Western Cold and Flu (WeCoF) aerosol study – preliminary results
© Savory et al.; licensee BioMed Central Ltd. 2014
Received: 8 March 2014
Accepted: 18 August 2014
Published: 23 August 2014
Influenza virus is responsible for annual deaths due to seasonal epidemics and is the cause of major pandemics which have claimed millions of human lives over the last century. Knowledge about respiratory virus transmission is advancing. Spread is likely through the air, but much work remains to be done to characterize the aerosols produced by infected individuals, including viral particle survival and infectivity. Although coughs have been characterized, little work has been done to examine coughs from infected individuals. The WeCoF project aims at providing evidence to support prevention measures to mitigate person-to-person influenza transmission in critical locations, such as hospitals, and during pandemics.
A novel experimental cough chamber facility – the FLUGIE – has been developed to study the far-field aerodynamics and aerosol transport of droplets produced by the coughs from humans naturally-infected with influenza. The flow field of each cough is measured using Particle Image Velocimetry (PIV). A preliminary study involving 12 healthy individuals has been carried out in order to quantify the strengths of their coughs at a distance of 1 m from the mouth. The spatially averaged maximum velocity was determined and the average value was 0.41 m/s across 27 coughs of good data quality. The peak value of velocity was also extracted and compared with the average velocity.
Preliminary results show that there is significant air motion associated with a cough (on the order of 0.5 m/s) as far away as 1 m from the mouth of the healthy person who coughs. The results from this pilot study provide the framework for a more extensive participant recruitment campaign that will encompass a statistically-significant cohort.
We have gained extraordinarily detailed knowledge in the past decade about the molecular nature of influenza virus and other respiratory viruses yet surprisingly little is known about how respiratory viruses are transmitted from person to person. Mathematical modelling of households, containing infected individuals, showed aerosol transmission to be more significant than contact transmission for influenza virus and that airborne transmission may be a significant contributor[1, 2]. Recent reviews of the literature support this important possibility[3, 4].
Aerosols consist of particles in a range of sizes. Traditionally droplets of >5 μm diameter have been implicated in short range (<1 m) spread, and droplet nuclei of <5 μm are believed to be responsible for longer range or airborne transmission (>1 m). However, larger-sized particles may be responsible for wider pathogen spread depending on other factors. For example, particles of the same size may travel different distances, depending on the velocity of the jet propelling them. These particle dynamics remain undefined in the clinical setting, and the implications are significant from an infection prevention and control perspective. In addition to its effect on dispersion, particle size has implications for the inhaling host. Larger particles (>10 μm) will be deposited by impaction in the upper respiratory tract and smaller particles (0.003-5 μm) may penetrate the tracheobronchial and alveolar regions, principally through sedimentation and diffusion. By studying the aerosols produced by infected individuals, we hope to precisely characterize how long virus infectivity persists in suspended aerosol droplets of various sizes. Recent work has begun to address this question for bacterial transmission by patients with cystic fibrosis[8, 9]. The question of droplet survival duration pertains to droplet size distribution and, on this front, there are discrepancies across the literature which are likely due to the varying measurement methodologies and techniques. Solid impaction with micrometry, a method that is insensitive to smaller droplets, revealed particle sizes from 1 to 2,000 μm. Size distribution analysis using an optical droplet counter[11–13] is reportedly less accurate for particles >2 μm but is useful in the submicron size range. Other previous techniques to measure size distribution include aerodynamic droplet sizer[15, 16], scanning mobility particle spectrometer, Andersen six-stage cascade impactor, electrical low pressure impactor, interferometric Mie imaging and laser diffraction[13, 20]. Aside from the dependency on initial size, the change in droplet size due to evaporation and condensation is also directly related to their chemical composition. Recent findings suggest a tri-modal size distribution during speech or voluntary coughing, with bronchiolar fluid film burst and laryngeal modes both contributing to a distribution peak centred near 1 μm size and with an oral mode yielding a distribution peak centred near 100 μm size due to droplets produced between the epiglottis and the lips.
Measurements near the mouth (0.17 m distance) of healthy non-smokers, giving best-effort voluntary coughs, indicate that 99% of expelled droplets are inhalable (<10 μm). For infected individuals, significant influenza RNA also appears to be contained in droplets with diameters in the respirable size range - 35% of influenza RNA was found in droplets of greater than 4 μm diameter, 23% in droplets of 1 to 4 μm diameter and 42% in droplets of less than 1 μm diameter - and symptomatic subjects appear to emit more particles. However, few results are available for the relationship between droplet size and infectivity for influenza virus.
The viability of airborne pathogens also depends on other factors, including environmental humidity, temperature and the presence of ultraviolet light. The relationship between these factors and infectivity is poorly understood. A recent study using a breathing mannequin and bioaerosol samplers indicated that high relative humidity (RH) was associated with reduced infectivity of influenza virus. Earlier studies using small settling chambers, influenza bioaerosols and guinea pigs found inactivation at high RH[25–28]. Evaporation and particle shrinkage are expected since particles typically enter an environment at lower RH than the respiratory tract. The evaporation process is fast; particles of 10 mm diameter or less typically evaporate in less than 0.5 seconds. At low RH, droplets evaporate more quickly and remain suspended longer compared to droplets generated under conditions of higher RH, thereby increasing the probability of ensuing inhalation[6, 21, 29]. Temperature has also been shown to enhance or interrupt transmission at low (5°C) or high (30°C) temperature, respectively, for all values of RH. Assessment of data in the literature suggested a relationship between absolute humidity (AH) and influenza survival. When extended to a human population level study, negative local daily deviation of AH from its 31 year mean was found to be associated with the start of influenza outbreaks during the winter[30, 31]. Studies on the effect of ultraviolet light irradiation showed avian influenza A (H7N9) virus was inactivated after at least thirty minutes of exposure, while adenoviruses appeared to be UV-resistant[33, 34]. The vaccinia virus was found to be less susceptible to ultraviolet radiation at high RH than at low RH. The precise mechanisms through which RH, AH, temperature and UV light exposure affect virus transmission and survival require clarification.
Since the Severe Acute Respiratory Syndrome coronavirus (SARS CoV) outbreak in 2003, the scientific and medical communities, as well as the general public, have gained an appreciation of the public health importance of understanding respiratory virus transmission. Although cough droplet sizes have been characterized, more research is needed to examine cough flows from infected individuals. Much is known about airflow rates during coughing[37–39], including parameters such as Cough Peak Flow Rate (CPFR), Peak Velocity Time (PVT) and Cough Expired Volume (CEV), that is the area under the flow rate versus time curve. A study of 12 female and 13 male subjects showed that the non-dimensional airflow rate (Flow rate/CPFR) versus non-dimensional time (Time/PVT) curve could be defined by two gamma-probability functions based on the medical parameters of CPFR, PVT and CEV that were themselves related to height, weight and gender. Also, a sequential cough was found to be a combination of two single coughs, with the first being approximately the same as a single cough, whilst the second was a scaled down version of the first. Visualizations showed that the airflow direction did not vary greatly amongst the subjects and, although mouth opening remained constant during a cough, there was a considerable variation of area across the subjects with no correlation to other parameters such as height. Their research examined the bulk parameter of flow rate, whilst earlier studies examined the flow field qualitatively using strobe photography or thermal plume imaging by Schlieren. More recently, quantitative analysis of shadowgraph images indicated that this technique requires a significant temperature difference (10°C) between exhaled air and room temperature and that coughs were detectable out to a maximum distance of 0.6 m from the source. Quantitative analysis of high-speed video images of a cough were limited to a similar range whilst also requiring the cougher to expel cigarette smoke as a tracer substance, which is problematic to apply in the study of humans with respiratory illness due to the unacceptable possibility of causing harm to participants. Accurate velocity measurements at greater distances in the far-field of a cough require a different measurement technique and approach.
Particle Image Velocimetry (PIV) velocity measurements have been undertaken using an artificial cough flow simulator[45, 46], a thermal mannequin with simulated breathing[47, 48] and healthy human subjects[19, 49–51]. Such measurements have revealed a peak cough velocity of 6 to 22 m/s, with an average of 11.2 m/s, but usually mouth area is merely assumed and not measured[50–52]. Kwon et al. reported average initial cough velocities of 15.2 m/s (males) and 10.6 m/s (females), with the angle of exhaled air being 38° (males) and 32° (females), although it is doubtful whether this difference in angle is statistically significant. Singh et al. found the peak flow rate produced by women to be 60% that of men and Chao et al. reported the maximum cough velocity of women to be approximately 77% that of men. On the other hand, VanSciver et al. found no significant difference in maximum cough velocity related to sex and weight of the cougher. Furthermore, they noted that, from the fluid dynamic point of view, a cough may be considered as a short-duration transient jet, being notably unlike a very-short-duration jet in which much of the cough would be entrained in a single vortex ring. Their PIV data showed a wide range of maximum cough velocities (1.5 to 28.8 m/s) and that the self-similarity of flow profiles associated with a transient jet was not applicable to coughs, such that it is necessary to develop an envelope of cough profiles rather than attempting to define a “typical” cough. The measurements by Zhu et al. showed that some saliva droplets produced during a cough can travel further than 2 m and (using the Lagrangian equation) that the transport characteristics of expelled saliva droplets change with size. Furthermore, a recent study of patients with cystic fibrosis emitting cough aerosols, which were collected with an Anderson Impactor in a wind tunnel of modest cross-sectional area, reported that viable bacteria can travel 4 m from the patient or remain aloft for up to 45 minutes. These findings call into question the “3 feet/1 metre rule” or “6 feet/2 metre rule”, which have been considered to be safe separation distances for preventing droplet transmission, and motivate further study of virus-laden bioaerosols and the velocity field at extended distances from the source. Indeed, all previous PIV flow measurements have been taken near the mouth, where velocities are highest, rather than far downstream near the limits of possible person-to-person transmission.
to understand the penetration of viral droplets into the ambient environment,
to rigorously test the “3 feet/1 metre” and “6 feet/2 metre” rules and
to identify host determinants of individuals who emit higher quantities of virus which disperse further,
all of which are important for implementation of future transmission prevention measures. The measured data will also be of use to other researchers who are attempting to develop realistic theoretical or computational fluid dynamics (CFD)[50, 56] models for cough jets/plumes and virus transmission. Such models require reliable modeling of the transport of aerosol droplets and virus particles. In addition, data from human subjects may be used to test simpler models that use the spatial distribution of expiratory aerosols and the viability functions of airborne viruses to estimate exposures to airborne viruses in the indoor environment, where previously such models were based on artificial puff sources, e.g.. This introductory review has covered issues such as cough droplet sizes and the influence of environmental parameters, notably temperature and relative humidity, which will be studied as part of the WeCoF project. However, the present paper focuses on the experimental facility and methods and presents the results from the initial experiments using healthy human volunteer subjects.
The position of the participant’s head is fixed by a chin rest and a forehead rest, such that the angle of the cough is horizontal and consistent over multiple trials. Although it may be argued that a more natural cough would be observed by permitting unrestricted head motion, it is likely that such freedom would permit a significant cough velocity component due to forward translation of the participant’s upper body during the expulsive phase, as well as introduce a greater unpredictability to the cough flow trajectory, which would be problematic for any experimental technique with a limited measurement site or window. From the perspective of achieving the present research aims in a controlled laboratory experiment, it is acceptable to examine the cough velocity produced by pulmonary effort alone.
The identity of the pathogen acquired by each study participant is confirmed by asking for a self-collected mid-turbinate swab (MTS) and these specimens are interrogated by multiplex polymerase chain reaction (multiplex-PCR) assay for a panel of respiratory viruses (RVP Fast, Luminex). The viral content from the membranes is quantified using a virus-specific monoplex quantitative real-time PCR assay and calculated using quantitative curves and number of litres of air sampled.
Particle Image Velocimetry of coughs
Separate measurements are performed to quantify the cough flow field. Optical access areas into the test chamber are outlined in light blue in Figure 1. A beam of green light (532 nm wavelength) horizontally emitted from the laser head (120 mJ, Nd:YAG crystal) is re-directed to vertical by a 45°-angled mirror and expanded into a narrow light sheet (~1 mm thickness) with cylindrical and spherical lenses. The light sheet enters the test chamber through a glass window in its floor, and illuminates a centreline plane from the test chamber floor to the test chamber roof. The Vieworks VA-4 M32 camera is a charge-coupled device (CCD) with a resolution of 10.0 pixels per mm and a sensor array of 1,752 pixels by 2,336 pixels, where the longer side is oriented vertically for this experiment. The camera is focused upon the light sheet at the chamber centreline and optical access is through a glass window on a chamber wall.
The test chamber is seeded with titanium dioxide particles (rutile mineral form). The product specifications indicate a particle size distribution ranging from 0.15 to 0.47 μm, where 69% of the particles are in the 0.34 to 0.43 μm size bin and 29% of the particles are in the 0.27 to 0.34 μm size bin. The titanium dioxide (TiO2) powder is dried in a vacuum-oven, stored in a vacuum container to minimize clumping and aerosolized using a custom-crafted version of the Pitt 3 aerosol generator. This device consists of a cylindrical drum with small inlet and outlet ports near its bottom and top ends, respectively. The drum is filled with TiO2 powder, which is carried up and out of the drum by the flow driven by a 30 kPa air line attached to the inlet port. The drum is placed on top of a loudspeaker, which generates sound waves to vibrate and break up the powder. From the outlet port of the aerosol generator, the aerosolized particles enter a settling chamber mounted on top of the test chamber through a tube with perforations. The FLUGIE settling and test chambers are separated by a fine mesh, which permits TiO2 particles, under the action of gravity and local airflow, to gently enter the test chamber along its centreline. The cough jet generated by the participant disturbs the TiO2 particles which are imaged to obtain quantitative information of the flow field.
Thus, this setup achieves an intersection of the tracer particles, the light sheet illuminating the tracer particles, the focused field-of-view of the camera recording the illuminated tracer particles and the cough flow, over a sizable region of space and time (400 cm2 and 5 s, respectively). A pulse generator (Berkeley Nucleonics Corporation, Model 555-4C) is used to control the timing and synchronizing of the laser and camera. Image pairs are captured at a rate of 16.7 Hz, from which instantaneous velocity fields are calculated using commercial software (TSI Incorporated, Insight3G) that cross-correlates the image pairs. In the tests reported here the field-of-view of the camera is centred at 1.00 m from the cough inlet. For each study participant, following the aforementioned cough droplet sampling, PIV is performed for another three independent single coughs with a settling time of 30 s between each cough.
A preliminary study involving healthy individuals has been carried out in order to assess the performance of PIV for measuring the far-field region of this transient and turbulent air flow. In addition, this work has provided the framework for a more extensive campaign that will encompass a statistically significant cohort. The velocity fields associated with the coughs from 12 healthy young adults were quantified at a distance of 1 m away from the mouth. This limited study with healthy volunteers leads into the recruitment and study of individuals who are naturally-infected with influenza virus. Written informed consent was obtained from all participants. The University of Western Ontario Research Ethics Board for Health Sciences Research Involving Human Subjects (HSREB) reviewed and approved this study. The HSREB is registered with the U.S. Department of Health & Human Services under the registration number IRB 00000940.
In an ongoing pilot study at a university student health clinic (Student Health Services at Western University), a small cohort of undergraduate students, who were naturally-infected with influenza, are being referred by clinicians to the WeCoF aerosol study. Written informed consent is being obtained from all participants in this ongoing study. The HSREB reviewed and approved this study.
A germicidal lamp, which produces continuous light in the Ultra-Violet B range, is used to disinfect the test chamber between study participants. In addition, an outlet has been retrofitted with a HEPA filter through which chamber air can be withdrawn to further reduce the risk of viral contamination between subjects. The experiments are repeated several weeks later, with the same participants, after recovery from the respiratory illness to permit an assessment of the differences in the coughs between an infected and a healthy person.
Discussion of preliminary results
Summary of 36 measured coughs from 12 healthy participants
Max Mean velocity (m/s)
Mean # of Valid vectors (%)
Strong; wide; partially high
In field of view
Missed, too low
In field of view then low
Strong; in field of view
Strong; horizontal flow
Strong; partially high
Weak; wide; multiple jets
In field of view; intermittent
In field of view
Sharp front; wide
Sharp front; wide; intermittent
Low seeding; spurious vectors
Low seeding; spurious vectors
Low seeding; spurious vectors
Low seeding; spurious vectors
Low seeding; spurious vectors
Low seeding; spurious vectors
Wide; some spurious vectors
Turbulent; in field of view
Low seeding; spurious vectors
Low seeding; spurious vectors
Low seeding; spurious vectors
Weak; partially low
Weak; partially low
Best for M8; intermittent
Very weak and low
Weak; partially low
A novel experimental facility – the FLUGIE – has been designed to study the far-field aerodynamics of human coughs produced by subjects naturally-infected with respiratory viruses, together with measurement of the viral content of the droplets produced by those coughs, in order to quantify the factors relating to person-to-person airborne transmission of virus. A preliminary study involving 12 healthy individuals has been carried out in order to quantify the strengths of their coughs at a distance of 1 m away from the mouth. The velocity fields were measured using Particle Image Velocimetry and the results indicate an important finding, namely that there is significant air motion during a cough, of the order of 0.5 m/s, even at a location as far away as 1 m from the person who is coughing.
The facility development and preliminary study were funded by the Ontario Thoracic Society and the Workplace Safety and Insurance Board. Thanks are due to University Machine Services at Western for the construction of the FLUGIE facility and to the 12 anonymous experimental participants. Dr. Sydney Siu provided key assistance with the medical research ethics and safety approvals. The PIV system was kindly made available by Dr. Kamran Siddiqui. Dr. Qiuquan (Charles) Guo provided physical assistance in the laboratory and autoclave training.
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