Effects of surface material, ventilation, and human behavior on indirect contact transmission risk of respiratory infection.

Infectious particles can be deposited on surfaces. Susceptible persons who contacted these contaminated surfaces may transfer the pathogens to their mucous membranes via hands, leading to a risk of respiratory infection. The exposure and infection risk contributed by this transmission route depend on indoor surface material, ventilation, and human behavior. In this study, quantitative infection risk assessments were used to compare the significances of these factors. The risks of three pathogens, influenza A virus, respiratory syncytial virus (RSV), and rhinovirus, in an aircraft cabin and in a hospital ward were assessed. Results showed that reducing the contact rate is relatively more effective than increasing the ventilation rate to lower the infection risk. Nonfabric surface materials were found to be much more favorable in the indirect contact transmission for RSV and rhinovirus than fabric surface materials. In the cases considered in this study, halving the ventilation rate and doubling the hand contact rate to surfaces and the hand contact rate to mucous membranes would increase the risk by 3.7-16.2%, 34.4-94.2%, and 24.1-117.7%, respectively. Contacting contaminated nonfabric surfaces may pose an indirect contact risk up to three orders of magnitude higher than that of contacting contaminated fabric surfaces. These findings provide more consideration for infection control and building environmental design.

Use of risk assessment and likelihood estimation to analyze spatial distribution pattern of respiratory infection cases.

Obvious spatial infection patterns are often observed in cases associated with airborne transmissible diseases. Existing quantitative infection risk assessment models analyze the observed cases by assuming a homogeneous infectious particle concentration and ignore the spatial infection pattern, which may cause errors. This study aims at developing an approach to analyze spatial infection patterns associated with infectious respiratory diseases or other airborne transmissible diseases using infection risk assessment and likelihood estimation. Mathematical likelihood, based on binomial probability, was used to formulate the retrospective component with some additional mathematical treatments. Together with an infection risk assessment model that can address spatial heterogeneity, the method can be used to analyze the spatial infection pattern and retrospectively estimate the influencing parameters causing the cases, such as the infectious source strength of the pathogen. A Varicella outbreak was selected to demonstrate the use of the new approach. The infectious source strength estimated by the Wells-Riley concept using the likelihood estimation was compared with the estimation using the existing method. It was found that the maximum likelihood estimation matches the epidemiological observation of the outbreak case much better than the estimation under the assumption of homogeneous infectious particle concentration. Influencing parameters retrospectively estimated using the new approach can be used as input parameters in quantitative infection risk assessment of the disease under other scenarios. The approach developed in this study can also serve as an epidemiological tool in outbreak investigation. Limitations and further developments are also discussed.

Personalized ventilation as a control measure for airborne transmissible disease spread.

The protective role of personalized ventilation (PV) against plausible airborne transmissible disease was investigated using cough droplets released from a 'coughing machine' simulating the human cough at different distances (1, 1.75 and 3 m) from the PV user. Particle image velocimetry was used to characterize and visualize the interaction between the cough-generated multiphase flow and PV-induced flow in the inhalation zone of the thermal breathing manikin. A dose-response model for unsteady imperfectly mixed environment was used to estimate the reduction in infection risk of two common diseases that can be transmitted by airborne mode. PV was able to both reduce the peak aerosol concentration levels and shorten the exposure time at all the examined injection distances. PV could reduce the infection risks of two diseases, influenza A and tuberculosis, by between 27 and 65 per cent. The protection offered by PV is less effective at a distance of 1.75 m than the other distances, as shown in the risk assessment results, as the PV-generated flow was blown off by the cough-generated flow for the longest time. Results of this study demonstrate the ability of desktop PV to mitigate the infection risk of airborne transmissible disease.