Coronavirus and Open Windows: some Science from the Cold War

by Richard Hobday

In the 1960s, scientists at Porton Down proved outdoor air is germicidal. What does that tell us now about opening our windows?

During the great influenza pandemic, the order went out to open windows. A century on, and the same recommendation is being made again. But there is a difference. Back then, rather more importance was attached to the germicidal effects of fresh air than is the case today. That’s why old hospital wards and tuberculosis sanatoria often had big windows on both sides. They could be cross-ventilated. At the time, scientific evidence for the benefits of this approach was limited. But it was thought that high ventilation rates not only diluted and dispersed germs in hospital wards, but also disinfected.

This photograph was taken in Cincinnati, Ohio in 1918. Trolley car windows were kept open to prevent the spread of influenza. The practice of keeping windows open was nationwide. (National Archives)

Open Windows

In common with the 1918 influenza virus, the coronavirus that causes Covid-19 seems to spread most effectively in indoor spaces where exposure is prolonged. This may be one of the reasons medical staff are so susceptible to it. [1] But much remains unknown about its natural history. Clearly government health policy, such as self-isolating and staying away from other people when outdoors, must be followed.

A high standard of hygiene is essential — above all, regular hand washing. This is what Florence Nightingale advocated in 1859, in her classic `Notes on Nursing’, at a time when life-threatening infections were common in hospitals. Miss Nightingale insisted on access to sunlight and fresh air wherever the sick were being cared for. She wrote that the `first canon’ of nursing was that air breathed indoors had to be as fresh as it was outside — without chilling the patient.

`Always air from the air without, and that, too, through those windows, through which the air comes freshest.’ [2]

Perhaps she was referring to what later became known as the Open Air Factor. After all, old-style Nightingale wards were cross-ventilated. But we can’t be sure. It was some time later, in the 1960s, that the Open Air Factor was discovered and got its name. And that name did not come from the world of medicine.

What is the Open Air Factor?

In the 1960s, during the Cold War, the Ministry of Defence was getting Britain ready to face a biological weapon attack. Scientists at its research centre at Porton Down, Wiltshire started a series of experiments to find out if airborne viruses and bacteria stay viable when released into the environment. Tests began with a harmless strain of the E. coli bacteria. The research team compared the survival times of E. coli samples out in the open air with those of samples in a closed vessel containing clean air. These tests had to be carried out at night because E. coli are rapidly killed by sunlight. [3]

E. coli exposed to outside air usually died off rapidly. In some cases samples in free air, in the dark, lost viability in just 30 minutes. By contrast, those in enclosed air at the same humidity and temperature survived for several hours. [4] Samples of more dangerous bacteria, such as group C Streptococcus and Franciscella tularensis, were tested and lost viability at similar rates to E. coli. [5] Following these discoveries, the scientists involved coined the term `Open Air Factor’ or OAF. They used it to describe the properties of outdoor air that reduced the survival of airborne pathogens, their infectivity, and their transmission through the air. [6] They couldn’t identify what the agent, or agents, involved were. No equipment then available was sensitive enough to identify and measure the OAF in the air. But one thing they did discover is that viruses are very sensitive to the Open Air Factor. [7]

The Influenza Virus and Open Windows.

The viability of the influenza virus was tested at Porton Down, and the results lend support to a belief widely held during the 1918 pandemic. It is that the risk of contracting influenza indoors is far higher than the risk of catching it outside. [8] Also, the Porton Down research team found they could preserve the germicidal properties of outdoor air in containers if they ventilated them at high rates. [9] Whether these properties can be retained in buildings at comparable rates is not known. No one appears to have done the tests. But from the Porton Down experiments it seems ventilation rates of 30 air changes per hour, or more, may be needed to fully retain the OAF’s ability to kill germs. These ventilation rates are — by accident or design — similar to those achievable in old cross-ventilated hospital wards with all the windows open. [10] Significantly, case studies suggest that cross-ventilation was effective in controlling the spread of infections in hospitals during the SARS coronavirus outbreak in 2003. [11]

Whether the more modest rates typical of modern buildings keep any of the OAF’s properties active is another unknown. But keeping windows open, and encouraging a flow of air through a building, should dilute and disperse germs. The air may also disinfect the indoor environment to some extent, as was thought to be the case 100 years ago. And possibly by Florence Nightingale some time before that.

References

1. Wang J, Zhou M, Liu F. Exploring the reasons for healthcare workers infected with novel coronavirus disease 2019 (COVID-19) in China, J Hosp Infect, https://doi.org/10.1016/j.jhin.2020.03.002.
2. Nightingale F. Notes on Nursing; What It Is, and What It Is Not. New York: Dover Publications; 1969. p13.
3. Druett HA, May KR. Unstable germicidal pollutant in rural air. Nature 1968;220:395–396.
4. Druett HA, May KP. The open air factor. N Sci 1969;41:579–581.
5. May KP, Druett HA, Packman LP. Toxicity of open air to a variety of microorganisms. Nature 1969; 221:1146–1147.
6. Cox CS. The open air factor. In: The aerobiological pathway of microorganisms. Chichester: John Wiley & Sons; 1987. p. 218–229.
7. Benbough JE, Hood AM. Viricidal activity of open air. J Hyg 1971;69:619–626.
8. Hood AM, Stagg AJ, Willis HI. Survival of airborne influenza virus in open air. First International Conference on Aerobiology, Munich. Berlin: Erich Schmidt Verlag; 1978. p. 257–262.
9. Hood AM. An indoor system for the study of biological aerosols in open air conditions. J Hyg 1971;69:607–617.
10. Hobday RA. The open-air factor and infection control. J Hosp Infect 2019;103:e23-e24
11. Qian H, Li Y, Seto WH, Ching P, Ching WH, Sun HQ. Natural ventilation for reducing airborne infection in hospitals. Build Environ 2010;45:559e65.