Relative effectiveness of ventilation in community indoor environmentsfor controlling infection

Abstract

The existence, probability and control measures of airborne infections

have been widely discussed for centuries. Although public belief regarding

airborne infection kept on altering throughout the entire history of medicine

and is still controversial, many airborne transmission experiments and airborne

infection outbreak analyses have been carried out. Different airborne

transmission models have been built and various airborne control measures

have been evaluated. One of the major knowledge gaps obstructing

applications of some airborne control measures in clinical practices and public

applications is that there is a lack of evidence in proving the effectiveness of

such measures.

Ventilation as an important airborne infection control method can be

achieved by opening windows, or increasing the outdoor air supply rate in

mechanical ventilation systems or indirectly by using filters and ultraviolet

equipments. However the applications of ventilation in infection control were

largely restricted to isolation rooms rather than regarded as a public control

measure. In this study we focus on evaluating the effectiveness of ventilation

as a community measure. Results, therefore, can provide evidence for using

ventilation as a public health measure for controlling respiratory diseases

transmitted by the airborne route or multi-routes.

Two mathematical modeling approaches (deterministic model and social

network model) are adopted to estimate different airborne diseases outbreaks

with a focus on ventilation and a corresponding analysis of their relative

effectiveness compared with other public health measures. A comprehensive

understanding of detailed control strategies (including both engineering and

public health control) will be achieved through gradually complicated and

realistic models.

It’s commonly believed that many respiratory infections are transmitted

through multiple routes including airborne, droplet-borne and contact routes.

Hence the effectiveness of airborne control measures was doubted when the

airborne route was not dominant. Therefore, we developed a model to simulate

partially airborne transmitted diseases outbreaks and evaluated the relative

effectiveness of ventilation when the role of airborne transmission altered.

Knowing the complex transmission mechanisms of respiratory

transmission and the role of the airborne route in the transmission process is

essential in determining the effectiveness of airborne control measures. Hence

in this study we also tested the virus exposures dose to infectious patients at

different distances when patients were carrying out different respiratory

activities. A complex model considering transmission mechanisms of

respiratory infections was also built to evaluate the influence of the

transmission route in large scale outbreak simulations.

The results showed that increasing ventilation rate especially in homes,

offices and classrooms is an effective control method for controlling airborne

and partially airborne transmitted infections. Combining isolation and

increasing ventilation rate can reach similar or even better control effect

compared with other general public health interventions such as vaccination.

This finding suggested the important role of ventilation in airborne infectious

disease prevention and intervention. The ventilation rate required by existing

ventilation standards such as ASHRAE 62 might be too low for the purpose of

controlling possible airborne outbreaks.