What ’ s become evident more recently is that simply by breathing , we produce an aerosol ; that ’ s thought to occur by small airways opening up as people inhale , and then the airways tend to close up and the fluid forms a film and then when you reopen during inhalation , an aerosol is produced that is then breathed out .”
other viruses . We know this virus could survive on hands and on surfaces , and that antiseptics and disinfectants eliminate it . Regarding improved ventilation , we talked about the reduction in viral density , but there are no studies available in healthcare facilities to demonstrate that improved ventilation reduces the risk of COVID transmission .”
He added , “ We need more studies , and we need to keep an open mind . If research — which I certainly encourage — demonstrates the benefit of improved ventilation , we clearly should adapt improved ventilation strategies , if determined to be clinically efficacious and cost-effective .”
Providing the perspective of an aerosol scientist to the IDWeek presentation , William Bennett , PhD , of the University of North Carolina at Chapel Hill , addressed the confusion in the literature around airborne versus droplets
“ Particles come in different sizes ,” Bennett confirmed , “ anywhere from 0.1 micron up to 100 microns . How fast they settle in a given amount of time depends on their weight ; for example , in still air , a 100-micron particle will take about 3 seconds to settle in 1 meter whereas a 0.1-micron particle could hang around for as long as 300 hours in the case of no ventilation . Someone sneezing or coughing produces a range of particle sizes ; very large particles have a shorter trajectory because they ’ re going to settle quickly to the ground , as opposed to airborne particles which can be transmitted further , certainly beyond the 6 feet of physical distancing that ’ s been suggested for avoiding SARS-CoV-2 transmission .”
Bennett addressed the three primary sites of origin and mechanisms of aerosol generation from the human respiratory tract .
“ Many of the largest particles — 100 microns or greater – are emitted by sneezing , coughing and speaking ,” Bennett explained . “ They tend to come from the large airways by shearing a liquid off the airway surface , or in the case of speaking , it would probably
be from the vocal cords in the larynx opening and closing and producing particles , and these tend to be pathogen-laden droplets of 1 , 2 and up to eight microns in size , according to the literature . What ’ s become evident more recently is that simply by breathing , we produce an aerosol ; that ’ s thought to occur by small airways opening up as people inhale , and then the airways tend to close up and the fluid forms a film and then when you reopen during inhalation , an aerosol is produced that is then breathed out . These tend to be very small particles less than 1 micron in size .”
Bennett continued , “ The next question is once they ’ re emitted and breathed in , where do they go in the non-infected individual ? This calls into play the upper airways leading to the nose and mouth , as well as the bronchial airways leading to the deep lung , the alveolar region , where gas exchange occurs . So , depending on the size of the particles , they can go to any of these locations to various degrees . How much deposits in the respiratory tract is a function of the particle size ; for example , 0.1-micron particles tend to deposit least in the respiratory tract , and that ’ s because they don ’ t rapidly diffuse . They don ’ t rapidly settle ; they tend to follow the airstream and go in and out of the lung . Larger-sized particles have a 10 percent to 20 percent deposition rate ; as you get above 5 microns , you go up to a 100 percent deposition rate – meaning they ’ re very efficient at depositing in the lung . If you breathe through your nose , you get even more because the nose is more effective than the mouth .”
He added , “ Anything 5 to 10 microns or larger tend to predominantly deposit in the nose or the mouth , those large particles that you encounter when somebody sneezes or coughs could be deposited immediately in your nose and mouth if you encounter them . On the other hand , if you ’ re looking at the size of a virion , the predominant region where these deposits will be in the alveolar region of the deep lung . So , there are two very distinct regions where infection could take hold .”
The answer to stopping deposition , Bennett said , was wearing face masks , but not all face coverings are made equal , he emphasized , pointing to an electron micrograph of face mask filtration , specifically a layer of nonwoven polypropylene from an N95 mask . “ That ’ s supposed to trap particles ; what ’ s nice about non-woven material is that it ’ s randomly distributed . It ’ s not in a nice cookie cutter pattern where you can see light through the material . So , there is a higher probability of depositing particles in these filters . If large particles spend enough time in the mask , they ’ ll settle . The primary way that large particles deposit is by impaction . They can ’ t follow the airstreams around these fibers , so they sort of bang into the filter material , the fiber interception method that people think of as a major route of the mechanism of deposition . But it requires that it be close to the fiber , so that the particle runs into it . The very small particles tend to deposit by diffusion or electrostatic attraction in these filter materials . When you add the two mechanisms together , you get an overall efficiency .”
Bennett asked , “ Do masks really protect you from being infected via aerosol exposure ?” He relayed his experience with an experiment : “ When the pandemic first began , we pondered this question and worked with our EPA colleagues who had already been thinking about this — they had set up a small chamber where we could introduce an aerosol at a large enough concentration to fill up this chamber ; the particles were about . 05 microns . We tested what was getting through the mask and then we had an ambient sampler to compare the two results .