The breath you just took contains about 400 parts of carbon dioxide (CO2) per million molecules (ppm) of air. 350 ppm is generally considered safe. People living at the start of the Industrial Revolution would have inhaled about 278 ppm. Since then, levels of CO2—the leading greenhouse gas driving changes in the climate—have doubled from the relentless burning of fossil fuels. Carbon dioxide is born of cellular respiration in animals and plants. Its accumulation from anthropogenic emissions in the atmosphere and oceans over the past two centuries now poses a direct threat to living beings on Earth. In a worst-case scenario that is increasingly likely, CO2 concentrations will reach 1,450 parts per million by 2150. 

But what does this mean for the proximate biological body?[1] In human bodies, levels of carbon dioxide in arterial blood and exhaled breath are always proportional. When the gas accumulates in blood during sleep, we experience this accumulation subconsciously: we can breathe in more deeply, snore, or be roused from sleep. Psychological researchers have learned that by exposing human subjects to carbon dioxide-enriched air, they can simulate a variety of mental infirmities, from anxiety and panic disorders to combat-related stress reactions. Hospitalists use CO2 as a gauge for prognosticating “time to death” in terminal patients (Hwang et al. 2013). Too much CO2 in the bloodstream is a sure sign of imminent cardiac arrest or death. In other words, when this invisible and colorless gas carves its way into our bodies, it can set off a host of biologically significant changes that unfold in both concealed and palpable ways.

To engage this dynamism a bit more, I purchased a carbon dioxide monitor online and measured levels of the gas in various spaces I occupy throughout the day. The monitor detected tremendous variability in CO2 levels in the spaces in which I spend most of my time. My office measures 837 ppm; a large empty lecture hall, 955 ppm; a small recently cleared seminar room, 1,027 ppm. Just outside my window, it measures 388 ppm. At home, the readings can vary from 402 ppm to 1,339 ppm. When I exhale right into the monitor, levels jump to 3,994 ppm and then to over 8,000 ppm, and I wonder how this is all coming from me. If CO2 is too high in one room, say, in a classroom or office, I will know (perhaps not always consciously) to open a window for some fresh air. If I start feeling dizzy, I can leave the room. Recovery, strictly speaking, means relying on the variability of a milieu: open air pockets in which I can adjust “my” partial pressure of carbon dioxide (PCO2) to a normal level. In this sense, life is made up of tangential hyper-arousals, not intentions. At a more general level, variability is the very thing that makes ambient carbon dioxide levels livable. “To live is to spread out,” Canguilhem writes (2001, 21). This spreading is what allows land animals to live in relatively high levels of CO2 and humans as a species to geographically dominate the planet.

But can such species tolerate even wider swings? If Canguilhem has taught us anything, it is that the living (le vivant) and the milieu are fundamentally relational concepts; each is never what it is without changing the other.[2] Yet where I differ with Canguilhem is on the direction and, in some significant respects, the whereabouts of this relationship in contemporary life. For Canguilhem, the milieu is an “elective extraction,” predicated on a living thing organizing it and, in some sense, constituting it according to its vital needs.[3] The milieu is always locatable vis-à-vis individuals. All senses are turned on, so to speak, creating opportunities for extractive agency—the living making the milieu into a livable space—and reconstitutions of some life-affirming norm. This reconstitution of le vivant is said to occur along some stable horizon of inseparability—between itself and the milieu, reflex and situation, symptom and context, the organism and the environment.[4] Yet, as one translator of Canguilhem’s seminal essay “The Living and Its Milieu” notes, le vivant is not the same thing as “organism” (as it is has been translated into English). The word organism implies a strict boundary between the living thing and its milieu that Canguilhem takes pains to avoid.[5]

But what happens when a milieu becomes hostile to exchange, to le vivant, and no longer receptive to the autonomic tangents, motions, and reflexes of certain animals like humans? Although Canguilhem indicates in his essay that the “notion of milieu is becoming a universal and obligatory mode of apprehending the experience and existence of living beings,” erratic experiences in partial pressure milieus rely on variability: the human (and animal) body’s propensity for movement, not extractive agency, is a vital outcome. Yet what makes me move, switch rooms, or open a window can just as easily immobilize me, make me faint, or induce cardiac arrest. In the face of an overwhelming milieu like this, le vivant (that which “does not stop at its ectodermic borders any more than it begins at the cell” [Canguilhem 2008, 111]) is lost or at least hits a wall. Without variability, “my” partial pressure of carbon dioxide is now constrained, i.e. theoretically limited (2008, 111), precipitating anything between immobilizing fear and certain death.

Walter B. Cannon, a professor of physiology at Harvard (from 1906-1942), is best known in medical anthropology for his 1942 essay called “‘Voodoo’ Death.”[6] The very idea of such a death, as Cannon showed, begins with a social displacement or ostracism (an ominous bone pointing, for example), but one that might be hard for the targeted individual, at least initially, to grasp. Realization that one is indeed the subject being pointed to “may induce a disastrous fall of blood pressure” that ends with “the victim’s last gasp” (1942, 179, 181). The cruelty of such displacement from a given milieu is undeniable, and its biological manifestation involves CO2. In a precursor to that work, Cannon experimentally demonstrated how an inhalation of carbon dioxide in dogs manifests distress and panic, which he famously called the fight-or-flight response.[7] “Great exertion, such as might attend flight or conflict,” he wrote, “would result in an excessive production of carbon-dioxide” (Cannon 1915, 205). He attributed these autonomic responses to a dysfunction of a biological “CO2 sensor.”

Canguilhem would have critiqued such mechanistic language, as his essay on the history of the milieu attempts to reintroduce “sense” (thermal, tactile, etc.) into what he called a “living” conception of the biological domain. But with Cannon’s work, I have been pointing to the limits of sense, even to a kind of mortal confusion, as part of that living conception. A seemingly anomalous event (bone pointing) points to milieus rendered suddenly non-adaptive, in which the living is blocked from “bring[ing] its own norms of appreciating the situation” (Canguilhem 2001, 21). To be sure, the insensible in Cannon’s rendering is not necessarily lost as dysfunction in a process in which living things become conditioned machines (Canguilhem 2008, 108); rather, the insensible speaks to varieties of displacement of the living from its milieu that Canguilhem does not address. From the standpoint of the individual, the milieu is simply no longer locatable: its whereabouts are unknown.

Image 1. An example of a sudden displacement or “regime shift” in a lake ecosystem (Carpenter 2003; Petryna 2015). One day the lake seems normal, the next day fish underbellies cover it as far as the eye can see, in this case, due to human pollutants and agrotoxins which have starved the lake of sufficient oxygen. Photograph from Clipart Library, “Dead Fish #1443522” (
Image 2. Rapid adaptation often yields what ecologists call a temporal and spatial “mismatch” between organism and environment, predator and prey: a debilitating confusion among ecological actors that can lead to unanticipated extinction scenarios. These emaciated sea lion pups were inadvertently abandoned by mothers in pursuit of fish that were themselves forced out of habitable zones due to rising ocean temperatures. Photograph from Jack Healy, “Starving Sea Lions Washing Ashore by the Hundreds in California,” New York Times, March 12, 2015,

How to conceptualize le vivant when the milieu is no longer what or where we expect it to be? Let me restate my points so far. The variability of the milieu is precisely what has allowed certain land species to thrive in, redefine, or expand their milieus. In these milieus, potential adaptability is only as good as the tangential arousals into open air pockets these milieus afford, so that we move within and build livable relations but are never displaced or overcome by them. Yet what if the unlivable or unexpected milieu is not the kind of milieu Canguilhem had in mind? How do rapidly changing milieus disrupt livable relations, and what biological senses might accrue in this other domain?          

With these questions in mind, I want to highlight the problem of abrupt climate change, which reimposes the milieu as a vital question,and the living’s attempt to survive in an uncalibrated or abruptly changing milieu. In this milieu, “truly a pure system of relations without supports” (Canguilhem 2008, 103), le vivant would have to engage, not in extractive agency, but in a form of expectation that was not present in the twentieth-century’s organicist’s vision of “the living bring[ing] its own proper norms of appreciating situations” (2008, 113): a negotiation of the proximal and the distal that allows certain organisms to co-exist with these abruptly changing milieus. Here we can think of the ingenuity of mussels and clams whose tenacity (in both senses of the word) and bivalve feet respectively make them master creatures of a constant inconstancy—enduring abrupt wave crashes, tidal shifts, drags and lifts, all of which can shear, destabilize, or overturn them. These organisms have biomechanically “solved” the problem of co-existence with abrupt shifts by establishing a grip and a foothold (literally). Yet, with the exception of the lungfish, whose gills extract oxygen from both water and air, evolution has provided few examples of mastery of the problem of wild partial pressure swings. Which brings me back to the image of a foothold. Can a milieu outpace life? How can runaway change—understood as a condition of radical departure from established baselines, systemic patterns, or historical trends—become more than an accretion of surprises, and come to comprise any living thing’s milieu?

Earth systems, stable for millennia, are now threatened by anthropogenic climate change. Droughts, storms, and wildfires exceed known metrics of frequency and scale. The scope of pending instability is poorly understood. Projections and policies are faltering with respect to the pace of these changes, and our heuristics are proving insufficient for grasping them. The problem of rapidly changing milieus requires schemes and sensibilities that can reckon with a form of violent displacement that is no longer slow or seemingly distant, but close, material, and thus right at hand (Petryna 2018). As wildfires, for example, enter a year-round regime, the very notion of a distinct fire “season” approaches obsolescence. When a wildland firefighter is seconds away from being entirely engulfed in flames—or, entrapped—he or she may deploy a “fire shelter,” i.e. a heat-resistant tent, to test certain limits or to avoid certain death: “It’s almost like crawling into your coffin to see if it fits.”[8] Today, as COlevels increase at a record rate and rising temperatures cause more areas to burn, these fire shelters do not hold up against fire temperatures on Earth. Changing conditions threaten to separate organism and milieu, bringing about untimely ends that are also interspecies events with largely unpredictable timing and scope: what will remain and what will disappear is really anyone’s guess. 

If the milieu is a relational concept, Nietzsche writes of this relation as a fundamental displacement that can only be remedied with a capacity to “lose its own view” in another’s (1997[1874]). In his view, the“living thing,” with a too-rigidly held and hemmed in “horizon” and a certain lack of imagination, is not a subject of vital optimism, but of horizon deprivation. Losing oneself in a different view and in a different horizon is at the crux of my thinking about ever-widening biophysical registers of instability, and about how we inhabit milieus that science has never seen (Petryna, forthcoming). When destructiveness obliges us to revise knowledge calibrated to conditions that no longer fit, marking horizons beyond which worlds as we know them disappear is itself an exercise in delimiting knowable, and thus livable, milieus.

Read another piece in this series here.

Works Cited

Canguilhem, Georges. 1989. The Normal and the Pathological. Translated by Carolyn R. Fawcett. Introduction by Michel Foucault. New York: Zone Books. First published in 1943.

Canguilhem, Georges. 2001. “The Living and Its Milieu.” Translated by John Savage. Grey Room no. 3 (Spring): 6-31. First published in 1952.

Canguilhem, Georges. 2008. “The Living and Its Milieu.” In Knowledge of Life, edited by Paola Marrati and Todd Meyers, translated by Stefanos Geroulanos and Daniela Ginsburg, 98-120. New York: Fordham University Press. First published in 1952.

Cannon, Walter Bradford. 1915. Bodily Changes in Pain, Hunger, Fear and Rage: An Account of Recent Researches into the Function of Emotional Excitement. New York and London: D. Appleton and Company.

Cannon, Walter Bradford. 1942. “‘Voodoo’ Death.” American Anthropologist 44: 169-181.

Carpenter, Stephen. 2003. “Regime Shifts in Lake Ecosystems: Pattern and Variation.” Center for Limnology (July).

Han, Clara and Veena Das. 2015. “Introduction: A Concept Note.” In Living and Dying in the Contemporary World: A Compendium, edited by Veena Das and Clara Han, 1-37. Berkeley: University California Press.

Hwang, Cheol, Hong Yup Ahn, Sang Min Park, Jae Yong Shim, and Kyoung Kon Kim. 2013. “Clinical changes in terminally ill cancer patients and death within 48 h: when should we refer patients to a separate room?” Supportive Care in Cancer 21: 835–840.

Nietzsche, Friedrich. 1997. Nietzsche: Untimely MeditationsTranslated by R. J. Hollingdale. Edited by Daniel Breazeale. Cambridge: Cambridge University Press. First published in 1874.

Petryna, Adriana. 2015. “What is a Horizon? Navigating Thresholds in Climate Change Uncertainty.” In Modes of Uncertainty: Anthropological Cases, edited by Paul Rabinow and Limor Samimian-Darash, 147-164. Chicago: Chicago University Press.

Petryna, Adriana. 2018. “Wildfires at the Edges of Science: Horizoning Work amid Runaway Change.” Cultural Anthropology 33, no. 4 (December): 570–595. 

Petryna, Adriana. Forthcoming. What is a Horizon? Abrupt Climate Change and Human Futures.

Sternberg, Esther M. 2002. “Walter B. Cannon and ‘Voodoo’ Death: A Perspective From 60 Years On.” The American Journal of Public Health 92, no. 10 (October): 1564–1566.


[1]I thank Gabriel Coren and Cameron Brinitzer for their thought-provoking editorial work in addition to providing the prompt. I am also grateful to Paul Mitchell and Naomi Zucker for their helpful feedback. 

[2]In The Normal and the Pathological, Canguilhem writes, “it seems very artificial to break up a disease into symptoms or to consider its complications in the abstract. What is a symptom without context or background? What is a complication separated from what it complicates?” (1991, 88).

[3]Drawing on an example of a tick waiting (“without eating for eighteen years”) to feed off a warm-blooded mammal, Canguilhem notes that the senses (thermal sense, tactile sense) even moments of insensibility towards “excitations that emanate” from a milieu, comprise this well-timed agency (2008, 112). 

[4]Cf. Canguilhem, The Normal and the Pathological, 88.

[5]“The Living and Its Milieu” (2001). Translator’s note, 29. Canguilhem writes, “The individuality of the living does not stop at its ectodermic borders any more than it begins at the cell” (2008, 111). On the possibilities and limits of Canguilhem’s notion of individuality that is attentive to “human forms of life” and instability, fragility, and voice, see Han and Das (2015, 20).

[6]Voodoo death is the “dramatic suddenness of the illness following the threat, coupled with a lack of any apparent injury, exposure to toxins, or infection suggested to Cannon that merely the fear of death could, through physiological response mechanisms initiated by fear, precipitate death itself” (Sternberg 2002, 1564). 

[7]According to Cannon, “although respiratory and circulatory changes of emotional origin may have prepared the body for struggle, the emotional provisions for keeping the working parts at a high level of efficiency may not continue to operate, or they may not be adequate” (1915, 205). The milieu is no longer a reciprocal entity.

[8]Firefighter recollection, National Interagency Fire Center, 2014. “Wildland Safety Training Annual Refresher: 1994 South Canyon Fire on Storm King Mountain.” 

Adriana Petryna: contributions / / University of Pennsylvania