IAQ benchmarks of air-conditioned offices in Hong Kong

Transcription

1 Indoor Air 2008, 7-22 August 2008, Copenhagen, Denmark - Paper ID: 20 IAQ benchmarks of air-conditioned offices in Hong Kong K.W. Mui *, L.T. Wong ** and P.S. Hui Department of Building Services Engineering, The Hong Kong Polytechnic University, Hong Kong, China Corresponding * behorace@polyu.edu.hk, ** beltw@polyu.edu.hk SUMMARY This study investigates the feasibility of a simple 5-star IAQ benchmark derived from the exposure concentrations of three representative common indoor air pollutants in typical airconditioned offices of Hong Kong, i.e. carbon dioxide (CO 2 ), respirable suspended particulates (RSP) and total volatile organic compounds (TVOC). It assigns star to the top 0% samples in the highest levels, 2 stars to the next 22.5%, 3 stars to the next 35%, 4 stars to the next 22.5% and 5 stars to the bottom 0% of the three air pollutants. The application of this IAQ benchmark was tested in 422 air-conditioned offices of Hong Kong, and a follow-up measurements of six more common air pollutants, including carbon monoxide, nitrogen dioxide, ozone, formaldehyde, radon and airborne bacteria count were performed to investigate the accuracy of 5-star IAQ benchmark. This simple benchmark would promote better IAQ and serve as a screening assessment for IAQ. KEYWORDS Indoor air quality (IAQ), Benchmarking, Air-conditioned offices, Star rating INTRODUCTION It is not economically to monitor all air pollutants over a general community for any indoor air quality (IAQ) programme, a few number of assessment parameters would be selected based on the severity of their adverse effects on human health and the simplicity in promoting the IAQ to general public. Various environmental parameters and surrogate indicators were suggested and adopted for IAQ assessment in workplaces (Jokl, 2000; Kraenzmer, 999; Lee et al., 2002; Montoya et al., 2007; Tchepel et al., 2007). In Hong Kong, the office IAQ would be certified by an IAQ Certification Scheme (HKEPD, 2003) by a full IAQ assessment assessing the 8-hour exposure limits of nine common indoor air pollutants, i.e. carbon dioxide (CO 2 ), carbon monoxide (CO), respirable suspended particulates (RSP), nitrogen dioxide (NO 2 ), ozone (O 3 ), formaldehyde (HCHO), total volatile organic compounds (TVOC), radon (Rn) and airborne bacteria count (ABC). The full IAQ assessment results are used in labeling offices with Good or Excellent IAQ. A considerable amount of resources and manpower are involved in the full IAQ assessments, however, only a small number of offices (<%) have been certified since the launch of the scheme in Apart from a number of technical difficulties in implementation such as the choice of representative parameters, sampling time period, sampling point density and influence of professional judgment on measurement uncertainty (Wong et al., 2006; Mui et al., 2006; Hui et al., 2006, 2007), the interpretation of the assessment results posed a barrier of understanding to the general public for promoting the IAQ care of the indoor environment. On top of assessing the workplace IAQ acceptance, a simple and readily understandable IAQ benchmark of the relative performance of an office environment would offer incentive of promoting office IAQ. This study proposes an IAQ benchmark presented by -star to 5-star

2 ratings by assessing the exposure concentrations of some common indoor air pollutants in Hong Kong offices (Wong et al., 2007). In order to illustrate the performance office IAQ, benchmarked offices of -star and 5-star ratings are compared with the full IAQ assessment. This proposed benchmark could be useful in promoting IAQ with quantitative assessment criterion for air-conditioned offices in Hong Kong or similar environment elsewhere. IAQ BENCHMARKS A five stars rating was selected in this IAQ benchmark study because the star ratings have been widely adopted in daily applications, such as financial analysis and hotel rating and the system is familiar to the public. Follow the criteria proposed for performance assessed by a continuous benchmarking parameter by Blume (998), the benchmarking system assigns star to the top 0% samples of IAQ benchmarking value (B i 0.9), 2 stars to the next 22.5% (0.675 BBi<0.9), 3 stars to the next 35% (0.325 B ib <0.675), 4 stars to the next 22.5% (0. BBi<0.325) and 5 stars to the bottom 0% (B i <0.), where B j is the integral of a benchmarking parameter, known as the IAQ index θ i of the office sample i from all offices in Hong Kong, θ is the distribution of the benchmarking parameters for all offices over a region with estimators of mean μ θ and standard deviation σ θ. B θ i = θ dθ i ; θ θ ( μ, σ θ θ ) () It is noted that among all spaces in the region, an indoor environment with B i < 0. indicates that it has the best IAQ, i.e., this environment has the lowest exposure concentrations of the air pollutants contributed to the index among the database of the index. On the other hand, an environment with B i 0.9 indicates the worst IAQ. It was noted that, among the nine common air pollutants in air-conditioned offices of Hong Kong, the three representative indoor air pollutants selected for the overall IAQ acceptance were CO 2, RSP and TVOC (HKEPD, 2003; Wong et al., 2006, 2007). It is noted that for a typical office environment, RSP would be transported from the outdoors and is closely related to the filter efficiency of an air-conditioning system. A study showed that the RSP level also depends on the indoor activities (Lee et al., 2002). Therefore, the inclusion of RSP in IAQ assessments could address issues related to system filtration performance and activities in an office. Indoor CO 2 is generated by occupants and diluted by outdoor air and thus a good indicator for the ventilation rate and occupant load in the space (ASHRAE, 2004). Finally, the inclusion of TVOC can indicate those indoor pollutant emissions dominated by the building materials, finishing, and human activities, e.g. building renovation works (Hoskins, 2003). * An IAQ index θ in equation below determined from the fractional dose j of these representative air pollutants j= 3, i.e. CO2, RSP and TVOC, to the exposure limits of IAQ acceptance j,e in assessing the IAQ satisfactory level for indoor environment was proposed as the benchmarking parameters (Wong et al., 2006, 2007; HKEPD, 2003) where, Фj is the assessed average representative pollutant level over the exposure time period. θ = 3 3 j= * j * j ; j = (2) j,e

3 IAQ OF AIR-CONDITIONED OFFICES IN HONG KONG The IAQ assessment results in a regional cross-sectional study of the common indoor air pollutants of 422 randomly picked, independent Hong Kong offices (Hui et al., 2006; Wong et al., 2006, 2007) were used as a reference of the IAQ indices. It was noted that it was one of the extensive studies with the largest sample size in this region and was assumed to be representative for the overall picture of common air pollutant levels in offices of Hong Kong. Table shows a summary of the measurement results of 9 common indoor air pollutants in typical offices of Hong Kong, with the range, the arithmetic mean (AM), the arithmetic standard deviation (ASD), the geometric means (GM) and the geometric standard deviations (GSD) respectively. Table. IAQ of typical air-conditioned offices in Hong Kong (Hui et al., 2006; Wong et al., 2006, 2007). No. Air pollutant HKEPD Arithmetic Geometric Geometric Standard recommended mean mean standard Range deviation maximum concentration concentration deviation ASD level AM GM GSD CO 2 (ppm) CO (μg m 3 ) RSP (μg m 3 ) NO 2 (μg m 3 ) O 3 (μg m 3 ) HCHO (μg m 3 ) TVOC (μg m 3 ) Rn (Bq m 3 ) ABC (CFU m 3 ) Taking the recommended maximum pollutant exposure levels in the HKEPD certification scheme shown in Table, Figure shows the IAQ index θ determined from the CO 2, RSP and TVOC measured in these Hong Kong offices. It was reported that the range of IAQ index θ was between 0.9 and.99, with mean and standard deviation of 0.50 and The corresponding benchmarks B i of offices in Hong Kong, using the star rating system were determined as: 5 stars for offices with IAQ index θ 0.30, 4 stars for offices with 0.30<θ 0.40, 3 stars for offices with 0.40<θ 0.54, 2 stars for offices with 0.54<θ 0.72, and star for offices with θ>0.72 as shown in Figure. θ>0.72 Benchmark value Bj <θ <θ <θ 0.40 θ IAQ index θ Figure. Benchmarks of IAQ.

4 IAQ IN 5-STAR AND -STAR OFFICES The benchmarked offices of -star and 5-star ratings were conducted with the full IAQ assessment and the results were used to evaluate the performance of the benchmark in indicating the office IAQ. Table 2 shows the comparison of the measured levels of the nine common indoor air pollutants in the 5-star offices and -star offices, the first three were used as the parameters for benchmarking. Among the 42 offices of each of the two benchmarked office groups, using the HKEPD s criteria on the nine common air pollutants as reference (HKEPD, 2003), only 4 (equivalent to 9.5%) 5-star offices were considered to have unsatisfactory IAQ, but for -star offices, 4 (equivalent to 97.6%) offices in the group have the air pollutant level(s) exceeded the HKEPD recommended maximum limit(s). In the -star office group, TVOC and ABC contributed to most of the unsatisfactory cases; the exposure levels of these air pollutants were significantly higher than that in the 5-star office group (p<0.000). The benchmarks also reflected in the differences in the exposure levels of CO 2 and RSP between groups that 5-star offices had significantly lower exposure levels (p<0.000). For the other five air pollutants (CO, NO 2, O 3, HCHO and Rn), no significant differences of the average exposure levels between groups were reported (p>0.05). The proposed benchmark would give good indication on the IAQ satisfaction. Table 2. IAQ of 5-star and -star offices in Hong Kong (Hui et al., 2006; Wong et al., 2006, 2007). 5-star offices -star offices HKEPD recommended No. Air pollutant Observed Observed maximum AM ASD unsatisfactory AM ASD unsatisfactory level rate (%) rate (%) CO 2 (ppm) % % 2 RSP (μg m -3 ) % % 3 TVOC (μg m -3 ) % % 4 CO (μg m -3 ) % % 5 NO 2 (μg m -3 ) % % 6 O 3 (μg m -3 ) % % 7 HCHO (μg m -3 ) % % 8 Rn (Bq m -3 ) % % 9 ABC (CFU m -3 ) % % CONCLUSION Nine common indoor air pollutants were selected as the assessment parameters for many office IAQ assessments in Hong Kong and database was available for reviewing the assessment strategy. This study proposes a simple IAQ benchmarks for IAQ quantification by assessing only three representative pollutants in air-conditioned office buildings of Hong Kong. With the regional cross-sectional measurement at 422 offices in Hong Kong, the IAQ indices were compiled and used to benchmark office IAQ with a 5-star rating system. The application of the proposed IAQ benchmark in labeling office IAQ referred to the 5-star and -star groups was demonstrated with follow-up measurement on the other six pollutant levels. The results show that the overall IAQ unsatisfactory rate in -star offices were 98%, while it is only 0% for offices awarded 5-star rating. This simple IAQ benchmark can effectively distinguish offices IAQ as identified by comprehensive IAQ assessment. The proposed benchmark would be a useful tool for policymakers, building owners and professionals to quantify IAQ in offices and to make decisions on resources and manpower management for IAQ investigations elsewhere in order to achieve best IAQ control.

5 ACKNOWLEDGMENT The work described in this paper was substantially funded by a grant from the Research Grants Council of HKSAR, China (Project No. PolyU 5248/06E, Account code: BQ0G). REFERENCES ANSI/ASHRAE Standard : Design for acceptable indoor air quality. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers. Blume M.E An anatomy of morning star ratings, Financial Analysts Journal, 54(2): HKEPD Indoor Air Quality Information Centre. Indoor air quality certification scheme for offices and public places. Hong Kong Environmental Protection Department, the Government of the Hong Kong Special Administrative Region, China. Hoskins J.A Health effects due to indoor air pollution, Indoor and Built Environment, 2(6): Hui P.S., Mui K.W., and Wong L.T Feasibility study of an express assessment protocol (EPA) for indoor air quality of air-conditioned offices, Indoor and Built Environment, 5(4): Hui P.S., Wong L.T., and Mui K.W Evaluation of professional choice of sampling locations for indoor air quality assessment, Building and Environment, 42(8): Jokl M.V Evaluation of indoor air quality using the decibel concept based on carbon dioxide and TVOC, Building and Environment, 35(8): Kraenzmer M Modeling and continuous monitoring of indoor air pollutants for identification of sources and sinks, Environment International, 25(5): Lee S.C., Guo H., Li W.M., and Chan L.Y Inter-comparison of air pollutant concentrations in different indoor environments in Hong Kong, Atmospheric Environment, 36(2): Montoya T., Gurian P.L., Velázquez-Angulo G., Corella-Barud V., Rojo A., and Graham J.P Carbon monoxide exposure in households in Ciudad Juárez, México, International Journal of Hygiene and Environmental Health. In press. Mui K.W., Wong L.T., and Hui P.S A new sampling approach for assessing indoor air quality, Indoor and Built Environment, 5(2): Tchepel O., Penedo A., and Gomes M Assessment of population exposure to air pollution by benzene, International Journal of Hygiene and Environmental Health, 20(3/4): Wong L.T., Mui K.W., and Hui P.S A statistical model for characterizing common air pollutants in air-conditioned offices, Atmospheric Environment, 40(23): Wong L.T., Mui K.W., Hui P.S Screening for indoor air quality of air-conditioned offices, Indoor and Built Environment, 6(5):