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Air do we go from here?
In the coming weeks it looks like we will begin the return to normality and to start occupying the spaces that have been the reserved for the essential workforce. As the built environment begins to take higher occupancy levels there is a need to deal with the risk of a person-to-person transmission of Coronavirus.
It is generally accepted that when a person coughs/sneezes/talks the Coronavirus pathogen is spread in one of two ways. The expelled droplets either behave ballistically and land on another person/surrounding surface or the droplets are aerosolised and are subject to air-buoyancy effects. The later of these mechanisms is where HVAC systems play a key role in controlling infection.
Given the novelty of the Coronavirus there still exists considerable conversation over how best to manage the ventilation systems of occupied spaces in order to reduce this risk. I have collated below the information from key groups (CIBSE, PHE, REHVA and ASHRAE). As you can see there exists conflict around topics such as RH and ventilation rates. I would be interested to know if there are any other individuals who are dealing with the same risk and having to implement a new or revised HVAC/building utilities strategy.
| CIBSE: CoVID-19 Ventilation Guidance (Summary) v1 9 May 20 Well ventilated rooms dilute pathogens and poorly ventilated areas carry a higher risk of airborne transmission Fine aerosols (from coughing and sneezing) remain airborne for several hours Duct cleaning has little or no effect on the prevention of spread of pathogens Specific guidance: Open doors and windows as much as possible to increase natural ventilation. Keep WC windows open as much as possible Maintenance staff to wear adequate protection when changing filters on A/C units Avoid recirculation where practicable Heat recovery devices in AHUs to be turned off |
| Public Health England: Infection prevention and control guidance – 27 Apr 20 “In light of limited data for SARS-CoV-2, evidence was assessed from studies conducted with previous human coronaviruses including MERS-CoV and SARS-CoV. Human coronaviruses can survive for up to 5 days at temperatures of 22 to 25°C and RH of 40 to 50%. The rate of clearance of aerosols in an enclosed space is dependent on the extent of any mechanical or natural ventilation – the greater the number of air changes per hour (ventilation rate), the sooner any aerosol will be cleared. The time required for clearance of aerosols, and thus the time after which the room can be entered without a filtering face piece (class 3) (FFP3) respirator, can be determined by the number of air changes per hour (ACH) as outlined in WHO guidance.” Specific guidance: “…A single air change is estimated to remove 63% of airborne contaminants, after 5 air changes less than 1% of airborne contamination is thought to remain. A minimum of 20 minutes (recommend 2 air changes) in hospital settings where the majority of these procedures occur is considered pragmatic” |
| REHVA: CoVID-19 Guidance document (Summary) – 3 Apr 20 Large and small particles (aerosols) are ejected from an infectious person. It’s the latter which are of concern for HVAC systems. Well ventilated rooms dilute pathogens. Poorly ventilated areas carry a higher risk of airborne transmission Heat recovery devices in AHUs usually work poorly and leak. Areas which have comfort cooling or heating need intervention. RH has little or no effect virus susceptibility. Specific guidance: At nights and weekends, do not switch ventilation off, but keep systems running at lower speed Ensure regular airing with windows (even in mechanically ventilated buildings) Avoid open windows in toilets to ensure the right direction of ventilation. Mechanical ventilation to run 24/7. Instruct building occupants to flush toilets with closed lid Switch air handling units with recirculation to 100% outdoor air or 400 ppm max of CO2 Do not plan duct cleaning for this period Regular filter replacement and maintenance works must be performed with PPE including respiratory protection Maintain social distancing measures to guard against person-to-person transmission |
| ASHRAE: Position document on infectious aerosols (Summary) – 14 Apr 20 “…Even the most robust HVAC system cannot control all airflows and completely prevent dissemination of an infectious aerosol or disease transmission by droplets or aerosols. Ventilation with effective airflow patterns is a primary infectious disease control strategy through dilution of room air around a source and removal of infectious agents. However, it remains unclear by how much infectious particle loads must be reduced to achieve a measurable reduction in disease transmissions.” Specific guidance: Air distribution patterns have an effect on distribution of aerosols Differential room pressurization can influence air flows Higher efficiency filtration (MERV-13) reduces concentrations of aerosols Increase outdoor ventilation as close to 100% as possible, 24/7 UV light with 265nm wavelength provides a good germicidal effect RH of between 40 and 60 RH is optimal for reducing microorganism survival Bypass energy recovery systems |
| World Health Organisation – Getting your workplace ready for CoVID-19 (Summary) – 3 Mar 20 No specific guidance on HVAC systems or strategies is provided in relation to COVID-19. |
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