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covid19 is airborne
4 April 2020

John Herbert, hong kong energy saving expert, BEAM expert
John Herbert

There is ample scientific and anecdotal evidence demonstrating that the novel coronavirus covid-19 is airborne, that means it can be transmitted between people 7 metres apart. Also the aerosol (water droplet with virus) may settle on surfaces more than two metres from an infected person.

aerosol, particle size compared, coronvirus sizeclick for larger image

Tiny particulates can float along the air, for example fine dust, aerosol (water droplets), even mould spores, for long periods of time indoors travelling upto 7 metres from the host[1] and will be carried in the direction of the airstream, whether the airflow is created by opening a door, or created through the normal operation of HVAC equipment, including air conditioning units, air grilles and diffusers, or fans.

Hong Kong HVAC air filters are typically wire mesh type filters (MERV 6) and DO NOT prevent the circulation of covid-19 in air conditioning or ventilation systems.

An aerosol (tiny water droplets carrying the covid-19 virus) in the air are approximate 5-10 micrometres in diameter, and they can carry duct, dirt, bacteria or covid-19 virus. To prevent circulation finer grade filters are need to capture those tiny particles.

If the air filters are blocked or partial blocked, it is possible that the aerosol may be trapped on the surface of the air filter, therefore, air filters must be handled with extra precautions.

Indoors, if the air and the aerosol in a room is left to settle, eventually those aerosols (water droplets) will land on various surfaces in the room, for stainless steel or plastic materials the best advice indicates that the covid-19 virus remains viable on the surface for around 72 hours[2].


1. Bourouiba L. Turbulent Gas Clouds and Respiratory Pathogen Emissions: Potential Implications for Reducing Transmission of COVID-19. JAMA. Published online 26 March 2020. doi:10.1001/jama.2020.4756 []
2. Aerosol and Surface Stability of SARS-CoV-2 as Compared with SARS-CoV-1, NEJM, 17 March 2020, doi: 10.1056/NEJMc2004973, []
3. Air, Surface Environmental, and Personal Protective Equipment Contamination by Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) From a Symptomatic Patient, JAMA, 4 March 2020. doi:10.1001/jama.2020.3227 []
4. COVID-19 Choir Outbreak, LA Times, 29 March 2020 []
5. Diamond Princess, Crew sealed up the corridor air grilles on the Diamond Princess Cruise ship at night, reported by guest.
6. SCMP Coronavirus can travel twice as far as official ‘safe distance’ and stay in air for 30 minutes, Chinese study finds, 11 March 2020.[] (study later withdrawn without reason)
7. Lu J, Gu J, Li K, Xu C, Su W, Lai Z, et al. COVID-19 outbreak associated with air conditioning in restaurant, Guangzhou, China, 2020. Emerg Infect Dis. __ July 2020 [prepublished 2 April 2020],

updated: 6-4-2020

About the Author

John Herbert is a veteran chief engineer with more than 30 years international engineering experience, educated in the United Kingdom, he has worked in the United Kingdom and then across Asia for more than two decades engaged by international and local companies. He is a Hong Kong Registered Energy Assessor (REA), a BEAM Professional, and stationed in Hong Kong.


tags: airborne, air transmission, air conditioning, air conditioning units, indoors, spread indoors, COVID-19, COVID19, HVAC, fans, airflow, Hong Kong

date: 15-04-21
kelcroft consultants hong kong