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Sunday, June 16, 2024

Airborne transmission and household ventilation: An informed discussion needed

A lot has been said about the need for good ventilation for prevention of COVID. Undoubtedly, good ventilation is vital for preventing build-up of high levels of SARS-COV2 in crowded / public places. Weddings have acted as super-spreader events as hundreds of people were cooped up indoors together for several hours, often in poorly ventilated or air-conditioned halls. However, does the same principle apply to residential homes? Is leaving your doors and windows open a sound strategy against prevention of COVID in family members?

This is a complex subject that needs more discussion especially since it is accepted that SARS-COV2 indeed has airborne mechanism of spread. Social distancing alone is no longer a solution. This must-read article describes the struggle researchers in the West faced for close to a year before the scientific community finally accepted that SARS-COV2 indeed was airborne. This was in spite of the fact that Chinese researchers had pointed out airborne transmission as early as June 2020.

My goal by writing this post is to stimulate a healthy discussion on how air flows transmit or hinder the transmission of airborne diseases.

Misunderstood guidelines

(Pic source: CDC guidelines on ventilation)

Many of the guidelines have not been understood properly. For example, the guidelines issued by the CDC in USA for improving ventilation at home specifically say that the improved ventilation at home is needed “if a visitor needs to be in your home”. Otherwise, “staying at home with members of your household” is the best way to avoid COVID.

In other words, if there is a suspected COVID or a known COVID patient in your house (and that too during the phase of active infection), it would be better to keep the windows open and the home well ventilated to reduce the viral load at home and safeguard all other occupants. But with the windows open, the breeze would definitely carry the virus out of the room and into the environment – and probably into the home of your unsuspecting neighbor, if the wind direction is favourable.

One can easily understand now, that the guidelines have been made keeping in mind the possibility of an infected person being indoors to reduce risk for other occupants and not really to account for the likelihood of the disease entering through the same open window to infect occupants of the house. This is probably because we had not factored in the “airborne” hypothesis at the time these guidelines were written.

Studies on effect of environmental factors on airborne transmission

There are numerous scientific papers and articles showing how wind patterns affect the spread of infectious diseases including COVID 19. A study from University of Texas showed that SARS-COV2 could survive outdoors for over 30 minutes and be transmitted for distances up to one mile in low wind conditions. However, if the wind was too strong, the viral particles would be rapidly dispersed and likelihood of transmission was greatly reduced.

Another study suggested that calm, cold, dry and overcast conditions were favourable to the transmission of COVID-19. A computational model suggested that if there was no wind, transmission would be negligible but a gentle wind of 4-15kmph could carry droplets over more than 6 meters in just five seconds. A study from India showed that (aerosol optical depth) AOD actually increased during lockdown period of March-April 2021 due to stagnant circulating climatic conditions.

The wind direction too plays an important role. For example, if the wind is blowing into an infected person, the likelihood of transmission decreases dramatically but if the wind is blowing from behind the person, the viral particles would be carried over much longer distances.

The location of the home also matters. It is well known that transmission is higher in densely populated areas and reduces in sparsely populated areas, although the mortality is lower in densely populated areas.

Plume Dispersion

The “plume dispersion” model was originally used to study pollution, but it was found that the patterns applied to dispersal of infectious organisms too. The wind direction, speed, temperature and humidity collectively interact in producing a variety of spread patterns or “plumes”. Higher the atmospheric humidity, lower is the outdoor transmission as the aerosolized droplets become heavier.

During dry conditions, the “plume” containing the viral particles can go higher in the atmosphere and pass over people’s heads, but settle lower during wet conditions and expose more people to risk. These are complex but important concepts that cannot be reduced to one-line guidelines. Those scientifically inclined to study such factors can read this detailed paper on the subject.

An Informed Discussion Needed

In summary, the following points need to be considered while discussing the issue of household ventilation during pandemic situations:

  1. An “open windows for reducing risk” advice in summer and winter, when wind speeds dip substantially, may put the occupants of the house to higher risk especially if the home is in a containment zone.
  2. If you have a strong wind blowing, it would help clean micro-organisms from your house. But if the breeze is light or non-existent, viral particles can more easily waft into your home along with the breeze and settle there.
  3. “Plumes” play an important role in transmission. The wind direction determines where the spread occurs. It may carry the micro-organisms to great heights or push it down closer to the ground, depending on a variety of constantly changing environmental circumstances.
  4. People are off-guard at their own home and not really expected to wear masks. To give it a perspective, keeping home windows open in a crowded locality would be much like walking around outdoors without a mask – something most people would consider an unacceptable risk.
  5. Household air conditioning is not a problem if none of the family members are infected, but air conditioned public spaces can act as enablers for spread of disease.

While formulating policies for advising the population at large on household ventilation during a pandemic, it is better to be specific with regard to social, climatic and environmental concerns rather than issue blanket advisories that may or may not be applicable at that time and place.

The bottom line is that we can only do risk reduction against an invisible enemy, and risk elimination is not possible without hurting your health in some other unintended way. I am not advising people not to keep their windows open out of fear of contracting COVID. A well ventilated home that lets in fresh air and adequate sunlight is essential for good health. If the number of active cases in vicinity is low and environmental conditions are favourable, it makes perfect sense to keep the windows open.

But it is certainly debatable whether one can reduce their risk by keeping windows open all the time during a pandemic wave when environmental viral load is high, or in a containment zone with several actively infected persons in the vicinity. Some prudence is needed in taking these calls based on common sense approach and individual risk assessment.

(This article was first published on the author’s blog on June 7, 2021 and has been republished here with consent.)

(Featured image source: CDC guidelines on ventilation)

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Dr. Amit Thadani
Dr. Amit Thadani
MBBS MS FMAS, Practising surgeon, musician, social activist, amateur photographer. Consulting General & Laparoscopic Surgeon Director, Niramaya Hospital, Kharghar, Navi Mumbai Consultant, Sushrut Hospital & Research Center, Mumbai Consultant, Apollo Spectra Hospital, Mumbai


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