The risk of airborne cross-infection in a room with vertical low-velocity ventilation Nomenclature a 0 area of the manikin´s mouth (m 2 ) a diff area of the textile diffusers (m 2 ) c tracer gas concentration (ppm) c 10 tracer gas concentration 0.10 m above the head (ppm) c chest tracer gas concentration at the chest (ppm) c exp tracer gas concentration in the inhalation (ppm) c R tracer gas concentration in the return (ppm) H height of the test room (m) K exp characteristic velocity constant n 1 characteristic velocity exponent q 0 volume flow rate in the room (m 3 /s) T absolute air temperature (K) T 0 exhaled air temperature (K) T amb air temperature at the height of the manikin´s mouth (K) T in supply air temperature (K) T out return air temperature (K) u 0 peak velocity of exhalation in the manikin´s mouth (m/s) u x peak velocity of exhalation at a horizontal distance from the manikin´s mouth (m/s) x horizontal distance (m) z vertical distance (m) Introduction In recent years, there has been a great interest in finding the most efficient ventilation strategy to pro- duce a comfortable indoor climate for people, and at the same time preventing the spreads of contaminants Abstract Downward flow ventilation systems are one of the most recommended ventilation strategies when contaminants in rooms must be removed and people must be protected from the risk of airborne cross-infection. This study is based on experimental tests carried out in a room with downward flow venti- lation. Two breathing thermal manikins are placed in a room face to face. One manikinÕs breathing is considered to be the contaminated source to simulate a risky situation with airborne cross-infection. The position of the manikins in relation to the diffuser and the location of diffuser in the room as well as the distance between the manikins are being changed to observe the influence of these factors on the personal exposure of the target manikin. The results show that the DWF in different situations often is unable to penetrate the microen- vironment generated by the manikins. The downward ventilation system can give an unexpected high level of contaminant exposure of the target manikin, when the distance between the manikins is reduced. I. Olmedo 1 , P. V. Nielsen 2 , M. Ruiz de Adana 1 , R. L. Jensen 2 1 Department of Chemical Physics and Applied Thermodynamics, Córdoba University, Córdoba, Spain, 2 Department of Civil Engineering, Aalborg University, Aalborg, Denmark Key words: Airborne cross infection risk; Vertical ventilation; Downward flow; Upward flow; Manikins; Exhaled contaminants. I. Olmedo Department of Chemical Physics and Applied Thermodynamics Córdoba University Córdoba Spain Tel.: +34 957 212203 Fax: +34 957 212236 e-mail: ines.olmedo@uco.es Received for review 10 February 2012. Accepted for publication 24 May 2012. Practical Implications Several guidelines recommend the downward ventilation system to reduce the risk of cross-infection between people in hospital rooms. This study shows that this recommendation should be taken into careful consideration. It is important to be aware of people position, position to other thermal loads in the room, and especially be aware of the distance between people if the exposure to the exhaled contaminants wants to be reduced. Indoor Air 2013; 23: 62–73 wileyonlinelibrary.com/journal/ina Printed in Singapore. All rights reserved Ó 2012 John Wiley & Sons A/S INDOOR AIR doi:10.1111/j.1600-0668.2012.00794.x 62