There’s Something in the Air, Part 2 – But It’s Not a Miasma

In a previous post, I examined the claims of advocates of interventions to reduce airborne transmission of COVID-19 that they were the heirs to a long tradition of work on ‘bad air.’ In context, however, ancient Greek and Roman writers meant something very different from the present notion of infective particles moving through space on air currents. The same is true of miasmatic theory, which is also claimed as a precursor of the movement to reform ventilation standards. This was not, as some claim, unjustly dispensed with in the latter part of the 19th century but displaced, after a long struggle, by germ theory, which offered a more useful account of infectious disease.

Greek and Roman writers saw the nature of air, and water, as aspects of the physical geography of a place. They were a potential threat to those who had not grown up in that location, like travelers and immigrants, and were unaccustomed to its specific environment. By the early nineteenth century, however, the notion of space had changed somewhat. Although the original idea was retained as an explanation of why migrants from the countryside into the slum areas of industrial cities suffered from an apparent excess of diseases, there was also a more micro view that buildings should be well-ventilated, at least by day, to prevent the accumulation of miasmas. The risks of damp, evil-smelling air from swamps and marshes had been extended to any source of bad odors – rotting vegetation, decaying corpses of animals or humans, the fumes from industrial and human waste. The hazard, however, was the vapor or the particles of decaying matter carried through the air and recognizable by the smell rather than a distinct biological agent (germ).

Public health interventions based on miasmatic theory could, of course, be effective for the wrong reasons. Paving and cleaning city streets, constructing sewerage systems or relocating noxious trades away from residential areas also removed potential sources of germs. This accounts for some of the resistance to germ theory – miasmatism worked to some extent and was seen to have triumphed over previous theories of infectious particles (contagium animatum) which had developed since the 16th century. These particles were not living entities, like germs, and were transmitted only over short distances by direct contact or immediate inhalation from an infected person. As such, that theory did not account very well for the appearance and disappearance of apparently unconnected disease outbreaks separated by long distances. It was also associated with socially and economically disruptive quarantine policies that attempted to control local transmission.

Miasmatic theory might require costly measures, as in the construction of the London sewerage system after the Great Stink from the River Thames during the summer of 1858. British legislators simply could not bear the smell that permeated the Houses of Parliament and approved loans to fund the work. However, this created opportunities for private contractors and employment in a wide variety of trades over a long period rather than damaging industry and commerce with closures and restrictions on movement.

The contest between theories of contagion and theories of miasma appeared to have been settled in favor of the latter by the 1850s. It is important to remember this when figures like Florence Nightingale are invoked to legitimize beliefs in miasma. Her work during the Crimean War (1854-55) preceded by a decade the publications of Pasteur and Koch that introduced germ theory. She could not have operated on any other basis. Her correspondence from the 1880s shows that she had accepted germ theory in step with most other leaders in public health. Nightingale’s efforts to remedy the emissions from a cracked and blocked sewer under the hospital at Scutari might be placed alongside John Snow’s studies of cholera outbreaks in London (1854-59) as a pivotal moment in reopening the contest.

In practice, especially in colonial public health, miasmatic thinking continued to be important until the very end of the 19th century. It informed the design of hospitals, where ‘Nightingale Wards’ were constructed with high ceilings and large windows to promote cross-ventilation. Many other public buildings, notably the elementary schools built after legislation in 1870 paved the way to compulsory education, incorporated the same principles.

Despite its ultimate triumph, however, germ theory has left several puzzles. While Robert Koch’s postulates serve to verify the link between a specific organism (bacteria, virus, etc.) and a specific infection under experimental conditions, they do not address questions of transmission. What does it actually take for infection to occur in the wild rather than in the lab?

Even in John Snow’s time, critics observed that some people could drink contaminated water and not develop cholera. Today, with a much greater understanding of variations in genotypes and immune responses, different responses to exposures remain difficult to explain. This is particularly the case for airborne infections. It is one thing to demonstrate the movement of particles under experimental conditions, and build models on that basis. It is another to show that this translates into the wider world with its infinite variability. The controls that work in experiments are difficult and costly to implement compared with established human practices like preferred social distances with intimates and strangers. Mostly, they lack validation from human challenge experiments that actually attempt to infect participants rather than simply describing the movements of particles.

There are many good reasons to ensure high standards for ventilation in public, commercial, and domestic spaces. However, ever more stringent standards inevitably come at a cost. This may be justifiable for high-risk areas like operating theatres or confined spaces like aircraft cabins. As a mass measure, however, filtration to the degree necessary to trap particles as small as viruses lacks the evidence of benefit that would make it an appropriate investment. The case will not be advanced by selective and contentious readings of the history of biomedical science and public health.