key: cord-0689610-3q3sktuq authors: Correia, G.; Rodrigues, L.; Silva, MG.; Gonçalves, T. title: Airborne route and bad use of ventilation systems as non-negligible factors in SARS-CoV-2 transmission date: 2020-04-25 journal: Med Hypotheses DOI: 10.1016/j.mehy.2020.109781 sha: 73b09458f180f11b782304353cffbb3c40479541 doc_id: 689610 cord_uid: 3q3sktuq Summary The world is facing a pandemic of unseen proportions caused by a corona virus named SARS-CoV-2 with unprecedent worldwide measures being taken to tackle its contagion. Person-to-person transmission is accepted but WHO only considers aerosol transmission when procedures or support treatments that produce aerosol are performed. However, transmission mechanisms are not fully understood and there is evidence for an airborne route to be considered as the virus remains viable in aerosols for at least 3h and that mask usage was the best intervention to prevent infection. Heating, Ventilating and Air Conditioning Systems (HVAC) are used as a primary infection disease control measure. However, they may contribute to the transmission/spreading of airborne diseases as proposed in the past for SARS. The authors believe that airborne transmission is possible and that HVAC systems when not adequately used may contribute to the transmission of the virus, as suggested by descriptions of from Japan, Germany, and the Diamond Princess Cruise Ship. Previous SARS outbreaks reported at Amoy Gardens, Emergency Rooms and Hotels, for example, also suggested airborne transmission. Further studies are warranted to confirm our hypotheses but the assumption of such way of transmission would cause a major shift in measures recommended to prevent infection such as the disseminated use of masks and structural changes to hospital and other facilities HVAC systems. The world is now facing a pandemic caused by a novel coronavirus, named Severe Acute Respiratory Syndrome virus corona virus 2 (SARS-CoV-2). The disease was called COVID-19 by the World Health Organization (WHO) (1) . Although it is the third epidemic caused by coronavirus in the 21st century, the current number of infected individuals has already surpassed the previous two (2) . So far only compulsory social isolation and mass mask usage have shown to potentially control the spread of the disease(3). The contagiousness and rapid spread has been a main characteristic of this outbreak but the transmission routes for person-to-person virus infection are not fully understood(4) (5) . It is highly contagious as it spread to almost the whole China in just 14 days (2) , and it is continuing to spread all over the world. As the mechanisms of transmission are not completely known it is difficult to evaluate the effectiveness of different measures for controlling the pandemic (6) . So far, WHO considers that the route of human-to-human transmission of SARS-CoV-2 is either via respiratory droplets or contact and recommends the use of N95 masks only in the context of aerosol generating procedures based on findings suggested by available information(7)(8). Aerosol transmission refers to the possibility that fine aerosol particles, called droplet nuclei, remain suspended in the air for prolonged periods. True aerosol transmission involves particles of less than 5 microns(8). Airborne transmission for corona virus has not yet been clearly established but there is growing evidence for aerosol-driven infection(9)(4)(10)(11)(12) (13) . The authors believe in airborne transmission of the SARS-CoV-2 in addition to droplet transmission, as an important cause of infection and spreading. We address the issue of aerosol transmission of the disease in closed environments but also consider the possibility of community transmission. Specifically, the authors reviewed the possible impact of Heating, Ventilating and Air Conditioning Systems (HVAC ) in buildings, such as hospitals or other healthcare facilities, as contributors to the spread of the virus. SARS-CoV-2 belongs to the betaCoVs category. It has a diameter of approximately 60-140 nm and round or elliptic and often pleomorphic form (14) . Still, standard ventilation systems use filters with pore diameter often larger than 1micron. Only specific locations, such as isolation rooms or intensive care units are equipped with more efficient filters, such as HEPA. The same applies to exhausted air from health care facilities. Production of infectious droplets and subsequent spread to surrounding environment is determined by generation and annihilation processes (16) and may be affected by many variables such as air temperature, relative humidity and ventilation rate(VR) (17) (18) . Thus, airborne particles that diffuse can be carried by air movements due to ventilation systems (16) . Still, there is no definite consensus about the ideal VR (18) . Sanitary facilities must also be addressed as a major factor of virus propagation (15) , since it has been proven that the virus is excreted on urine and feces (19) and sanitary facilities ventilation systems are potentially quite sensitive for the possible propagation of SARS-CoV-2. Through recirculating air in building ventilation systems; Every HVAC system comprises a pool of recirculating air to allow for systematic renovation of that air by exchanges with air outside the building. This type of propagation depends on the characteristics of settings and air circuits for HVAC systems. This is a possible mechanism by the which the virus can spread to different floors or to different compartments on the same floor (20) . C) Every HVAC system comprises a pool of recirculating air to allow for systematic renovation of that air by exchanges with air outside the building. Usually these units are placed on top of the buildings and can be a potential threat to the environment surrounding healthcare facilities. Although posing a smaller probability for transmission, we believe this hypothesis should be addressed in case adequate exhaust filtering systems are not mounted. Transmission of infectious diseases can occur through direct contact with infected individuals, indirect contact via fomites, droplet transmission, and aerosolized fine particles (airborne transmission). WHO and CDC define droplets as being > 5 microns and airborne pathogen transmission to occur from desiccated droplets < 5 microns in size (7) . The latter are both air transmission routes but differ in the way that large droplet (>10 microns) formed from coughing and sneezing typically fall to surfaces no further than 1-2 m from the infected person. On the contrary, airborne transmission is due to small particles (< 5 microns) that may stay airborne for hours and therefore can travel long distances. These particles form from droplets which evaporate and desiccate (15) . For the current pandemic, WHO considers 3 main routes for transmission: infective respiratory droplets, fomites and specific circumstances and settings in which procedures or support treatments that generate aerosols are performed (14)(1) But, other routes for transmission cannot be excluded (21)(20)(5)(22). Corona Virus have a lipid envelope and therefore survive longer in conditions with lower (<50%) relative humidity (RH) and their persistence eliminated at RH>80%. During the first stages of the current outbreak, two case descriptions point out to recent transmission of the disease because of identical genome of the virus, albeit there is no direct link between cases or between cases and Huanan Seafood Market (24) . Also in the current outbreak, 18 patients with confirmed infection, in Zhuhai, Guangdong, China, were assessed for viral loads in their noses and throats. Loads detected in symptomatic patients with SARS-COV-2 were high, comparing to those with SARS-CoV-1. Authors stated that the viral nucleic acid shedding pattern of these patients resembled that of patients with influenza, and found an asymptomatic patient with a similar viral load (25). This finding further supports the hypothesized transmission from an asymptomatic patient as also suggested by a case report in Germany (26) and was later confirmed that asymptomatic individual could be shedding the virus 2-3 days before the onset of symptoms (27) . Such cases reinforce the hypothesis of airborne transmission in the sense that transmission is less likely to occur through droplets as people transmitting the disease are not coughing or sneezing. The hypothesis may be further supported by the evidence that most cases of infection remain undocumented (28) A study designed to evaluate the use of N95 masks in medical staff (doctors and nurses) in hospital facilities concluded that none of the 278 medical staff using N95 masks became infected with SARS-CoV-2 whereas 10 out of 213 doctors or nurses became infected, regardless of their lower risk of exposure (39) .In 2011, a Cochrane review highlighted that masks were the best intervention in infection prevention across population, settings and threats. The authors found no evidence for the superiority of FFP over surgical masks (40) . Aerosol transmission allows for the virus to be transported over longer distances by airflow (21) . Also, small aerosols are more likely to be inhaled deep into the lung and cause infection in the alveolar tissues of the lower respiratory tract(41)(42), leading to more severe forms of the disease (4). Likewise, as in SARS, high viral load was associated with more severe forms of the disease (43) in COVID-19 (44) . Ventilation is a primary infectious disease control strategy in hospitals and other facilities (45) . In healthcare facilities the density of infected individuals increases the rate that pathogen-loaded droplets are shed (16) , which can be problematic in overcrowded hospital rooms and lead to more severe disease(43)(44). In the current outbreak, the Diamond Princess, a cruise ship, is a study case for the propagation of the disease. The main route for transmission was considered to be from person-to-person but other routes should not be neglected such as aerosol transmission via central air supply or drainage systems (50) . Transmission by aerosols was also addressed regarding airplanes, but has been put aside, because the disease only spread from infected passengers to others in close contacts, with lower transmissible rates(51) (24) . This supports our airborne transmission hypothesis since airplane cabin ventilation systems have HEPA filters which will allow the extraction of air viral particles (52) . Legionelosis is the prototype for aerosol transmission for community acquired infection transmitted by buildings cooling towers (53) . Legionella is an aerosol transmitted pathogen that has a unique water-to-air transmission route and cooling towers are perhaps the most widely known source of legionellosis (46)(54). It is possible for the HVAC systems, including ductwork, to have a role in its transmission, although they remain unexplored (46) . Likewise, there is a theoretical possibility that the aerosolized particles containing the virus that enter the ventilation ducts in hospitals attending COVID-19 patients may be exhausted through ventilation system to the environment and contribute to further sporadic cases, as in the case of Legionella, with cooling tower associated transmission. This has already been hypothesized and verified through air flow analysis of transmission to other blocks of apartments in the Amoy Gardens outbreak in 2003 and was affected by wind direction(13). As a major threat to humans worldwide, strong unprecedent measures are being taken Coronavirus disease (COVID-2019) situation reports. 2020. 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Person-to-person transmission is accepted but WHO only considers aerosol transmission when procedures or support treatments that produce aerosol are performed.However, transmission mechanisms are not fully understood and there is evidence for an airborne route to be considered as the virus remains viable in aerosols for at least 3h and that mask usage was the best intervention to prevent infection. The authors declare no funding sources or grants were attributed to this work.