THE A DETAILED STUDY OF VOLATILE ORGANIC COMPOUNDS OF 5 AIR-CONDITIONED BUILDINGS IN SINGAPORE

Transcription

1 THE A DETAILED STUDY OF VOLATILE ORGANIC COMPOUNDS OF 5 AIR-CONDITIONED BUILDINGS IN SINGAPORE KWTham 1, SCSekhar 1, M Amanullah 2, HKLee 3, D Cheong 1 and NHWong 4 1 Department of Building, National University of Singapore, Singapore. 2 Department of Building, National University of Singapore, Singapore. 3 Department of Chemistry, National University of Singapore, Singapore. 4 Department of Building, National University of Singapore, Singapore. ABSTRACT A detailed study to identify and characterize the Volatile Organic Compounds (VOCs) in the indoor air of air-conditioned office buildings in tropical Singapore has been accomplished. This study reports a summary of the TVOC and formaldehyde levels for each building; the relative ranking based on frequency of occurrence and level of concentration; indoor-outdoor ratio of the ten most abundant VOCs and a summary of the ventilation parameters of all the buildings. Toluene and benzene, one suspected and the other a known carcinogen, have been found to be most frequent at over 90% of indoor locations. Sources of abundant VOCs are found to be mostly indoors. Air distribution systems in these buildings demonstrated a wellmixed flow pattern. INDEX TERMS Indoor air chemistry, Offices, Ventilation rates and strategies, VOCs, Measurement methods INTRODUCTION VOCs are major indoor contaminants of concern indicated by a lumped representative term Total Volatile Organic Compound (TVOC). VOC measurements were initiated in the mid 1980s mostly to determine the pattern of sensory perception with the presence of a single or a combination of VOCs. Berglund et al. (1986) reported 68 commonly detected VOCs based on their studies in Europe and the USA. World health organization (WHO, 1989) categorized organic chemicals based on the boiling point range. A major study of IAQ including VOCs was carried out by the US EPA. Air analyzed using EPA method TO-14 detected 50 compounds in indoors and forty-five at outdoors (Hawden et. al., 1997). Most of these compounds had indoor to outdoor ratio greater than one. Wargocki (1998) made a comprehensive review of studies and compiled a table of 10 highest VOCs based on frequency, concentration and odor index from a list of one hundred and thirty three different VOCs of 22 studies in 209 buildings. Based on this extensive literature review Wargocki presented a list of 25 ubiquitous VOCs, i.e. the most prevalent compounds and those having the highest mean weighted concentrations. Molhave et. al. (1997) proposed a new definition for calculating TVOC based on identification of as many compounds as possible (IAMCAP) or at least the ten most abundant compounds in a sample. They also suggested a prescription of which compounds to include in the TVOC calculation. In the present study, five buildings are monitored using established protocols for the Gas- Chromatography Mass Spectrometry method. TVOC is calculated in light of the discussion presented by Molhave et. al. (1997). This study is mainly motivated by lack of data of VOC Contact author bdgtkw@nus.edu.sg 561

2 profile for air-conditioned buildings in the tropical climate and importance of formulating effective strategies for mitigating VOC contamination in air-conditioned buildings in the tropical climate as they operate with very high re-circulation rates, typically in excess of 90%. Buildings are selected based on age, location and air distribution methods so as to provide a representative coverage of the office building stock in Singapore. In each building, a minimum of three floors at the top, middle and low levels were measured during the occupied hours when the air-conditioning systems were in operation. Wherever possible, the spatial distribution of the measurement locations in each floor is determined by the physical bounds associated with air-handling units serving the floor. This facilitates the parallel evaluation of the ventilation characteristics (through the tracer gas concentration decay method), process and equipment schedules, fabric and occupancy profiles within the floor, supporting a closed system analysis. The observed TVOC profile, values and indoor-outdoor ratio are discussed in the light of the characteristics of the tropical climate, the building operation and ventilation strategies adopted, location, fabric, equipment and processes as well as occupancy patterns. METHODS In soliciting continuous real-time chemical data such as TVOC, formaldehyde and obtaining ventilation parameters a set of multi point sampler and multi-gas monitor were used. A multi point sampler is used to draw air continuously from sampling locations via polytetrafluroethylene tubings. The multi-gas monitor identifies and quantifies the gas concentration based on the photo-acoustic infrared detection method. Air exchange rate (ACH), quantity of fresh air, age of air, air exchange effectiveness (AEE) and air exchange efficiency are determined using tracer-gas techniques using sulphurhexaflouride (SF 6 ) as the tracer-gas. United States Environment Protection Agency (USEPA) compendium method TO-17 was used as a general guideline for detailed characterization of VOCs. This method describes a sorbent tube-thermal desorption-gas chromatographic based monitoring system. Active sampling using a pump with dual flow adapter was used for collecting air samples in an adsorbent tube (tube style 2 defined by USEPA TO-17, TS2). TS2 consisted of appropriate adsorbent materials separated by glass wool, capable of capturing compounds ranging in volatility from n-c 3 to n-c 16. Safe sampling volume is 2 L at relative humidity 65% and temperature below 30 0 C. Distributed volume pair of 1L and 2L of air sample was passed through TS2 over a period of 8 hours. This translated in a flow rate of 2 ccm and 4 ccm through port 1 and port 2 respectively. An Automatic Thermal Desorber (ATD) was used to transfer sample to a GC-MS system in which the separation of the VOCs present in the sample was obtained. Phenyl Methyl Siloxane (30m x 0.25 µm) as the capillary column together with mass selective detector was used for the detailed characterization of VOCs in the sampling tubes. A schematic diagram of the entire process is shown in Figure 1. RESULTS AND DISCUSSIONS The range of HCHO levels in five building are presented in Table 1. It is observed that building N (with the exception of location NL5), U and M have levels within the guideline value of mg/m 3 (0.1 ppm). Building T is also within limits except level 6. The HCHO level at building H is consistently high, which would suggest the presence of internal sources. A summary of the VOC values measured by different instruments for all five buildings is reported in the same Table 1. It is noticed that the TVOC PAS levels are within the recommended value of mg/m 3 (3 ppm) (ENV, 1996). TVOC GCMS values are also found to be within the acceptable limit of mg/m 3 (3 ppm) except in Building H. Level 2 of Building H showed high TVOC GCMS values, as is the case with TVOC PAS, strongly indicating 562

3 presence of internal sources. It is important to note that there exists little correlation between TVOC PAS and TVOC GCMS values. However, it is observed that TVOC GCMS predicts a conservative estimate compared with TVOC PAS since 80% of the TVOC GCMS values are below its TVOC PAS counterpart as can be seen from Table 1. Ventilation parameters AIR PAS HCHO SF 6 TVOC PAS AIR Adapter P U M P TS2 TS2 A ATD T D G C M S D Definition of Molhave et. al. TVOC GCMS Figure 1. Schematic representation of measurements Table 1. Summary of Volatile organics in all buildings Location TVOC GCMS TVOC PAS HCHO mg/m 3 mg/m 3 STEL 1 (mg/m 3 ) NL NL NL TL TL TL TL UL UL UL HL HL ML ML ML Short Term Exposure Limit based on 8 hours averaging time (ENV, 1996). A list of most abundant compounds based on concentration and frequency is prepared by adding the concentration and frequency ranks with the lowest sum being most abundant. This list is compared with US review and European audit in Table 2. It is observed from the frequency-ranking list list that toluene and benzene are the most frequent compounds found 563

4 in buildings studied in this work. The presence of toluene in most places is in line with the findings of other studies such as European audit and US review (Bernhard et al., Holcomb and Seabrook, 1995). However, its presence towards the top of the list may be due to the urban nature of Singapore. Crude oil is a natural source of toluene and 92% of toluene produced are used as a component of gasoline. It has frequent use in paints and inks also. The presence of benzene at 93% locations is alarming since it s a known carcinogen and has a low TWA value of 0.96 mg/m 3 (0.1 ppm). However, individual level of maximum benzene ranged between mg/m 3 ( ppm) being within the acceptable limit. Source of benzene is primarily gassing vehicles and urban nature of Singapore may explain its widespread prevalence. Table 2. Comparison of ubiquitous indoor VOCs of Europe, North America and Singapore in order of ca. 15 most abundant in descending order Singapore 1 European audit 2 US review 3 1 o-xylene Acetone o-xylene 2 Toluene Isoprene Benzene 3 1,4-Dichlorobenzene 2-Methyl pentane Tetrachloroethylene 4 1,3,5-Trimethyl benzene Hexane m/p Xylene 5 Ethylbenzene 2-Methyl hexane/benzene Ethylbenzene 6 Styrene Heptane Trichloroethylene 7 1,1,1 Trichloroethane Toluene Toluene 8 m/p-xylene M/p Xylene 1,1,1 Trichloroethane 9 Benzene o-xylene Dichlorobenzenes 10 Cyclohexane Decane Styrene 11 Naphthalene Trimethyl benzene Undecane 12 Limonene Limonene Dodecane 13 Biphenyl Octane 14 1,2,3-Trimethyl benzene 15 m/p-ethyl toluene 1 Present work; 2 Bernhard et al.; 3 Holcomb and Seabrook, A summary of indoor-outdoor ratio for 10 abundant compounds is presented in Table 3. An entry of ID refers to negligible contribution of this chemical from the outdoors. It can be seen from this table that sources of VOCs in most levels are in indoor. It's only building U, where outdoors most likely contributes to indoor pollution. However, for all the buildings, benzene, 1,2,3-trimethylbenzene, toluene and ethylbenzene from outdoor contributed to indoor VOC levels as can be seen from the table indicated by low indoor-outdoor ratio values. 564

5 Table 3. Indoor-outdoor ratio of 10 abundant compounds 1,1,1 Trichloroethane Benzene Cyclo- Hexane Toluene Ethyl benzene m/p- Xylene Styrene o- Xylene 1,2,3- Trimethylbenzene 1,4-Dichlorobenzene NL2 ID ID NL3 ID ID 3.3 ID ID NL5 ID ID 49.6 ID ID 16.4 ID ID TL3 ID 1.8 ID TL4 ID 2.3 ID ID 8.9 TL ID ID TL7 ID 3.8 ID UL ID UL14 ID ID UL ID HL HL3 ID ID ID ID ID ID ID ML10 ID ID ID ID ID ML14 ID 9.7 ID ID ID ID ID ID ID ID ML ID ID ID 0.4 ID The ventilation characteristics, such as the ACH, fresh air provision and the air exchange effectiveness parameters (AEE) are presented in Table 4. It is seen that the ACH ranges quite substantially amongst the five buildings investigated. It is also observed that the fresh air provision on the basis of design occupancy is generally acceptable in most buildings. However, actual fresh air received at these buildings is below the ENV guideline (ENV, 1996) in many locations indicating overcrowding. The AEE parameters observed in these buildings are indicative of a well-mixed flow pattern, which is typical of conventional designs of air distribution systems. Table 4. Summary of ventilation parameters in all the buildings Building ACH Fresh Air (lps/person) Fresh Air (lps/person) AEE Design Occupancy * G Actual Occupancy 1 AEE L AEE OL N T U H M * Based on the value of 10 m 2 /person AEE G, AEE L and AEE OL = 1 and η ACG = 50% imply perfectly mixed flow pattern and thus could be considered good from dilution perspective. Such flow patterns are normal design practices in office air-conditioning systems. Values of AEE G, AEE L and AEE OL > 1 and η ACG > 50% are even better as they indicate increased dilution. AEE significantly <1 is indicative of short-circuiting. 565

6 CONCLUSION AND IMPLICATIONS Detailed characterisation of VOCs in indoor air in office buildings using the Gas Chromatography - Mass Spectrometry (GCMS) method has been accomplished. This provided a blueprint of VOCs present in indoor environment of Singapore. A list of most frequent and strong VOCs is reported in Singapore context. These abundant compounds are found to be within their individual acceptable limits. However, an eye should be kept on benzene since its observed concentration levels are near the allowable limit. Benzene is a known carcinogen and it has been established in this study that sources of this compound are both indoor and outdoor relating to the urban nature of Singapore. Sources of abundant VOCs are however, mostly indoors. Although fresh air provision on the basis of design occupancy is generally acceptable in most buildings, actual fresh air received at these buildings is below the local guideline in many locations indicating over crowding. Air distribution systems in these buildings demonstrated a well-mixed flow pattern. ACKNOWLEDGEMENTS This study has been carried out under a National University of Singapore sponsored research project number R REFERENCES Berglund, B., Berglund, U., Lindvall, T. (1986) Assessment of discomfort and irritation from the indoor air, In: Proceedings of IAQ 86 Managing Indoor Air for Health and Energy Conservation, Atlanta, USA, ASHRAE, Molhave., L., Clausen., G., Berglund., B., Ceaurriz., J.D., Kettrup., A. Lindvall., T. Maroni., M., Pickering., A.C., Risse., U., Rothweiler., H., Seifert., B. and Younes., M Total Volatile Organic Compounds (TVOC) in Indoor Air Quality investigations. Indoor Air, Vol. 7. pp Wargocki, P Human Perception, Productivity and Symptoms Related to Indoor Air Quality. Ph.D. Thesis. Technical University of Denmark. WHO, World Health Organization (1989) Indoor Air Quality: Organic Pollutants, Report on a WHO Meeting Berlin(West), Euro Report and Studies number 11, WHO regional office, Copenhagen, Denmark. ENV, Guidelines for good Indoor Air Quality in Office Premises, Ministry of Environment, Singapore. Hawden, G.E., J.F. McCarthy, S.E. Womble, J.R. Girman and H.S. Brightman, Volatile Organic Compound Concentrations in 41 Office Buildings in the Continental United States, Proceedings of Healthy Buildings/IAQ >97 International Conference, Washington D.C. 566