Carl Zimmer on the Hidden Life in the Air We Breathe

Carl Zimmer — science writer, New York Times columnist, and author of Air-Borne — joins Tyler Cowen for Ep. 235 to trace the history of airborne disease, from the miasma theory of ancient Greece to the WHO’s delayed acknowledgement of COVID-19’s aerosol transmission, and to explore the hidden microbial world that drifts through every breath we take.

Key ideas

1. The idea that air carries disease was resisted for centuries, not through ignorance but through competing consensus. Miasma — the notion that corrupted air itself caused illness — held scientific authority from Hippocrates into the early twentieth century. When germ theory arrived in the 1880s, it dismantled miasma only to replace it with a new orthodoxy: diseases spread by water, food, or contact, and the air was exonerated. That replacement consensus became the inertia public health carried into COVID-19.
2. William and Mildred Wells demonstrated airborne transmission in the 1930s, and were almost entirely ignored. The Wells team showed, by experiment, that measles spread through the air and that ultraviolet lamps could prevent outbreaks in schools. Their work was absorbed into classified biological-warfare research during the Second World War — what Wells called ‘the suicide of bacteriology’ — and the findings never re-entered mainstream public health.
3. The WHO and CDC delayed acknowledging COVID’s airborne spread because of institutional path dependence, not bad faith. Airborne transmission carries a far more demanding response than surface or droplet transmission: it cannot be solved by hand-washing or wiping surfaces, it requires ventilation, filtration, and ultimately indoor-air standards. Acknowledging it meant committing to interventions public health agencies were not ready to mandate.
4. Indoor air quality is a tractable engineering problem with proven tools. Ventilation, HEPA filtration, and ultraviolet germicidal irradiation (UV-C light that destroys airborne pathogens without exposing people) are all available now. Carbon-dioxide sensors — whose scientific basis dates to the 1850s — give a real-time proxy for how much air a room’s occupants are re-breathing. The constraint is standards and coordination, not technology.
5. The aerobiome — the community of living things in the air — extends far beyond pathogens. Pollen travels thousands of feet up and hundreds of miles on the wind; cloud droplets contain millions of microbes per cubic metre; and it is even plausible that life in Venus’s upper atmosphere persists in sulphuric acid clouds, the way Earth’s clouds host microbes that cycle in and out. The air is an ecosystem, not a void.

Content

Why airborne disease took so long to accept

Zimmer opens by inverting the assumption that scientific blindness is unusual. Great ideas, he argues, ‘can just sit around for a long time waiting to be discovered’ — Darwin is his example, where the evidence preceded the synthesis by decades. The specific obstacle for airborne disease was that the air already had a scientific story: miasma. Hippocrates had explained epidemic clusters (many people in one place falling ill at once) by supposing the air had ‘gone bad,’ and that framework was sophisticated enough to generate predictions and survive two millennia of scrutiny.

When germ theory arrived in the 1860s–1880s — bacteria in the water for cholera, mosquitoes carrying viruses for yellow fever — it defeated miasma precisely by assigning each disease a specific, non-aerial route. The lesson drawn by early-twentieth-century public health authorities was that the air could safely be set aside. “Some leading public health experts in the early 1900s just said, ‘All this concern about the air — this is just this obsolete miasma concept. Just rest easy, just put it aside.’” That dismissal, Zimmer notes, was ‘in print’ and became a strong consensus. Even the most prominent experiments of the 1930s were conducted against this headwind.

The Blackley episode and the structure of scientific proof

Charles Harrison Blackley, a British doctor in the 1860s, noticed that walking past hay fields triggered his hay fever, rubbed grass pollen into his nose and sneezed, and then asked where the pollen went. He built a kite, flew it over 300 metres (1,000 feet), and retrieved pollen grains — even when the wind was coming from the sea, meaning pollen had crossed the ocean. The scientific community dismissed him. Hay fever, they decided, was caused by bacteria or was a neurosis.

Blackley’s problem, Zimmer says, was the structure of proof. ‘Science really works by numbers. You actually have to study lots and lots of people, and then, if you’re going to try to tie people’s symptoms to the environment in some way, well you’re actually going to have to track them over months, years.’ One man flying a kite and sneezing cannot satisfy that standard, no matter how clever the experiment. It was not until the early 1900s — after Blackley died — that immunologists realised hay fever was the immune system mounting an attack on an inhaled allergen (a substance that triggers an overreaction), not a microbial infection at all.

The military capture of aerobiology

The field of aerobiology was effectively named in 1937 by Fred Meier, a researcher Zimmer describes as positioned to ‘lead aerobiology into the modern age.’ Meier died in a plane crash, and the field’s subsequent growth was absorbed by the military. Fort Detrick (then Camp Detrick) classified most of what was known about how pathogens travelled through the air, and developed that knowledge for biological weapons rather than public health.

William Firth Wells, the scientist who had pioneered indoor airborne transmission research, discovered after the war that his foundational ideas had been weaponised without his knowledge. He described the effect on science as ‘the suicide of bacteriology.’ Zimmer draws a direct line from this militarisation to the public-health posture that persisted into COVID-19: instead of treating indoor air as a public good to be managed like water or food safety, American public health came to frame infectious disease as an adversarial problem — fighting an enemy — and those who built the modern CDC were simultaneously consulting on biowarfare.

COVID and the WHO’s inertia

Zimmer treats the WHO and CDC’s reluctance to name COVID-19 as airborne — a delay of several months into 2020 — as the accumulated product of a century of institutional path dependence (the tendency of organisations to keep doing what they’ve always done, even when circumstances change), not bureaucratic bad faith. Three forces combined. First, the scientific default said respiratory diseases spread by large droplets that fall within a metre — a model designed for pre-World War I diseases and never updated. Second, naming airborne transmission creates an obligation: you cannot ask people to fix it by washing their hands. Third, the evidence base for aerosol (tiny airborne droplet) transmission requires different study designs from the ones epidemiologists (scientists who track how diseases spread through populations) had spent decades refining.

“If something is just spreading by dry droplets on surfaces, you can tell people, ‘Just wash your hands, disinfect surfaces, and you’ll be fine.’ But it’s quite something else when indoor air is starting to become rife with these pathogens that we are exhaling into it.” The phrase ‘rife with pathogens’ means crowded with disease-causing microorganisms — which is exactly what happens in a poorly ventilated room full of infected people.

Ultraviolet light, ventilation, and indoor-air standards

Zimmer makes a practical case for UV-C germicidal irradiation — short-wave ultraviolet light that destroys airborne viruses and bacteria before they can be inhaled. The Wells team used it in the 1930s. Research stalled partly because of the military capture of aerobiology and partly because it requires careful engineering (placed too low, UV-C can damage eyes and skin; placed at ceiling level, it irradiates the air before it recirculates downward). The US government was, at the time of recording, funding a $150 million project to develop real-time airborne pathogen sensors.

Cowen raises the property-rights problem: any single building owner who installs UV lamps only redistributes infections rather than reducing them, because unprotected buildings absorb the surplus risk. Zimmer concedes the point but argues the solution is indoor-air standards — enforceable targets for how clean building air must be, analogous to water-quality standards, met through any combination of ventilation, filtration, and UV. The analogy to water is load-bearing: we do not debate whether the landlord has the right not to filter drinking water.

Carbon dioxide concentration is the simplest practical proxy for ventilation quality: if people are breathing in a room and CO₂ is rising, they are re-breathing each other’s exhalations. Zimmer credits Max von Pettenkofer, a nineteenth-century German hygiene scientist, with the discovery — noting with some irony that Pettenkofer was simultaneously one of the fiercest opponents of germ theory, swallowing a tube of cholera broth to prove it was not waterborne (he survived, and considered himself vindicated). The CO₂ threshold he identified — roughly 1,000 parts per million — is still called the Pettenkofer number.

The aerobiome beyond pathogens

Zimmer broadens the frame: the air is a habitat. Clouds contain viable microbes — fungi, bacteria — that circulate in and out as clouds form and dissipate. This has a direct bearing on astrobiology: the sulphuric acid clouds of Venus exist at altitudes where temperature and pressure are mild enough for microbial life, and Zimmer considers it a genuine (if unlikely) possibility that life began on Venus’s surface and retreated upward as the planet dried and heated.

On extraterrestrial life generally, Zimmer is cautious but not dismissive. On a moon-by-moon basis, life anywhere in our solar system is ‘highly unlikely,’ but he would accept 1,000:1 odds in favour of life somewhere — not 100:1 — given how many planets and moons exist in the galaxy. His preferred candidates are the subsurface oceans of icy moons such as Enceladus or Europa, where liquid water has been stable for billions of years.

DNA, heritability, and the limits of genetic prediction

The conversation moves, characteristically for Conversations with Tyler, across Zimmer’s other books. On She Has Her Mother’s Laugh (about heredity) and Cowen’s hypothetical of employers using DNA in hiring decisions: Zimmer defends the scientific complexity of heritability — the fraction of variation in a trait that correlates with genetic variation — while warning against the political history of such ideas. Early-twentieth-century America categorised entire immigrant groups as genetically inferior on the basis of IQ tests administered in a language they did not speak; Zimmer treats that as the appropriate cautionary frame for any confident extrapolation from population-level genetic correlations to individual hiring decisions.

On embryo selection, he is circumspect: current technology can reliably exclude embryos carrying dominant single-gene disorders (such as Huntington’s disease), but moving beyond that to polygenic traits such as intelligence carries risks he thinks are underappreciated. The most personal: ‘people will convince themselves that the information we have about DNA is going to ensure things about their kids that they can’t be sure about. Then you’re going to have this lifetime of watching your kid not live up to your genetically encoded expectations.‘

Related

• Carl Zimmer — speaker; science writer and author of Air-Borne
• Tyler Cowen — host
• Evolution — Zimmer has written on evolution extensively; the aerobiome and astrobiology discussions extend the evolutionary frame to airborne and extraterrestrial life