News outlets have reported some recent research on airborne viral spread and the World Health Organization has warned of emerging evidence that the coronavirus may spread through the air via “microdroplets”.

In a scientific brief issued on 9th July the WHO advise that larger respiratory droplets produced when a person coughs or sneezes are still likely to be the main way that SARS-CoV-2, the virus that causes COVID-19, is transmitted. However, they highlight the remaining uncertainty over the contribution of very small airborne particles to the spread of the virus. The concern is particularly relevant to closed, unventilated spaces when aerosol-generating devices create small microdroplets that can circulate in the air. Other experts from around the globe have also warned that the effect of this aerosol transmission has been underestimated, and called for more widespread use of face masks to control infection spread.

The WHO has advised the use of fabric masks ‘in closed, overcrowded spaces,’ alongside other infection control measures such as physical distancing, handwashing and environmental disinfection. This may help to protect others if the wearer is carrying the virus, even if asymptomatic themselves. The UK government advised on 13th July that wearing of face masks/coverings in shops in England will be mandatory.


Where did the story come from?

On 9th July 2020, the World Health Organization (WHO) issued a scientific brief on the different modes of transmission of SARS-CoV-2. Recent studies published in journals such as the Clinical Infectious Diseases and PNAS have also covered airborne transmission of the virus.

What is the basis for the claim?

The WHO brief explains that aerosol particles may result either from normal exhaled breath or can be generated from the evaporation of larger respiratory droplets (e.g. when someone coughs). These airborne particles can be suspended in the air for long periods and spread over long distances. In laboratory experiments where aerosol particles carrying SARS-CoV-2 were distributed using a nebuliser, the virus was detectable in air samples for between 3 and 16 hours. The WHO caution that this may not reflect real-life situations. Tests of the air found in healthcare facilities caring for people with COVID-19 have given inconsistent results. Some found the detectable virus in air samples, while others found none – and importantly none found viable virus that could infect people.

Essentially the ‘infectious dose’ of aerosol particles in COVID-19 remains unknown. The WHO still consider that viral transmission occurs via respiratory droplets through close contact with an infected person.

Other experts have highlighted concerns over airborne transmission. An international group of experts writing in the journal Clinical Infectious Diseases ‘appeal to the medical community and to the relevant national and international bodies to recognize the potential for airborne spread of COVID-19.’ They stated that studies had ‘demonstrated beyond any reasonable doubt’ that microdroplets small enough to remain aloft pose a risk at distances of several metres from the infected individual, and urged the use of protective equipment.

Meanwhile, US researchers published in the journal PNAS looked at viral transmission rates in China, where ‘aggressive measures’ including mandatory face masks in public were instituted. They rightly acknowledge that the effectiveness of these individual measures has not been rigorously evaluated. However, they put forward the theory that the unpopularity of face masks in the western world could underlie the consistent number of cases in places such as Italy and the US, compared with the steady decline in cases in China.

What do trusted sources say?

The WHO conclude that the evidence suggests that transmission of SARS-CoV-2 occurs mainly through respiratory droplets – either close contact with an infected person or from surfaces contaminated with them.  However, they say that airborne transmission of the virus can occur in health care settings where aerosol generating procedures are used. Some outbreak reports related to indoor crowded spaces have suggested the possibility of aerosol transmission for example, in restaurants or in fitness classes.

Among other infection control measures, they advise ‘wear fabric masks when in closed, overcrowded spaces to protect others.’

The latest UK government guidance on face coverings dates from 24th June when they advised face coverings were compulsory on public transport and when attending hospital. This is likely to be updated to reflect new recommendations for compulsory face masks in shops.

Analysis by EIU Healthcare, supported by Reckitt Benckiser



  1. Morawska L, Milton DK. It is time to address airborne transmission of covid-19. Clinical Infectious Diseases. 2020 Jul 6.
  2. Zhang R, Li Y, Zhang AL, Wang Y, Molina MJ. Identifying airborne transmission as the dominant route for the spread of COVID-19. Proceedings of the National Academy of Sciences. 2020 Jun 11.

Reading list

  1. World Health Organization. Transmission of SARS-CoV-2: implications for infection prevention precautions. Scientific brief, 9 July 2020.
  2. UK. Coronavirus (COVID-19). Guidance: Staying safe outside your home. Updated 24 July 2020