Building Energy & Environment Laboratory
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Chen, C.*, Zhao, B., Lai, D., Liu, W. (2018). A simple method for differentiating direct and indirect exposure to exhaled contaminants in mechanically ventilated rooms. Building Simulation, 11, 1039-1051.

Many airborne infectious diseases can be transmitted via exhaled contaminants transported in the air. Direct exposure occurs when the exhaled jet from the infected person directly enters the breathing zone of the target person. Indirect exposure occurs when the contaminants disperse in the room and are inhaled by the target person. This paper presents a simple method for differentiating the direct and indirect exposure to exhaled contaminants in mechanically ventilated rooms. Experimental data for 191 cases were collected from the literature. After analyzing the data, a simple method was developed to differentiate direct and indirect exposure in mixing and displacement ventilated rooms. The proposed method correctly differentiated direct and indirect exposure for 120 out of the 133 mixing ventilation cases and 47 out of the 58 displacement ventilation cases. Therefore, the proposed method is suitable for use at the early design stage to quickly assess whether there will be direct exposure to exhaled contaminants in a mechanically ventilated room. 


Chen, C., Lin, C.-H., Jiang, Z., Chen, Q. (2014). Simplified models for exhaled airflow from a cough with the mouth covered. Indoor Air, 24, 580-591.

Covering a cough can be useful in reducing the transmission of airborne infectious diseases. However, no simple method is available in the literature for predicting the exhaled airflow from a cough with the mouth covered. This investigation used smoke to visualize the airflow exhaled by 16 human subjects. Their mouths were covered by a tissue, a cupped hand, a fist, and an elbow with and without a sleeve. This study then developed simplified models for predicting the airflow on the basis of the smoke visualization data. In addition, this investigation performed numerical simulations to assess the influence of mouth coverings on the receptor’s exposure to exhaled particles. It was found that covering a cough with a tissue, a cupped hand, or an elbow can significantly reduce the horizontal velocity and cause the particles to move upward with the thermal plumes generated by a human body. In contrast with an uncovered cough, a covered cough or a cough with the head turned away may prevent direct exposure. 

Chen, C., Zhu, J., Qu, Z., Lin, C.-H., Jiang, Z., Chen, Q. (2014). Systematic study of person-to-person contaminant transport in mechanically ventilated spaces (RP-1458). HVAC&R Research, 20, 80-91.

It is essential to investigate person-to-person contaminant transport in mechanically ventilated spaces to improve air distribution design and reduce the infection risk from airborne infectious diseases. This paper provides a systematic study of the effects of ventilation mode, ventilation rate, and person-to-person distance on person-to-person contaminant transport. This study first collected available cases of person-to-person contaminant transport from the literature to create a database. Then this investigation identified the limitations of the existing data and added a number of cases to complete the database. The additional cases were generated by using a RANS-Eulerian model that was validated by experimental data from an occupied office with under-floor air-distribution (UFAD) systems. The database shows that the overall performance of displacement ventilation and the UFAD systems was better than that of mixing ventilation. A higher ventilation rate was beneficial in reducing person-to-person contaminant transport to some extent. Person-to-person contaminant exposure increased rapidly with a decrease in person-to-person distance when the distance was smaller than 1.1 m. Generally speaking, person-to-person distance is an important parameter when compared with ventilation mode and ventilation rate. 


Chen, C., Zhao, B., Yang, X., Li, Y. (2011). Role of two-way airflow owing to temperature difference in severe acute respiratory syndrome transmission: revisiting the largest nosocomial severe acute respiratory syndrome outbreak in Hong Kong. Journal of the Royal Society Interface, 8, 699-710.

By revisiting the air distribution and bio-aerosol dispersion in Ward 8A where the largest nosocomial SARS outbreak occurred in Hong Kong in 2003, we found an interesting phenomenon. Although all the cubicles were in “positive pressure” towards the corridor, the virus-containing bio-aerosols generated from the index patient’s cubicle were still transmitted to other cubicles, which cannot be explained in a traditional manner. A multi-zone model combining the two-way airflow effect was used to analyze this phenomenon. The multi-zone airflow model was evaluated by our experimental data .Comparing with the previous CFD simulation results, we found that the air exchange due to the small temperature differences between cubicles played a major role in SARS transmission. Additionally, the validated multi-zone model combining two-way airflow effect could simulate the pollutant transport with reasonable accuracy but much less computational time. A probable improvement in general ward design was also proposed.


Chen, C., Zhao, B., Cui, W., Dong, L., An, N., Ouyang, X. (2010). The effectiveness of an air cleaner in controlling droplet/aerosol particle dispersion emitted from a patient's mouth in the indoor environment of dental clinics. Journal of the Royal Society Interface, 7, 1105-1118.

Dental health-care workers (DHCWs) are at high risk of occupational exposure to droplets and aerosol particles emitted from patients’ mouths during treatment. We evaluated the effectiveness of an air cleaner in reducing droplet and aerosol contamination by positioning the device in four different locations in an actual dental clinic. We applied computational fluid dynamics (CFD) methods to solve the governing equations of air flow, energy, and dispersion of different-sized airborne droplets/aerosol particles. In a dental clinic, we measured the supply air velocity and temperature of the ventilation system, the air flow rate and the particle removal efficiency of the air cleaner to determine the boundary conditions for the CFD simulations. Our results indicate that use of an air cleaner in a dental clinic may be an effective method for reducing DHCWs’ exposure to airborne droplets and aerosol particles. Further, we found that the probability of droplet/aerosol particle removal and the direction of airflow from the cleaner are both important control measures for droplet and aerosol contamination in a dental clinic. Thus, the distance between the air cleaner and droplet/aerosol particle source as well as the relative location of the air cleaner to both the source and DHCW are important considerations for reducing DHCWs’ exposure to droplets/aerosol particles emitted from patient’s mouth during treatments.

Chen, C., Zhao, B. (2010). Some questions on dispersion of human exhaled droplets in ventilation room: answers from numerical investigation. Indoor Air, 20, 95-111.

This study employs a numerical model to investigate the dispersion characteristics of human exhaled droplets in ventilation rooms. The numerical model is validated by two different experiments prior to the application for the studied cases. Some typical questions on studying dispersion of human exhaled droplets indoors are reviewed and numerical study using the normalized evaporation time and normalized gravitational sedimentation time was performed to obtain the answers. It was found that modeling the transient process from a droplet to a droplet nucleus due to evaporation can be neglected when the normalized evaporation time is less than 0.051. When the normalized gravitational sedimentation time is less than 0.005, the influence of ventilation rate could be neglected. However, the influence of ventilation pattern and initial exhaled velocity on the exhaled droplets dispersion is dominant as the air flow decides the droplets dispersion significantly. Besides, the influence of temperature and relative humidity on the dispersion of droplets can be neglected for the droplet with initial diameter less than 200μm; while droplet nuclei size plays an important role only for the droplets with initial diameter within the range of 10μm-100μm.