VENTILATION C


OVID-19 is caused by the SARS-CoV-2 virus, which is transmitted in two ways by infected people. Firstly, viruses can survive on surfaces for up to several weeks, especially at cooler room temperatures. Consequently, fomite transmission is possible when people touch infected surfaces and transfer the virus to their mouth, nose or eyes. Secondly, the virus can spread from an infected person’s mouth or nose in small liquid particles when they cough, sneeze, speak or breathe. These liquid particles vary from larger respiratory droplets to smaller aerosols of less than 5µm diameter. According to the World Health Organisation


(WHO): Aerosol transmission can occur in specific settings, particularly in indoor, crowded and inadequately ventilated spaces, where infected person(s) spend long periods of time with others, such as restaurants, fitness classes, nightclubs, offices and/or places of worship.


Supporting the hypothesis that SARS- CoV-2 is transmitted primarily by the airborne route, a recent paper in the Lancet, provided: Ten scientific reasons in support of airborne transmission of SARS-CoV-2. By understanding the modes of transmission,


governments have been able to define appropriate strategies to combat viral transmission with measures such as facemasks, social distancing, hand washing and surface disinfection. Importantly, governments have also


recognised the increased threat from indoor environments, with recommendations for outdoor activity and increased ventilation. In November 2020, the UK Government published a video highlighting the importance of ventilation in reducing the spread of Covid- 19. Their report said: “Research shows that being in a room with fresh air can reduce the risk of infection from particles by over 70%” In January 2021 hundreds of Canadian experts (physicians, scientists, occupational health and safety experts, engineers and nursing professionals) wrote an open letter to their Prime Minister urging him to: “to update provincial COVID-19 guidelines, workplace regulations and public communication to reflect the science — COVID-19 spreads through inhaled aerosols.” One of the key recommendations in the letter was to: “Recommend and deploy carbon


dioxide (CO2 ) monitors as a surrogate measure


in case of inadequate ventilation to reduce long-range transmission risk in shared room air. During a TB outbreak, CO2


concentrations


above 1000 PPM significantly increased the risk of becoming infected with TB. Improving the building ventilation to a CO2


concentration of


600 PPM stopped the outbreak in its tracks.” The WHO says that infected people appear to be most infectious just before they develop symptoms. In addition, some infected people are asymptomatic, so it is logical to assume that in an office environment, for example, the main threat will NOT come from people with severe symptoms such as coughing and sneezing, but from those who do not realise that they have the disease. These people are more likely to exhale viral aerosols of less than 5µm diameter – particles which do not respect social distancing. These fine aerosols are roughly equivalent in size to the particles in cigarette smoke, which, as we know, do not settle readily and are able to spread widely in poorly ventilated spaces. A recent paper published in The Lancet described studies of cough aerosols and exhaled breath from patients with various respiratory infections which showed striking similarities in aerosol size distributions, with a predominance of pathogens in small particles (<5 μm).


These particles are immediately respirable and can remain airborne indefinitely under most indoor conditions - unless there is removal by air currents or dilution ventilation. Humidity also affects the spread of aerosols because low levels of humidity cause aerosols to become lighter and therefore better able to remain airborne.


Humidity has also been shown to affect


vulnerability to viral infection because exposure to dry air impairs host defense against influenza infection, reduces tissue repair, and inflicts cell breakdown.


Risk reduction measures Traditional health and safety risk assessments address hazards such as slips and trips, heavy objects, repetitive injury, falling, stress, electric shock, fire and lone working, but to create Covid-secure environments, organisations will need to also include an assessment of microbiological risk. It will therefore be necessary to


identify potential sources of pathogenic microorganisms as well as their modes and paths of transmission. Hand sanitiser can be made available and surfaces can be frequently disinfected. Procedures can be established to reduce the chance of disease transmission, with measures such as screens, social distancing and even disinfectant fogging. However, even with all of these measures in place, one infected person can quickly contaminate large areas. Effective ventilation will therefore be


essential, and the control system will need to undertake accurate and timely measurements from each room or space so that it can respond promptly.


Some systems may simply monitor CO2 in


the exhaust gas, but this does not provide the ability to detect poor ventilation issues in specific spaces. One of the main functions of a building automation/management system (BMS) is to control thermal comfort and optimise energy usage, so temperature is undeniably the most important control parameter in occupied spaces.


Some systems also measure and control humidity to maintain a level of 40-60% RH. This is for health and comfort reasons as well as the protection of computer systems and the avoidance of structural or mould-related issues in the building.


Temperature measurements do not generally suffer from drift, but traditional humidity sensors do, so products such as Vaisala’s HUMICAP sensors are preferable because of their long-term stability and insensitivity to interferences, for example dust and condensation. Increased humidity levels can be an


indication of human activity and poor ventilation. However, humidity varies considerably as a result of external factors– freezing dry conditions or rainy humid conditions – rather than as a result of human exhalation.


To summarise, temperature and humidity


monitoring play an important role in the optimization of a BMS, but where facility managers need to take into account the occupancy of people and reduce human- generated pollution in spaces, CO2 is the ideal additional parameter for automatic ventilation control.


www.acr-news.com


May 2021 17


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