There seems to be a lot of confusionin this whole aerosol versus dropletdiscussion, even in the medical community. So I thought I’d take a closer look. What actually is a dropletand what is an aerosol?There are different definition, butit’s most useful to differentiate themaccording to their behaviorin the environment. Droplets are both 20 micrometers or microns in size. They’re usually produced during thingslike coughs, sneezes, shouting, et cetera,and usually succumb to gravity, meaningthat they fall down after traveling in theair for one to two meters. Aerosols on the other hand, is made upof fine particles under 10 microns indiameter, and they can travel from manymeters before they fall to the ground orsome other surface. So an aerosol is below 10 micronsand can travel far in the air. Droplets are larger than 20 microns. They usually succumb togravity and fall to the ground. And then there are the in between sizedparticles of 10 to 20 microns that canhave somewhat of an intermediate behavior,but are generally thought to fall to theground, like droplets. And there’s an even more granulardistinction between the different aerosolparticles. Those below five microns are so small thatthey can travel all the way down into thealveolar space wherethey can cause pneumonia. Whereas particles below 10 and above fivemicrons can only penetrate down below theglottis and are thought to landsomewhere in the tracheal branch. When droplets fall on surfaces, uninfectedindividuals can pick them up and bytouching their face can get infected. That’s why hand washing is so crucial. To make matters more complicated when thewater component of droplets dries up inthe air, when the wind and temperatureconditions are right, the remaining bitsof floating virus are called dropletnuclei, and these can then behave likeaerosols too. Also, when, wind conditions are right,even droplets might travel much furtherthan two meters. When you go to the ocean on a windy dayand feel the sea spray on your face. You’ve just encountered dropletsthat have become airborne. What does that mean forCOVID-19 or influenza?Well, it means that actual suspensiontimes of droplets will be far higher whenthere are significant crossflows, which isoften the case in healthcare environmentswith doors, opening beds and equipmentmoving and people walking back and forthconstantly. So the general wisdom is that for stuffthat flies around in the air that we mustinhale, in order to get sick, we needmasks to protect ourselves and others. When we’re dealing with droplets thatare falling to the ground and on surfacescalled fomites, we need hand hygieneand we need to keep a distance. So what about COVID-19and the SARS-CoV-2 virus?Is it airborne and inhaled ordroplet based via fomites and hands?While the uncomfortable truth is that wehave evidence for both, which is also thecase for influenza, by the way. Let’s have a look at this paper from WuhanUniversity — Aerodynamic Characteristicsand RNA Concentration of SARS-CoV-2Aerosol in Wuhan hospitals During theCOVID-19 Outbreak. These authors looked at aerosols andsurface samples at the Renmin Hospital ofWuhan University, which was and isdesignated for the treatment of severecases of COVID-19, and the WuchangFangcang Field Hospital, one of the firsttemporary hospitals, which was renovatedfrom an indoor sports stadium toquarantine and treat mildly symptomaticpatients, and from outdoor public areas inWuhan during the coronavirus outbreak. They then measured the viral RNAconcentrations in these specimen. It’s important to know that these authorsdid not look at whether these viralspecimen could infect cells in turn, theyonly looked for the presence of viral RNA. And here’s what they found. In the patient area of Fangcang Hospitalairborne viral load was minimal and wasentirely absent in the intensivecare unit of Renmin Hospital. The negative pressure ventilation andhigher air exchange rate inside the ICU,CCU and ward room of Renmin Hospitalseem to have been effective in minimizingairborne SARS-CoV- 2. Fangcang Hospital hosted over 200 mildlysymptomatic patients in each zone duringthe peak of the COVID-19 outbreak. However, the SARS-CoV-2 aerosolconcentrations inside the patient hall arejudged to be very low withranges between one to nine. They also took the position samples fromtwo spots of the floor of the ICU roomsand there they found apretty high concentration. The deposited virus probably comes fromthe respiratory droplets or virus-ladenaerosol transmission. They also found elevated airborneSARS-CoV-2 concentrations inside thepatient mobile toiletin Fangcang Hospital. This may come from either the patient’sbreath, or the aerosolization of patientsfeces or urine during use. We know that SARS-CoV-2 has been isolatedfrom patients, stools and bladders, andit’s very much in line with anotherpaper that we’re going to get to shortly. The authors call for extra care andattention on the proper design use anddisinfection of the toilets in hospitalsand in communities to minimize thepotential for transmission. What was particularly concerning in thispaper is the high airborne concentrationof virus in staff rooms, especially inchanging rooms where staff removed theirprotective gear. The authors believe that one directsource of the high SARS-CoV-2 aerosolconcentration in these changing roomsmay be the resuspension of virus-ladenaerosols from the surface of protectiveapparel while they are being removed. These resuspended aerosols originallymay come from the direct, deposition ofrespiratory droplets or virus-ladenaerosols, onto the protective apparel,while medical staff are workinglong-hours inside the patient area. Another possible source, of course, isthe resuspension of floor dust, aerosolcontaining virus that were transferredfrom the patient area to the staff areavia the staff’s shoes. In public areas outside the hospital theyfound that the majority of sample siteshad undetectable or very lowconcentrations of SARS-CoV-2 aerosol. Except for one crowded gathering siteabout one meter to the entrance of adepartment store with customers frequentlypassing through, and the other site nextto the Renmin Hospital entrance wherethe outpatients and passengers passed by. Similar findings werereported by these authors. They performed ear and surface samplesof three COVID-19 patients in Singapore. Samples were taken in the patient’sroom, the anteroom, and the bathrooms. The samples of patients A and B were takenafter cleaning and were all negative. For patient C whose samples were collectedbefore routine cleaning, they foundpositive results with 13 out of 15 roomsites or 87% testing positive, and threeout of five toilet sites or 60%testing positive for the virus. All air samples werenegative in this study. Now we have to consider that these wereall special isolation rooms with a specialkind of ventilation. The fact that air exhaust outlets testedpositive suggests that small virus-ladendroplets were displaced and landed there. But it’s important to remember that all ofthese studies looked at viral RNA or viralparticles, but we don’t know if theseviral particles were still viable and ableto infect humans or cells in culture. So how long will viral particles survive?How long after they fall on a surface orget suspended in the air will they stayviable and able to infect cells or humans?That’s what these authors looked at. They suspended the virus in air and onvarious surfaces like copper, cardboard,steel, and plastic, and tooksamples at various time points. They then look to see whether thatvirus was still able to infect cells. So that’s way stronger than justmeasuring RNA concentration. They found that SARS-CoV-2, was moststable on plastic with viable virusdetectable up to 72 hours afterapplication, seen here in the right mostpane, followed by stainless steelwith 24 hours, cardboard and copper. Aerosolized virus remained viable forthe entire experiment, which lasted threehours. So in summary, when it comes to viral loadin the air, the data suggests that theconcentration of suspended virus in theair increases from almost no virus inuncrowded public places ICUs and isolationrooms, to a little more in crowdedoutdoors areas, even more in medical staffrooms and patient toilets, to a lot instaff changing rooms where theytake off their protective apparel. In general, the concentration of virus inthe air inside hospitals seems to be low,but may be significantly elevated whenstaff having spent long hours taking careof patients with aerosol and dropletsbeing deposited on their protective gear,when they then take off the protectiveequipment deposited materials might becomeresuspended in the air. Medical staff might have a false senseof security when they’re outside thepatient’s rooms, like in medical staffrooms or changing rooms, but the datasuggests that these are the places wherethey’re most likely to be infected. What the data also shows is that patienttoilets seem to be particularly prone tocontamination, and heightened cleaningmeasures in these toilets seem to benecessary to prevent transmission. One last statement coming from this paper. These authors say that if they’re ongoing,contradictory finding in multiple studiesas with influenza and potentially alsoSARS-CoV-2, it may be more likely that thevarious transmission routes maypredominate in different settings, makingthe airborne route for that particularpathogen, more of an opportunistic pathwayrather than the norm. This means that the airborne route isprobably mainly relevant for certainsituations. And I’d say that would be the hospitalsand hospital staff as well as crowded andbadly ventilated public spaces. Everyone else is probably more likely toget the virus through touching surfaces,bad hand hygiene and touching their face.