*2.3. Simple Ventilation Practices for Improving IAQ*

After obtaining the results of the one-year observation, four different systems were implemented in June 2018 during working hours (9:00–17:00) to discover the simplest and most efficient means of removing stale air from inside the room, and introducing fresh air from outside. The experimental condition was conducted in real practice where people in the office were working and doing activities as usual. The four systems tested were as follows: Case 1: the AC was turned on all day (normal case) from 1–7 June, Case 2: the AC was turned off during the lunch hour (12:00–13:00) from 8–14 June, Case 3: the AC was turned off for half a day (9:00–13:00) from 15–21 June, and Case 4: the AC was turned off all day (9:00–17:00) from 22–28 June. The statistical analysis was analyzed by one way ANOVA to consider among four cases at confidential level 95% (p < 0.05) by Statistic 8 Software (Version 8, USA).

For Case 3, it was decided to turn off the AC in the morning, because the high T in the afternoon [2] had a negative effect on work efficiency. Turning off the AC all day (Case 4) could not realistically be applied, since it would probably result in problems, such as heat strain. This system was included to establish the maximum rate of full-day ventilation with the AC turned off, the door constantly opened, and an electric fan mounted on the ceiling turned on.

An assessment of the envelope air permeability of the room was obtained by the infiltration rate, which was calculated by the following equation:

$$Q = -\frac{V}{t} \times \ln\left[\frac{\mathbf{C}\_t - \mathbf{C}\_{ext}}{\mathbf{C}\_0 - \mathbf{C}\_{ext}}\right]$$

where, *Q* is infiltration rate of air entering the room, *V* is volume of air in the office (m3), *t* is time interval (s), *Ct* is indoor concentration of CO2 at time *t* (ppm), *Cext* is concentration of CO2 in the ambient air (ppm), and *C0* is indoor CO2 concentration at time 0 (ppm) [29,30].

The volume of air in the office (V) was calculated from the size of the room (4 m x 10 m x 3 m), Interval (t) was 3600 seconds from hourly average data, *Cext* was average monitoring outdoor CO2 concentration at 430 ppm, *Ct* was the monitored CO2 concentration at time t (i.e., 18:00), and *C0* was the monitored CO2 concentration at one hour before *t* (i.e., 17:00). The frequency of measurement was every one minute, so the raw data were calculated to hourly data for both indoor and outdoor CO2 concentration. The indoor CO2 concentration was obtained by monitoring inside the office at 1 m height or nose level while people were sitting. The condition in the room was no plant and no ventilation for a long period. The door was opened when people came in and went out in a short time, not over one minute. The outdoor CO2 concentration was monitored at 1.5 m above ground level in the ambient air.

#### *2.4. Sansevieria Trifasciata for IAQ Improvement*

Another option tested for improving IAQ was locating the mother-in-law's tongue plants in the office to reduce the CO2 concentration in the ambient air through their photosynthesis. In this experiment, the *S. trifasciata* was put in a pot that contained soil. The plant was watered twice a week. The experiments were conducted by monitoring the air quality for six conditions, as follows: 0, 2, 3, 4, 5, and 6 Mother-in-law's tongue plants with three replicates for each case. The plants were placed on the floor near the desks where people worked, as shown in Figure 1. The number of plants was limited by the space of the rooms in which they were located. The IAQ was monitored from March to April 2019 for 24 hours each day to observe the amount of CO2 that the plants consumed for photosynthesis during the daytime and to establish the amount that they released through respiration at night. RH, T, CO, and CO2 were monitored by indoor air meter (FLUKE 975 AirMeter, USA) every minute. We monitored CO2 in a real situation to represent working activities or business as usual in tropical areas. Only the room temperature was controlled by air conditioner to comfort people at 25-degree Celsius, which was the general setting temperature in tropical countries.
