Volatile organic compounds (VOCs) are a group of major indoor air pollutants that has been associated with many health problems. The influence of the operation of mechanical ventilation systems on the accumulation of the levels of VOCs is an important subject as people spend a significant amount of time indoors. Mechanically ventilated buildings are also environments that occupants are least able to control. This study aimed at collecting information about the levels of 43 individual VOCs using US-EPA Method TO-14 and the level of total volatile organic compounds (TVOC) using real time monitoring by a Photo-acoustic Spectroscopy Infrared (PAS-IR) gas analyzer. The PAS-IR method was further verified with US-EPA Method TO-12 by means of regression analysis and a satisfactory result was obtained. The site measurements were conducted in both the indoor and adjacent outdoor environments of a range of various mechanically ventilated premises.
This study covered both the office sector and the non-office sector which include customer service centers, institutional buildings, hospitals, etc. Samples and measurements were taken when the mechanical ventilation was turned on as well as when it was turned off. The 43 individual VOCs were distinguished and quantified by means of Gas Chromatography/ Mass Spectroscopy (GC/MS). The results were further analyzed by categorizing them into three classes, namely, aromatic hydrocarbons, chlorinated hydrocarbons and organohalogen. The weight % of chlorinated hydrocarbons (48%) and the weight % of aromatic hydrocarbons (38%) only differed by about 10% in the office sector. Organohalogen (14%) was contributed to the smallest fraction of the total on all the premises in the office sector on weight basis. A completely different distribution pattern was found in the non-office sector. The most abundant class of VOCs in terms of weight was aromatic hydrocarbons (80%). The second abundant class of VOCs was chlorinated hydrocarbons (14%) and was much less than the level of aromatic hydrocarbons in terms of weight. Organohalogen (6%) contributed to the smallest fraction of the total on all the premises in the non-office sector on weight basis.
Hundreds of VOCs have been identified in indoor air and more than 350 VOCs have been recorded at concentrations higher than lppb. In this study, statistical analysis was also carried out to investigate the correlation among 43 individual VOCs which were documented in the well-established US-EPA Method TO-14. This method has been adopted by many researchers worldwide. In this study, these 43 VOCs were chosen for comparison purpose. Among the 43 targeted VOCs in this study, 15 VOCs had good correlation with other VOCs in the office sector. For these 15 VOCs, 7 VOCs were aromatic hydrocarbons and they were benzene, ethylbenzene, m-xylene, p-xylene, o-xylene, 1,3,5-trimethylbenzene and 1,2,4-trimethylbenzene. The other 7 were chlorinated hydrocarbons and they were dichlorofluoromethane (12), methyl chloride, trichlorofluoromethane (11), dichloromethane, chloroform, trichloroethylene and tetrachloroethylene. The remaining VOC was an organohalogen and it was p-dichlorobenzene. While in the non-office sector, 16 VOCs had good correlation with other VOCs. Among these 16 VOCs, 9 VOCs were aromatic hydrocarbons and they were benzene, toluene, ethylbenzene, m-xylene, p-xylene, styrene, o-xylene, 1,2,4-trimethylbenzene, p-ethyltoluene. The other 5 were chlorinated hydrocarbons and they were dichloromethane, chloroform, 1,1,1-trichloroethane, trichloroethylene and tetrachloroethylene. The remaining 2 VOCs were organohalogen and they were p-dichlorobenzene and 1,2,4-trichlorobenzene.
Source apportionment analysis was also conducted based on a material balance equation. This analysis has identified the relative contributions to VOC indoor accumulation from building material related sources, occupant related sources, ventilation ductwork related and outdoor sources. This study revealed that TVOC level in the evening after the mechanical ventilation system was turned off could be generally higher than the day time level by 7 to 35%. In general, the emission rate of the contaminant from ventilation ductwork ranged from 0.01 to 0.99μg/s. The emission rate of the contaminant from building material ranged from 0 to 0.46μg/m
2s. The contaminant emission rate from each occupant ranged from 1.37 to 17.52μg/s per person. The ratio of the TVOC contribution from building material related sources to occupant related sources ranged from 1.7:1 to 56:1. In this study, it was estimated that the building material related TVOC sources and the occupant related TVOC sources each contributed 0 to 55% by weight (mean = 21%) and 0 to 73% by weight (mean = 20%) respectively to the total indoor TVOC sources. The remaining part came from the outdoor sources which ranged from 14 - ~100% by weight (mean = 60%). The TVOC level contributed by ventilation duct related sources of less than 1% was found to be negligible.
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