Investigating the airborne transmission pathway – different approaches with the same objectives

The advent of molecular diagnostics has allowed the investigation of the airborne transmission pathway to progress significantly in the past 10–12 years, since the 2003 outbreaks of severe acute respiratory syndrome (SARS). Much of this progress has been driven by researchers in some of the most affected countries, namely Hong Kong, Singapore, and Canada. Perhaps equally inspiring is that the researchers who have produced this wealth of new data have come from several different, traditionally quite separate disciplines and teams, including engineering, epidemiology, public health, hospital physicians, and infection control teams. At the time of writing, the current number of people infected by Ebola virus in the ongoing West African outbreak is estimated to be 15351, with 9596 laboratory confirmed and 5459 deaths (CDC, 2014). Concerns about the potential for possible airborne transmission of Ebola virus have been expressed in a detailed online commentary (CIDRAP, 2014b), and a similar one has been published regarding a recently emerged, novel respiratory virus, Middle East respiratory syndrome coronavirus (MERS-CoV) (CIDRAP, 2014a). The authors of these commentaries, particularly in the one on Ebola, have urged for a new approach to assessing the potential ‘aerosol transmissible’ capability of infectious agents produced by human respiratory activities performed by infected patients, and not to rely on assessments based on older, outdated experimental data. These authors almost certainly refer to the various, modern, technical advances in both pathogen detection as well as airflow visualization that have had a major impact on how the more modern studies have been conducted, which clearly separates them from the older, traditional studies. In particular, in diagnostic testing, using molecular methods (such as the polymerase chain reaction – PCR – as well as DNA sequencing), have allowed the detection of fewer and fewer organisms in both clinical and environmental samples which do not necessarily have to be viable at the point of testing (like those obtained by air sampling), which has altered how indoor air quality studies are now performed, and perhaps our awareness of the richness and variety of the organisms in the air around us (Blachere et al., 2009; Booth et al., 2005; Gangneux et al., 2006; Knibbs et al., 2014; Suzuki et al., 2004). Such molecular methods that detect specific targets in the pathogen genome (or nucleic acid) suffer from the disadvantage of not being able to distinguish inactivated from truly viable organisms. In addition, this level of sensitivity in the pathogen detection method raises some double-edged concerns. Although such sensitive assays can allow us to detect naturally occurring infectious agents earlier and at much lower concentrations, which may improve the assessment of ventilation and containment efficacy (at least in a hospital infection control setting), this needs to be balanced against whether such low levels of the pathogen are actually clinically relevant, and the level of clinical relevance will depend on the specific patient population, for example immunocompetent vs. immunocompromised, pediatric vs. adult, and hospitalized vs. community patients. On the engineering side, the increasing availability and accessibility of higher resolution temporal and spatial, Schlieren/shadowgraph, and other particle capture/visualization methods using high-speed cameras (Clark and de Calcina-Goff, 2009; Nishimura et al., 2013; Settles, 2001; Tang et al., 2009, 2011, 2013) or laser-based imaging (e.g. particle image velocimetry, PIV) (Chao et al., 2009; Hui et al., 2006; Pantelic et al., 2009; Tang and Settles, 2008; VanSciver et al., 2011; Wan and Chao, 2007; Zhu et al., 2006), and increasingly accurate particle counters, (Chao et al., 2009; Edwards et al., 2004; Fabian et al., 2008; Morawska et al., 2009; Yang et al., 2007) have allowed the detailed characterization and quantitation of droplets expelled during various forms of human respiratory exhalation flows. In addition, the improved design and sensitivity of modern air samplers (Blachere et al., 2007, 2011; Cao et al., 2011; Fabian et al., 2009) and various innovative and novel droplet capture methods (Hatagishi et al., 2014; Huynh et al., 2008; McDevitt et al., 2013; Stelzer-Braid et al., 2009), coupled with these more sensitive molecular techniques, have allowed more pathogens to be detectable at lower concentrations. So together with the increased molecular detection methods above, it is now possible to characterize the potential pathogen content of different sizes of particles/droplets produced during different human respiratory activities.