Aerosol transmission of human pathogens: From miasmata to modern viral pandemics and their preservation potential in the Anthropocene record

Abstract

Ongoing uncertainty over the relative importance of aerosol transmission of COVID-19 is in part rooted in the history of medical science and our understanding of how epidemic diseases can spread through human populations. Ancient Greek medical theory held that such illnesses are transmitted by airborne pathogenic emanations containing particulate matter ("miasmata"). Notable Roman and medieval scholars such as Varro, Ibn al-Khatib and Fracastoro developed these ideas, combining them with early germ theory and the concept of contagion. A widely held but vaguely defined belief in toxic miasmatic mists as a dominant causative agent in disease propagation was overtaken by the science of 19th century microbiology and epidemiology, especially in the study of cholera, which was proven to be mainly transmitted by contaminated water. Airborne disease transmission came to be viewed as burdened by a dubious historical reputation and difficult to demonstrate convincingly. A breakthrough came with the classic mid-20th century work of Wells, Riley and Mills who proved how expiratory aerosols (their "droplet nuclei") could transport still-infectious tuberculosis bacteria through ventilation systems. The topic of aerosol transmission of pathogenic respiratory diseases assumed a new dimension with the mid-late 20th century "Great Acceleration" of an increasingly hypermobile human population repeatedly infected by different strains of zoonotic viruses, and has taken centre stage this century in response to outbreaks of new respiratory infections that include coronaviruses. From a geoscience perspective, the consequences of pandemic-status diseases such as COVID-19, produced by viral pathogens utilising aerosols to infect a human population currently approaching 8 billion, are far-reaching and unprecedented. The obvious and sudden impacts on for example waste plastic production, water and air quality and atmospheric chemistry are accelerating human awareness of current environmental challenges. As such, the "anthropause" lockdown enforced by COVID-19 may come to be seen as a harbinger of change great enough to be preserved in the Anthropocene stratal record.

Keywords: Aerosol disease transmission; Anthropocene record; COVID-19; Coronavirus; History of epidemics.

Graphical abstract

Fig. 1

Examples of transmission clusters of viral epidemic diseases in buildings. A) Meschede Hospital, Germany 1970 smallpox outbreak, attributed to virus-bearing particles moving on air currents from index patient to infect others on all three hospital floors (Wehrle et al., 1970, Gelfand and Posch, 1971). B) Amoy Gardens residential apartments, Hong Kong 2003 outbreak of SARS-CoV-1 where infection was attributed to virus-bearing particles escaping through floor drains from the sewage-system into bathrooms and then driven as aerosols by ventilation fans into light wells serving the tower block (Yu et al., 2004;Li et al., 2004b;McKinney et al., 2006, Lee, 2012, ; photo: WMwiki, CC By Creativecommons.org/licenses/by/3.0> via Wikimedia Commons. https://commons.wikimedia.org/wiki/File:Amoy_gardens_2017.jpg). C) Apartment building in Seoul, South Korea in 2020 SARS-CoV-2 outbreak where movement of virus-bearing particles through vertical air ducts serving bathrooms was implicated in COVID-19 transmission (Hwang et al., 2021). D) Apartment building in Bilbao, Spain in 2021 SARS-CoV-2 outbreak where the viral transmission mode remains unclear (Galloway et al., 2021). https://english.elpais.com/society/2021-02-15/architecture-of-an-outbreak-the-spanish-apartment-building-hijacked-by-the-coronavirus.html.

Fig. 2

Cholera tramples the victor and the vanquished both: Title of a 1831 sketch by the famous illustrator Robert Seymour during the second 19th century cholera pandemic which spread west from Russia during the Polish Russian War 1830–31. Note the depiction of a black miasmatic cloud accompanying the figure of death: prevailing miasma theory did not adequately explain cholera transmission and there was much debate and speculation at the time amongst the medical and scientific community. Image in the collection of The London Library of Medicine, London. (Wikipedia Commons).

Fig. 3

Hypotheses born before their time are often sterile. Both Filippo Pacini and Joachín Balcells separately observed the cholera bacillus in 1854 during the third 19th century cholera pandemic but the subject received little attention until the rediscovery of the microrganism by Robert Koch in 1884. Top left: one of a series of publications on cholera by Pacini (Source: The Historical Medical Library of The College of Physicians of Philadelphia. Pacini F. Du Choléra Asiatique. Tr. Janssens E. Brussels: Librairie Médicale de. H. Manceaux, 1865); lower left: Pacini’s 1854 microscope slide of the cholera bacillus (Wikipedia Commons); top right: the Spanish publication in which Balcells reported his experimental observations; lower right: translation of the front page editorial announcing Balcells’ discoveries.

Fig. 4

From left to right, William Firth Wells (1886–1963), Richard L. Riley (1911–2001), Cretyl Inez Mills (née Crumb) (1919–1990). These three scientists were key to proving that tuberculosis can be transmitted via aerosols (see text for details). Kind permission to reproduce the Wells and Riley photos has been given by Professor Edward Nardell who was given them by the Riley family after 2001. Nardell writes of Cretyl Mills: Riley told me that she was writing the history of their classic aerosol studies when she died but her manuscript's location is unknown. Riley (also) told me that she deserved a lot of credit for keeping track of a huge amount of data that those studies generated.

Fig. 5

The similarities in messaging around the COVID-19 pandemic are striking (Randall et al., 2021). North American health posters and newspaper headlines during the 1918–20 Influenza A H1N1 pandemic. Although emphasis by the health authorities was based on disease transmission by near-range “Flügge droplets”, note a clear awareness of the importance of good ventilation (sources: Library of Congress https://www.loc.gov/item/rbpe.24101900/; Tuberculosis and the Cincinatti Irish by Jessica Heskett: https://libapps.libraries.uc.edu/exhibits/irish-cincinnati/cincinnati-irish-births-and-deaths/tuberculosis-and-the-cincinnati-irish/; newspapers in Chicago, Wikipedia Commons).

Fig. 6

Virologist June Almeida (née Dalziel Hart), the first person to observe and describe human coronaviruses. The photo shows her in the 1960s using a Philips EM300 electron microscope while working at St Thomas's Hospital Medical School, London. Reproduced by kind permission of Joyce Almeida. See https://www.whatisbiotechnology.org/index.php/people/summary/Almeida.