home environments after 2011 to determine if concentrations of these nine pollutants have changed since. Furthermore, we compare the post2011 results with up-to-date chronic health guidelines produced by the World Health Organization (WHO) and US EPA (Figure 1). Any pollutants with an average long-term concentration greater than the appropriate guideline will be designated a 2019 priority pollutant. The data collected in Figure 1 suggests that the mean concentrations of four indoor pollutants have increased in studies after 2010 namely, benzene, naphthalene, NO2 and PM2.5. Reductions in concentrations of acetaldehyde, acrolein, dichlorobenzene 1,4 and formaldehyde were measured, perhaps because of a large body of research focusing on lowering pollutant emissions from building materials. Acetaldehyde, benzene, formaldehyde, and NO2 are the indoor pollutants commonly measured at concentrations greater than the appropriate guidelines (Figure 1) causing long-term health issues and, thus, being classied as 2019s priority pollutants. Up to 10 Dracaena fragrans could be used in a green wall without raising ofce RH above 60% Chronic health guidelines (g m-3) 80.0 Arithmetic mean concentration (g m-3) INDOOR AIR QUALITY WHO IAQ guideline US EPA Rel 70.0 60.0 Logue et al (pre-2010) Post-2010 40.0 40.0 30.0 20.0 10.0 0 e e e ,4 xid yd -1 len 2.5 eh dio ne ha d t l e PM n h z a e n p g e rm Na tro ob Fo Ni lor ch i D Figure 1: Arithmetic mean concentrations of Logues priority pollutants, pre- and post-2010. Triangles and circles in the gure represent the appropriate chronic health guideline produced by the WHO and the US EPA. Butadiene has been omitted from the gure because no data was found post-2010 in home environments e yd eh d l a et Ac n lei ro Ac e en nz Be Indoor plants Forty studies have investigated numerous indoor plants for their ability to remove the 2019 priority pollutants benzene (> 45 species/cultivars) and formaldehyde (> 150 species/cultivars). The results from the most robust, well-cited and informative studies from these have been selected and are presented in Table 2. To the authors knowledge, no studies have investigated the potential of indoor plants to sequester either acetaldehyde or NO2 although the removal of NO2 by outdoor plants has been thoroughly studied, with promising results. Plants as a building service CO2 removal The main sources of CO2 indoors are human respiratory emissions and the outdoor air-supply rate. Several health guidelines exist for maximum safe CO2 concentrations, with the lowest eight-hour guideline being recommended by ASHRAE, at 1,000ppm. 12 June 2019 www.cibsejournal.com CIBSE Jun19 pp11-14 Green plants Supp.indd 12 24/05/2019 18:25