K.W. Mui

Feasibility study of an express assessment protocol for the indoor air quality of air-conditioned offices

It is costly to sample all major toxic pollutants in community air. Sampling should be conducted based on an assessment strategy and monitoring planning. Assessment methods that rely on monitoring some representative pollutants have been proposed variously at different times. In Hong Kong, the Environmental Protection Department (HKEPD) launched an Indoor Air Quality (IAQ) certification scheme to grade the IAQ in workplaces as “Excellent” or “Good” by measuring the levels of 12 indoor environmental parameters, i.e. carbon dioxide (CO2), carbon monoxide (CO), respirable suspended particulates (RSP), nitrogen dioxide (NO2), ozone (O3), formaldehyde (HCHO), total volatile organic compounds (TVOC), radon (Rn), airborne bacteria count (ABC), temperature (T), relative humidity (RH) and air velocity (V). The present study proposes an Express Assessment Protocol (EAP) to quantify IAQ problems by assessing the “dominant” contributors to unacceptable IAQ prior to any detailed assessment for benchmark purposes. To aid this, a comprehensive measurement of the 12 stated indoor environmental parameters was performed in some Hong Kong offices. The results were used to identify the dominant contributors and assess the performance of the proposed protocol in identifying the associated IAQ problems in an office environment. Comparing the results with the parameter measurements in the certification scheme they showed that for the “Excellent” IAQ level, 96% (94 to 98% for 95% confidence interval (CI)) of the “unacceptable” offices (414 samples) could be identified from the measurements of TVOC, RSP and HCHO; while those that achieved the “Good” level, 93% (90 to 97% for 95% CI) of them (201 samples) could be identified from the measurements of TVOC, ABC, RH, HCHO and O3. The EAP would be a useful tool for policymakers, building owners and professionals to quantify IAQ problems in offices and to make improved decisions based on resources and manpower management for efficient mitigation actions.

A Field Survey of the Expected Desirable Thermal Environment for Older People

In order to identify an indoor thermally comfortable environment for older people, this study examined the thermal acceptance of 384 older people of ages from 60 to 97 years under various thermal environmental conditions in 19 centers for older people in Hong Kong. The results were compared to those of the younger residents in Hong Kong. A probable decay of one predicted mean vote (PMV) for every 25.3 years was reported for older people beyond the age of 60 years ( p ≤ 0.05), which indicated that the older age group probably has a different expectation of a thermoneutral environment as compared with younger age groups. It was also noted that the expected PMV for a thermoneutral environment of older females was apparently higher than that for older males in the same age group. The results should be a useful reference for an acceptable thermally comfortable environment in centers for older people up to the age group over 90 years.

A New Sampling Approach for Assessing Indoor Air Quality

Continuous sampling has been widely adopted for assessing indoor pollutant level. It is believed that the longer the measurement time, the higher the accuracy and improved error rate of the measured average pollutant concentration can be achieved. This study proposes an alternative sampling scheme in which the average pollutant concentration is obtained from two short sampling periods in two sampling sessions when a building was occupied. Two indoor pollutant concentration databases for a 1-year continuous measurement of carbon dioxide (CO2) and radon concentration in a typical openplan office building in Hong Kong were used to investigate the probable errors of the proposed scheme regarding the sampling period. The results showed that these errors deviated from a long-term average value and correlated with the required measurement time. At certain confidence levels, the potential reductions in measurement time of the proposed sampling scheme would be up to 30% and 50% for CO2 and radon, respectively, as compared with an 8-h continuous one.

A method of assessing the acceptability of noise levels in air-conditioned offices:

This study examined the acceptable noise level in an office environment by interviewing 422 occupants about the aural environment being perceived in 61 air-conditioned offices in Hong Kong. The survey samples covered different types of building and prevalent mechanical air supply system. In particular, the measured equivalent continuous noise level Leq in the offices was chosen as an indicator and correlated with the subjective responses from occupants on a visual analogue assessment scale and a dichotomous assessment scale. The results showed that this acceptability was significantly influenced by the office noise level and it would be correlated with a logistic regression model. A satisfaction criterion of the noise level could be evaluated from the frequency distribution of the occupants’ response at certain Leq. In addition, the proposed satisfaction criterion of aural comfort in current design practice was confirmed by the survey samples: an indoor environment with an equivalent continuous noise level Leq of 57.5 dBA would be adopted as a design criterion for some offices. Practical application: The neutral criterion for aural comfort indicated by the equivalent continuous noise level Leq in air-conditioned offices was determined within the range of 45-70 dBA and the ‘optimum’ noise level La/57.5 dBA to be maintained in an office was determined from the survey. This study provides a template for applying a statistical frequency analysis and logistic regression model to examine the ‘optimum’ criteria for aural environment indicated by equivalent continuous noise level Leq in air-conditioned offices elsewhere.

Using Carbon Dioxide Concentration to Assess Indoor Air Quality in Offices

Carbon dioxide (CO2) concentration is commonly used as a surrogate indicator for assessing indoor air quality (IAQ) and ventilation efficiency. The measurement and analysis of average indoor CO2 concentration is useful to understand the performance of a ventilation system. However, uncertainties due to the ventilation rate effectiveness as well as the initial state, build-up, fluctuation, and decay of indoor CO2 concentration from occupant load variation have not been addressed in many measurements; considerable measurement efforts are obligatory. This study, taking the long-term CO2 measurement and occupant profile of a typical open-plan office building as reference, has developed a database to examine the probable errors of an alternative sampling scheme for assessing indoor CO2 level in which the average pollutant concentration is obtained from two short sampling periods in two sampling sessions when the building is occupied. The results show that the assessed CO2 level deviates from the long-term average and the probable errors correlate with the required measurement time and occupant load profile. At certain confidence levels, the potential reductions in measurement time of the alternative scheme, as compared with an 8-h continuous one, will be up to 30%, some 20% less than the reduction of measurement time for assessing indoor radon level.

Evaluation of the neutral criterion of indoor air quality for air-conditioned offices in subtropical climates

Occupant-generated carbon dioxide (CO2) is used as the tracer gas to determine ventilation rate. Although CO2 concentration may not provide a comprehensive indication of indoor air quality (IAQ), it can be a good indicator of the concentration of other human bioeffluents perceived as a nuisance and be used to identify the acceptability of IAQ in a space by its occupants. In this study, the acceptability of IAQ in air-conditioned offices was evaluated by the subjective responses of the office occupants with respect to indoor CO2 concentration. CO2 concentrations at 396 sample locations were measured and subjective responses at those locations were recorded by an electronic questionnaire. Specifically, an occupant’s indirect acceptability of the perceived IAQ indicated on a semantic differential evaluation scale was correlated to the occupant’s direct acceptability described by a dichotomous scale. The overall acceptability of IAQ from all occupants was then described by a logistic regression model and shown to correlate with indoor CO2 concentration.

Determining the Domestic Drainage Loads for High-Rise Buildings

The building drainage loads must be reasonably estimated to design an effective and healthy drainage system. Nowadays, data in various drainage design guides have not been examined for various architectural designs and user behavioral patterns in high-rise residential buildings in Hong Kong. A recent survey on the usage patterns and discharge flow rates from domestic appliances of 597 selected apartments in 14 high-rise residential buildings was performed in Hong Kong. The probable loads of the drainage systems in high-rise residential buildings are investigated and design figures are proposed for the ‘fixture-unit’ approach. The probable wastewater discharge flow rate in a group of domestic appliances, for a vertical drainage stack and the corresponding stack size for high-rise residential buildings are examined, and compared with the design guides used for local buildings. This study provides an updated database of drainage demands in high-rise residential buildings in Hong Kong which is very useful for evaluating the various drainage system designs of the buildings in developed cities having a high population density.

Modelling Transient Occupant Loads for Offices

Occupant load profile of the space is an essential parameter for planning facilities, operations, water and energy consumptions of a building. In view of growing concerns on the effectiveness of building designs, in terms of space utilization, equipment sizing, energy performance and consumption, occupant loads have been specified in some building regulations, design guides and codes of practice. However, an occupant load would be varied from time to time and difficulties would be encountered in determining it to certain accuracy at the building design stage. This paper proposes a mathematical model to generate time-variant design occupant loads using Monte-Carlo simulations, with the model parameters determined from a year-round occupant load assessment in a typical high-rise office building in Hong Kong. Example occupant load profiles for some offices were generated to demonstrate the application of the model. The model would be useful in determining the occupant load profiles of some office buildings in Hong Kong.

Thermal Environmental Interference with Airborne Bacteria and Fungi Levels in Air-Conditioned Offices

Indoor airborne bacteria and fungi levels can be selected as indicators of a healthy indoor environment. This study investigated the relationships between the airborne bacteria levels, fungi levels, and thermal environmental parameters, i.e., air temperature and relative humidity, in some offices with a Mechanical Ventilation and Air-Conditioning (MVAC) system operating. A total of 101 samples were collected from two typical Hong Kong air-conditioned office premises. There was evidence that the operation of the MVAC system would have significant influence on both of the indoor airborne bacteria and fungi levels. The results showed that no significant difference in airborne bacteria and fungi levels was observed between offices having similar thermal environments ( p > 0.05). However, significantly higher airborne bacteria and fungi levels were found in the same office during non-office hours when the air-conditioning system was shut down ( p<0.03). It was also reported that the airborne bacteria and fungi levels would be correlated with the thermal environmental parameters in some offices ( p<0.0001).

An Epistemic Indoor Air Quality Assessment Protocol for Air-Conditioned Offices

Long-term measurement could be the best approach to determine the average pollutant concentration of an indoor environment for decisions on mitigation measures. This approach requires considerable measurement effort for accurate results and hence indoor air quality (IAQ) assessment methods that need less effort have been developed. This study proposes an epistemic approach for assessing an acceptable office IAQ against certain indoor air pollutant exposure limits. Together with the prior understanding of failure rates of IAQ determined from a number of extensive survey studies, a sample test result of pollutant levels and the uncertainties of different sampling schemes, the probability of satisfactory IAQ for an assessed air-conditioned office could be quantified. In particular, assessment of an acceptable IAQ for some air-conditioned offices in Hong Kong was demonstrated by taking CO2 as an example pollutant. Accuracy of the failure rates was improved by the application of sampling correction factors and Bayesian statistics. The probabilities of an acceptable office IAQ were evaluated against a CO2 level limit. This approach would be useful for IAQ assessment in some air-conditioned offices.