Examination of Environmental Factors Influencing the Emission Rates of Semivolatile Organic Compounds

Some types of semivolatile organic compounds (SVOCs) that are emitted from plastics used in building materials and household appliances have been associated with health risks, even at low concentrations. It has been reported that di-2-ethylhexyl phthalate (DEHP)—one of the most commonly used plasticizers—causes asthma and allergic symptoms in children at home.,e amount of emitted DEHP, which is classified as a SVOC, can be measured using a microchamber by the thermal desorption test chamber method. To accurately measure the SVOC emission rates, the relation between SVOC and environmental factors should be clarified. Herein, we examined the effects of the temperature, relative humidity, concentration of airborne particles, and flow field in the microchamber on SVOC emission rates. ,e flow fields inside the microchamber were analyzed via computational fluid dynamics (CFD). ,e emission rate of SVOC released from PVC flooring increased under high temperatures and at high concentrations of airborne particles but did not depend on the relative humidity. From an evaluation performed using an index of air change efficiency, such as the air age and the coefficient of air change performance, we found that a fixed air exchange rate of 1.5 h in the microchamber is desirable.

e thermal desorption test chamber (TDC) method was developed in Japan. is technique can be employed to determine the emission rates of SVOCs under room-temperature conditions using a microchamber [16][17][18][19].Additionally, a TDCbased microchamber method approach was standardized as JIS A 1904 as a method for determining the emission of SVOCs from building materials [20].
We verified the accuracy of the measurements using a microchamber and the TDC method as well as the reproducibility of the microchamber method.ey determined the emission rate of SVOCs from various types of materials and investigated the recovery ratios of the microchamber using reference materials [18,21].Next, they selected a number of factors, including background concentration, loading factor, specimen orientation, time, type of gas supplied, and process effects on the inner surface of the microchamber, which were expected to affect the emission behavior of SVOCs with the microchamber method and reported the results [21,22].Meanwhile, the temperature, relative humidity, air exchange rate, and other factors have been considered to affect the emission rate of chemicals using JIS A 1901 and other general test chamber methods [23].Although it may be predicted that the aforementioned factors can also influence the measurement of SVOC emission rates using the microchamber method, there are a few investigated cases.Moreover, SVOCs have an extremely low volatility and are easily adsorbed to the surfaces of solids.To investigate the factors that influence the emission rate of SVOCs, it is essential to investigate the correlation between the SVOC emission rate and airborne particles in the room.Herein, we study the dependence of the emission rate of di-2-ethylhexyl phthalate (DEHP)-an SVOC-on the temperature, relative humidity, and airborne particles.Additionally, we investigated the in uence of varying the air exchange rate properties of the microchamber on its ow eld using computational uid dynamics (CFD).

Method
2.1.Microchamber System.Figure 1 shows a schematic of the microchamber, which is made of glass and has been treated with silane.
e microchamber is cylindrical (82 mm (diameter) × 120 mm (height)).Air enters the microchamber through an inlet (inner diameter 3 mm) at the lower side of the cylinder (13 mm from the bottom).Directly opposite to this inlet, on the upper side of the cylinder (96 mm from the bottom), there is a collection outlet (inner diameter 3 mm).On top of the microchamber, there is a 15 mm rim on which a glass lid (diameter 11.5 mm) is placed.A clamp is used to hold a seal and a ooring material in place while keeping the interior of the microchamber airtight.e emission area of the building material is 5.3 × 10 3 mm 2 .
Figure 2 shows a schematic of the thermal desorption test chamber system.SVOCs are easily adsorbed by the microchamber walls under room-temperature conditions.e thermal desorption test chamber method measures emissions based on the adsorbed constituents.After collecting the gas emissions from the specimen under room-temperature conditions, the sample is removed from the microchamber and heated at a high temperature.
e SVOC constituents adsorbed to the interior of the microchamber are collected, and the emission rate is determined by summing the two [16].

Recovery of the Microchamber.
Assuming that the SVOCs emitted from the material will adsorb to the microchamber walls, the recovery rate was calculated using a reference standard SVOC substance.A predetermined amount of the reference SVOC material was directly added to Tenax TA.In total, 2 μL of the reference SVOC solution was injected into the microchamber using a syringe.en, thermal desorption was performed immediately.From the amount of SVOC collected, it was con rmed that the recovery rate was at least 90%.Tables 1 and 2 list the measurement conditions of the thermal desorption test chamber system and the analytical conditions of the gas chromatography/ mass spectrometry thermal desorption system (GC/MS-TDS), respectively.

Measurements. Table 3 lists the test cases. In case 1, PVC
ooring was used as the specimen (this ooring has been reported to emit DEHP, which is an SVOC).e ooring specimen was placed inside a desiccator and cured.After one day of curing, it was removed.
e air supplied to the desiccator had a temperature of 28 °C ± 1 °C and a relative humidity (RH) of 50% ± 5%.PVC ooring was placed inside   2 Advances in Civil Engineering the microchamber, and the test was conducted under the following conditions: temperature 28 °C, RH 50%, and air exchange rate 0.5 h −1 .e sample was a 115 mm diameter circle of ooring cut from the center of a roll of ooring (as shown in Figure 1, the emission area had a diameter of 82 mm due to the seal).
To ensure the reproducibility of the volume of chemicals emitted from the ooring specimen, the test performed in case 1 was repeated three times under the same conditions.e temperature inside the chamber was set to 28 °C and 40 °C, and the dependence of the SVOC emission rate on the temperature was investigated.
e temperature in the high-temperature condition was set to 40 °C considering the rise in the surface temperature of building materials placed in a location with good exposure to sunlight and the rise in the temperature of the surface of consumer electronics when running. e RH was set to 40, 50, and 70%, and the in uence of the changes in the RH on the SVOC emission rate was con rmed.e RH was set to 40% and 70% because Japan's Act on Maintenance of Sanitation in Buildings sets the environmental standard for RH in o ces between 40% and 70%.Additionally, quartz wool was placed inside the microchamber, and the authors investigated the in uence of the changes in the adsorption area due to the presence or absence of airborne particles.
is wool was heated and cleaned before the experiment.As with the measurement of the recovery rate in the microchamber, we calculated the DEHP collection rate in the microchamber for cleaned quartz wool and con rmed a recovery rate of at least 90%.To con rm that the SVOC emission rate was in a steady state, the emissions were sampled ve times in case 1 (0.5, 3, 12, 24, and 48 h after the experiment began) and three times in case 2 (0.5, 3, and 24 h after the experiment began).In the other cases, the emissions were sampled 0.5 and 24 h after the experiment began, and the rate of SVOC emissions from the ooring specimen over time was investigated.

Numerical Method.
Figure 3 shows a schematic of the analytical mesh.Tables 4 and 5 list the CFD analysis cases and conditions, respectively.e inlet boundary conditions were set as follows: air exchange rate 1.5 h −1 (U in 2.1 ×10 −2 m/s), 0.8 h −1 (U in 3.9 × 10 −2 m/s), and 4.0 h −1 (U in 5.2 × 10 −2 m/s).Additionally, the air age and the coe cient of the air change performance η in the microchamber were investigated [23].e air age is de ned as the length of time t during which a speci c amount of outdoor air has been in a building, zone, or space, and the coe cient of the air change performance is de ned as an air distribution system's ability to deliver ventilation air to a building, zone, or space [24].A three-dimensional analysis was performed based on a ow eld of the low-Reynolds (Re)-type k-ε model (Abe-Nagano model) [25].Given the symmetry of the interior of the microchamber, only half of the area was subjected to analysis.
After analyzing the ow eld, the surface gas-phase concentration of DEHP was set in the specimen position and the di usion eld was analyzed.Table 5 lists the DEHP surface gas-phase concentration (C 0 ) and air di usion coe cient (D a ) values [26,27].When determining the amount of DEHP emitted from the ooring specimen, it should be set as an internal di usion-controlled building material; however, for computational convenience, this study's model is set as an evaporation-controlled building material.e concentration of the air owing into the microchamber via  the air inlet was set to zero, and the constant concentration was calculated under isothermal conditions (28 °C).

Results
Table 6 and Figures 4-6 show the measurements of the rate at which DEHP was emitted from the flooring specimen due to influencing factors, including temperature, RH, and airborne particles.

Relation between the DEHP Emission Rate and Temperature.
In case 1, 0.5 h after the initiation of the experiment, the rate at which DEHP was emitted from the flooring specimen was 49.8 μg/m 2 h.However, over time, the DEHP emission rate decreased.Approximately 10 h after the initiation of the experiment, the DEHP emission rate reached a steady state.At this point, the DEHP emission rate was 13.3 μg/m 2 h (Table 6 and Figure 4).In case 2, the temperature was set to 40 °C considering the rise in the temperature of the surface of the flooring specimen because of the exposure to sunlight or the like.In this case, 24 h after the initiation of the experiment, the rate at which DEHP was emitted from the flooring specimen was 42.2 μg/m 2 h.As shown in Figure 4, we confirmed that the emission rate of DEHP was dependent on the temperature.e DEHP emission rate determined 24 h after the initiation of the experiment is an average value over three measurements; the relative standard deviation (RSD) is 4%.

Relation between DEHP Emission Rate and RH.
In case 3, the RH was set to 40%.0.5 and 24 h after the start of the experiment, the rates of DEHP emissions from the flooring specimen were 37.7 and 10.2 μg/m 2 h, respectively.us, the DEHP emission rates obtained in this case were not considerably different from those obtained in case 1 wherein the RH was set to 50%.Furthermore, in case 4, the relative humidity was set to 70%, and 24 h after the initiation of the experiment, the rate at which DEHP was emitted from the flooring specimen was 9.2 μg/m 2 •h.Katsumata et al. reported that there were almost no differences in the DEHP emission rates, regardless of whether the air supplied to the microchamber was humidified or dry [16].
e present   Advances in Civil Engineering experiment also con rms that a certain amount of time after the initiation of the experiment, the DEHP emission rate was not considerably in uenced by the RH in the range 40%-70% (Figure 5).

Relation between DEHP Emission Rate and Airborne
Particles.In case 5, quartz wool (170 mg) was placed in the microchamber as airborne particles.In case 6,340 mg of quartz wool was used.Table 6 and Figure 6 show the experimental results.In case 5, when 170 mg of quartz wool was placed in the microchamber, 2,733.0ng of DEHP was collected and the DEHP emission rate was 20.6 μg/m 2 h.us, the collected volume and emission rate were considerably greater than those in case 1 wherein no quartz wool was used (amount of DEHP collected 1,686.4ng; DEHP emission rate 20.6 μg/m 2 h).In case 6 wherein 340 mg of quartz wool was used (amount of DEHP collected 3,723.0ng; DEHP emission rate 29.1 μg/m 2 h), the DEHP collection volume and emission rate were higher than those in case 5. is indicates that increasing the volume of airborne particles increases the area available for DEHP adsorption.It is thus believed that the SVOC emission rates are considerably affected by the adsorption surface area (the surface area of the interior of the microchamber and the surface area of the airborne particles).

Prediction Results for the Flow Field within the Microchamber.
Table 7 and Figure 7 show the results of a CFD analysis of the air ow distribution, average air age, coe cient of the air change performance, and other factors for each case.In case 7, the inlet speed of the microchamber supply was set to 2.1 ×10 −2 m/s and the air exchange rate was set to 0.8 h −1 .In this case, the average air ow in the microchamber was 4.7 × 10 −5 m/s and the air ow near the specimen was 3.5 × 10 −5 m/s.In cases 8 and 9, the air exchange rate was set to 1.5 h −1 .In the experiments performed in these cases, the increase in the air exchange rate exhibited an increase in the average air ow in the microchamber (Table 7).In cases 7 (air exchange rate 0.8 h −1 ) and 8 (air exchange rate 1.5 h −1 ), the air ows straight up from the center of the microchamber, whereas in case 9 (air exchange rate 4.0 h −1 ), there is a large S-shaped air ow vector distribution.In cases 7 and 8, there is almost no circulating ow in the microchamber area as a whole.In contrast, in case 9, the high air supply rate causes the ow to collide with the front of the air inlet.
Figures 7(a)-7(c) show the air age [23] in the microchamber.ese values are the average of local air ages at all points in the microchamber.Here, the local average air age is the amount of time required by air to move from the air inlet to an arbitrary point inside the microchamber.

Advances in Civil Engineering
In cases 7-9, the average air age values in the microchamber were 0.79, 0.87, and 1.78, respectively.us, as the air exchange rate increased, the average air age tended to increase (Table 7).In particular, the air age values near the top of the microchamber (where the specimen was placed) were 0.81 in case 7, 0.94 in case 8, and 1.96 in case 9. us, fresh air required the maximum amount of time to travel from the microchamber's inlet to the top of the microchamber in case 9 wherein the air exchange rate was 4.0 h −1 . is may be because the high air supply rate in case 9 caused a circulating ow in the microchamber, as described above.
e coe cient of the air change performance corresponds to the air change performance in the microchamber.For example, if the air in the microchamber is in a completely mixed state, the coe cient of the air change performance will be 1 [23].
In case 7, the coe cient of the air change performance was 1.6.As shown in Figure 7(a), almost no stagnant areas or circulating ows were formed in the microchamber in this case.In case 8, the coe cient of the air change performance was 0.8.In this case, there was a slight stagnant area in the microchamber; however, the coe cient of the air change performance was close to 0.9, which is the stipulated coe cient of the air change performance for a small chamber method in JIS A 1901 [23].In case 9, the coe cient of the air change performance was 0.1.us, despite the high air exchange rate, there apparently was a high formation of stagnant areas and circulating ows in the microchamber (Figure 7(c)).

Discussion
As shown in Figures 4-6, the rate at which DEHP was emitted from the ooring specimen was high at the beginning of the experiments (0.5 h after the initiation of the experiment); however it gradually declined over time.24 h after the initiation of the experiment, the emission rate stabilized.When experiments are conducted to measure SVOC emission rates using a microchamber, it is preferable to calculate the rates in the steady state that is reached 24 h after the initiation of the experiment.
Figure 8 shows a comparison of DEHP emission rates in the presence of factors in uencing this rate.It was con rmed that the rate at which DEHP is emitted from the ooring specimen considerably depends on the temperature.An increase in the temperature excites the thermal motion of the

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Advances in Civil Engineering molecular chains, which has a signi cant impact on the emission of chemicals.e DEHP and resin chains contained in the ooring specimen are bound via intermolecular forces.e increases in temperature may gradually free the DEHP molecules from the resin, thereby increasing the DEHP emission rate.In contrast, there were no signi cant changes in the DEHP emission rates when the RH was changed. is may be because DEHP, which is hydrophobic, is not considerably in uenced by humidity.
It was found that the increase in the adsorption area in the presence of airborne particles, that is, the increase in the surface area available for DEHP adsorption, is directly correlated to the increase in the DEHP emission rate.However, the increase in the DEHP emission rate was not directly proportional to the increase in the airborne particle volume because of the relation between the location of the emission source in the microchamber and the ow eld. Figure 9 shows the results of an analysis performed under the conditions same as those in case 1 but with the surface concentration of the ooring specimen set to 298.3 μg/m 3 .In this experiment, the DEHP emissions from the specimen were di used; however, a concentration boundary layer was formed at the top of the microchamber wall.Additionally, the DEHP emissions were considerably in uenced by the ow eld (Figure 7(b)), possibly because DEHP is only adsorbed to a considerably small part of the microchamber walls at the top of the chamber (Figure 9).Although the extreme conditions must be considered (e.g., the walls' boundary conditions having unlimited adsorption capacity), DEHP was preferentially adsorbed to a part of the microchamber's interior surface that was near the emission source.Additionally, in cases wherein quartz wool was used as airborne particles, DEHP only adsorbed to a portion at the top of the quartz wool (in cases involving the use of 170 and 340 mg of quartz wool).
When the air exchange rate in the microchamber increased, the water vapor-equivalent mass transfer coe cient of the specimen surface rose.Despite this fact, stagnant areas and circulating ows also formed inside the microchamber, resulting in a far worse air change e ciency.Conversely, when a low air exchange rate was set, the air change eciency in the microchamber was excellent; however, the mass transfer coe cient decreased to less than 1 m/h.e experimental ndings suggest that supplying ventilation air (or helium) with an air exchange rate of ∼1.5 h −1 is appropriate for the microchamber method.

Conclusions
Herein, we investigated the e ects of temperature, RH, airborne particles, and air change properties on the emission rate of DEHP-an SVOC-using the microchamber method.Our results are summarized as follows: (1) is research con rmed that the DEHP emission rate considerably depends on the temperature set for the experiment.When the microchamber method is utilized as the standard testing method, it is essential to set the temperature to a constant level to ensure that the performance of the experimental sample is evaluated uniformly.(2) Although the DEHP emission rate was not shown to depend on humidity in this research, there is a need to investigate DEHP emission rates in low-humidity environments below 40% RH. (3) Depending on the volume of airborne particles, we con rmed that placing quartz wool in the microchamber as airborne particles caused a considerable change in the DEHP emission rate.that nearly all SVOCs emitted by the specimen were only adsorbed to the inner walls at the top of the microchamber.(6) e changes in the DEHP emission rates in lowtemperature and low-humidity environments must be investigated.In the future, we plan to investigate the relation between emission rate and the factors influencing this rate for other types of SVOCs.We also plan to conduct experiments adapted to CFD analysis and verify the degree of adaptability to this type of analysis.

Figure 2 :
Figure 2: Schematic of the thermal desorption test chamber system.

Figure 3 :
Figure 3: Schematic of the analytical mesh: (a) vertical cross section (symmetrical) and (b) horizontal cross section.

Figure 4 :Figure 5 :Figure 6 :
Figure 4: A plot of emission rate versus time showing the emission of di-2-ethylhexyl phthalate (DEHP) from the ooring specimen with regard to the temperature.

2 ×Figure 7 :
Figure 7: Distribution of air ow vectors and air ages: (a) air change rate of 0.8 times/h, (b) air change rate of 1.5 times/h, and (c) air change rate of 4.0 times/h.

( 4 )Figure 8 :Figure 9 :
Figure 8: Comparison of DEHP emission rates in the presence of factors in uencing this rate.

Table 1 :
Measurement conditions of the thermal desorption test

Table 3 :
A list of test cases (air exchange rate 1.5 h −1 ).