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Fires of Life
A candle will burn some four, five, six, or seven hours.… Then what becomes of it? Wonderful is it to find that the change produced … is the very life and support of plants and vegetables that grow upon the surface of the earth.
—Michael Faraday
My next breath may very well be in your lungs. Store it wisely, because my life depends on it.
—Jarod Kintz
Take a breath, if you please. You're about to take one anyway, as you've done thirty thousand or so times during the last twenty-four hours. As a newborn you drew your first breaths automatically, perhaps forty times per minute, and even now that you've slowed down closer to half that rate and have the language skills to discuss the process, you still breathe without a thought. You'll die if you stop for too long, and you are hardwired to continue this labor reflexively, even in your sleep.
This time, however, try thinking about it for a moment. Notice how you tighten your diaphragm and relax the muscles in the walls of your chest. This effort alone consumes roughly 3 percent of your metabolic energy at rest, all in order to pull the equivalent volume of a grapefruit into your lungs. Trillions of air molecules are now trapped within your chest like fish in a net. Only a few of them, the oxygens, are what you're after. An average adult uses nearly two pounds of them every day, and this particular breath full will help to keep you alive for the next few minutes. It will also connect you to the rest of life on Earth and to the planet itself in surprising ways that we will soon explore.
People who anticipate having to hold their breath for a long time sometimes try to boost their oxygen stores by breathing purified gas. In 2010 a thirty-eight-year-old free diver named Peter Colat spent nineteen minutes and twenty-one seconds submerged in a water tank in Saint Gall, Switzerland. Shortly before plunging in to become the latest world champion of breath holding, Colat inhaled pure oxygen for several minutes, but the competitive edge that he gained in this manner was only partly due to super-enriched blood. Once your hemoglobin molecules hold their full allotment of oxygen, it becomes difficult to force more of the gas into your bloodstream. Most of the benefit to Colat came from the flushing of stale air from his lungs and the conversion of his inflated chest cavity into a temporary oxygen-storage facility while his mouth and nose were closed.
Your lungs are not necessarily the only route that oxygen takes to your blood, however. You also breathe a little through your eyes. So vital are these oxygen particles that the cells in the transparent surfaces of your eyes absorb them directly from the atmosphere to supplement the meager supply that your blood vessels send to them, as do many of the cells of your skin. And an even more direct way of oxygenating blood was recently developed by researchers at Boston Children's Hospital.
After watching a young girl suffer fatal brain damage before she could be connected to a heart-lung machine, the cardiologist John Kheir sought a way to bypass the lungs and inject oxygen straight into the blood. To do this without producing bubbles that could cause deadly embolisms, Kheir and his colleagues used sound waves to whip pure oxygen and oily lipids into a fine white froth. This trapped the gas in soft, flexible, microcapsules that could oxygenate red blood cells on contact, and when the froth was injected into rabbits they survived on it for fifteen minutes or more without breathing and without obvious distress. "This is a short-term oxygen substitute," Kheir told a reporter for ScienceDaily in 2012. "A way to safely inject oxygen gas to support patients during a critical few minutes." If the method can be perfected for human use, it could be deployed in emergency rooms—and presumably, in breath-holding competitions, as well.
For most of us, however, breathing is our primary link to airborne oxygen, and it is so universal and continuous that it can be easy to forget about—until we can't do it anymore. Then it becomes more obviously precious and symbolic of life itself. We take special note of words that are carried on final breaths, and sometimes we even cherish the physical substance of the breaths themselves. Henry Ford kept a small glass test tube of air in his home for many years, and inside the tube was said to be a sample from the last breath of his late friend and fellow inventor Thomas Edison. According to sources at the Henry Ford Museum in Dearborn, Michigan, several such tubes are believed to have been left open to the air of the room near Edison's deathbed. "Though he is mainly remembered for his work in electrical fields," Edison's son Charles reportedly said, "his real love was chemistry. It is not strange, but symbolic, that those test tubes were close to him at the end." After Edison's death, Charles had the tubes sealed and later passed one of them on to Ford as a memento.
But why do you breathe? Why do you so desperately need those invisible specks of oxygen? Where do they come from, what do they do inside of you, and where do they go after they leave? You can't even see them, although you live amid dramatic evidence of their presence in the seemingly empty spaces of your world, from rustling leaves and billowing sails to rusting metal and the soft glow of a candle. Until a mere century or so ago, many reputable scientists even doubted their very existence.
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Oxygen atoms lurk almost everywhere in your daily life. They are not only things that you breathe but also things that you are largely made of. Scientists sometimes describe human beings as "carbon-based life forms," but a strict tally of numbers says otherwise.
Roughly 60 percent of your weight comes from water, depending on your size, age, and health, and nine-tenths of the mass of a water molecule is dominated by the bulky oxygen atom within it. Therefore oxygen comprises most of your wet mass, and an adult body weighing 150 pounds contains about 95 pounds of it. By comparison that same adult's carbon supply would weigh only 35 pounds and be outnumbered by oxygen two-to-one on an atom-for-atom basis.
In the remaining dry parts of your body, oxygen atoms are also interwoven with other elements in the protein fibers of your tendons, the soap-bubble membranes of your cells, and the coiled tendrils of your genes. Oxygen represents a little more than half of the mass of the blood sugar in a human artery and of the lactose in a drop of human milk, as well as most of the mineral-forming atoms in human bone. If all of your oxygen atoms were to vanish you would still be visible, though not for long. The mist of leftover elements would scatter with the first puff of wind.
But that is not why you breathe. You collect most of your water-bound oxygen atoms by drinking, and your carbon-bound oxygen by eating. Breathing is another matter entirely. The target in this case is not merely oxygen atoms for their own sake, but pairs of them that are joined together in reactive molecules of oxygen gas. And unlike eating or drinking, you have to inhale this gas continuously because, apart from the inflatable bags of your lungs, you can't safely hold much of it inside of you.
Even if you could purify and compress a lot of it into some internal storage space, you wouldn't want to do so. Left unguarded within your body, it can attack and damage your cells, and it is toxic in high doses. You need to consume your oxygen in controlled sips, using it immediately and efficiently, and then take more from the sea of atoms that surrounds you.
The story of how scientists uncovered the atomic connections between us and the air includes a centuries-long sequence of incremental advances and dead ends. It is too long a tale to relate in depth here, but a short summary of some key discoveries during the eighteenth century can at least show that the knowledge was hard-won. Although many investigators made important contributions during that time of intellectual and cultural ferment, the work of three scientists in particular will help to show what can be accomplished with relatively simple but clever experiments.
To set the stage, we enter the eighteenth century armed with two legacies from the 1600s, one misguided and one prescient. The former was a hypothesis by the German alchemist Johann Becher that combustion is simply the release of a mysterious substance called "phlogiston." The latter was the work of Robert Boyle, an Irish-born scientist who proposed that air is not a pure element but a mixture of gases. In his book Suspicions about the Hidden Realities of Air, Boyle wrote, "I have often suspected that there may be in the Air some yet more latent Qualities or Powers.… For this is not as many imagine a simple and elementary body, but a confused aggregate … (and) perhaps there is scarce a more heterogeneous body in the world."
During the early 1770s the Swedish chemist Carl Scheele heated a powdered oxide of mercury and concluded that the "fire air" it produced also exists in the atmosphere, but he did not publish his results quickly. A few years later the British chemist Joseph Priestley did the same, called the emissions "de-phlogisticated air," published the finding before Scheele did, and also showed that the gas could keep a flame burning and a mouse breathing in a sealed glass jar. The French chemist Antoine Lavoisier, having heard of Scheele and Priestley's experiments, conducted similar tests and then claimed that the discovery was at least partially his.
None of these men were in fact the first to create oxygen gas experimentally—alchemists had already produced it during the previous century without fully understanding what it was. But Scheele recognized this vital gas as a distinct component of air; Priestley demonstrated its links to respiration and combustion; and Lavoisier gave it the name we use today. He called it "oxygène" (from the Greek words for "acid maker") because combining it with nitrogen and sulfur in solution yields nitric and sulfuric acids. Along with others Lavoisier also helped to overturn the phlogiston hypothesis. By heating tin in a closed container, he showed that the metal gained rather than lost mass, and that the gain in the oxidized metal matched the mass of the air that rushed in when the container was opened.
The personal lives of these men are also interesting in their own right, and their biographies are full of dramatic details. They struggled bitterly over credit for the discovery of oxygen, and all three met with great troubles later in life. Scheele died at age forty-three in 1786, supposedly from sniffing and tasting too much mercury, arsenic, and lead in his lab. Priestley was driven from England for his liberal religious views and for supporting the American and French Revolutions, and while living in exile in Pennsylvania he endured the deaths of his wife and young son, isolation from the scientific community of Europe, and accusations of sedition. Lavoisier, one of the king's hated tax collectors, was guillotined during the French Revolution, and the mathematician Joseph-Louis Lagrange famously wrote of the execution, "It took them only an instant to cut off his head, but France may not produce another such head in a century."
The discoveries made by scientists such as these led to later investigations on the atomic scale, but without high-tech equipment or scientific training, the invisible nature of atoms still makes it difficult to recognize how oxygen and fire work. It is still easy to overlook, for example, as Priestley and Becher did, the increase in total mass that fire produces in burning fuel, because the buoyancy and dispersal of the waste gases conceals it. Burning a six-pound gallon of gasoline releases about nineteen pounds of heat-trapping carbon dioxide into the atmosphere, and the U.S. Energy Information Administration estimates that transportation vehicles in the United States alone released more than a billion and a half metric tons of CO2 in 2012. Such perceptual limitations can make it difficult to notice our effects on our surroundings, and they can sometimes mislead us about the atomic nature of the air we breathe.
After watching mice respire easily in a jar of fire air nearly two and a half centuries ago, Priestley sampled some of the gas for himself. "The feeling of it to my lungs was not sensibly different from that of common air; but I fancied that my breast felt particularly light and easy for some time afterwards. Who can tell but that, in time, this pure air may become a fashionable article in luxury?" Who, indeed, could have foreseen that fashionable people of the twenty-first century might pay a dollar a minute to pump oxygen gas into their nostrils through plastic hoses?
First developed in large, pollution-plagued cities such as Tokyo and Beijing, "oxygen bars" spread during the 1990s to other cities around the world and have now spawned an additional market in portable oxygen dispensers for home use. Proponents say that the gas removes toxins from the body, strengthens the immune system, cures hangovers, and performs other medical miracles, but little or no empirical evidence supports most of these claims. As George Boyer of Mercy Medical Center in Baltimore explained in an interview for WebMD, "If your lungs are healthy, and you have no breathing difficulties, your body has all the oxygen it needs," adding that "for the vast majority of people there is little harm [in using an oxygen dispenser], but also absolutely no science of benefit."
Whether you buy your oxygen in a bar or inhale it for free, the question remains: Why do you do it at all? Many early investigators thought that breathing was done merely to cool the "animal heat" of one's body, and that lungs were little more than air conditioners. The reality lies deep within the cells of your body, where the differences between combustion and respiration are also more clearly revealed.
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The basic formula of "food plus oxygen yields CO2 and water" that is often presented in textbooks seems to imply a direct connection between the gases that enter and leave your lungs, as though the metaphorical fires of life operate on the atomic scale precisely as real fires do. Many scientists foster this impression, sometimes to simplify concepts for nonspecialists, and sometimes because similar explanations led them astray earlier in their careers. A recent article on molecular physiology in Science, for instance, described how blood sugar is "burned" with oxygen, and college professors often suggest in their lectures that the carbon dioxide we exhale is an exhaust gas that forms when breath oxygen combines with sugary fuel in the furnaces of our cells.
The image is appealing, but it is also wrong. A closer look at the subject shows why fire doesn't perfectly illustrate your use of oxygen.
Fire does resemble a living thing in many ways. Both emit carbon dioxide and—strangely enough—water vapor. Although a rush of liquid water can extinguish a flame or drown a person, the gaseous form of water in a puff of smoke or a human breath has no such effects on its source. Flames and life are also alike in that both can be snuffed out if deprived of oxygen. The light and heat that emerge from a candle arise from the breaking of chemical bonds in wax molecules, and the warmth of your skin is related to the breaking of bonds in food molecules. But although the basic equations of combustion and respiration are similar, the processes at work in a fire differ from those that sustain you.
In a candle flame, oxygen gas from the surrounding air attacks the melting wax directly, tearing electrons away from fuel molecules in a swirl of glowing carbon-rich particles and partially ionized gases. When the gaseous body of a flame is hot, dense, and ionized enough, as for example on the six-thousand-degree business end of an oxyacetylene torch, it is called a "plasma," a term that also applies to the incandescent sphere of the sun. Plasma is the fourth state of matter, a dynamic complement to the more commonly recognized solid, liquid, and gas states, and it may well be the dominant readily visible form of matter in the universe because stars are made of it. Here on Earth, less ferocious kinds of fire unwind carbon-based fuels into simpler particles much like those from which they originated. For instance, the petroleum from which candle paraffin is made was built from carbon dioxide and water by light-harvesting algae. Burning a candle returns its carbon and hydrogen atoms to the atmosphere in the grip of oxygen as carbon dioxide and water molecules. It also unleashes the solar energy that originally bound those raw materials together into living tissues, and it does this so rapidly that temperatures inside the hottest parts of a candle flame can reach 2500°F.
Inside the controlled confines of your cells, however, oxygen gas is usually not so much a fierce lion as a well-trained house cat that waits to be fed. When hot wax breaks down in a candle flame, for example, oxygen gas swoops into the blaze and emerges with carbon and hydrogen atoms in its clutches. When carbon-rich food oxidizes in your body, those same two waste products (CO2 and H2O) are made in two separate processes. The oxygen gas that you inhale will not carry carbon atoms off in later breaths as it might if you were a candle. In the relatively tame habitats of your cells, oxygen specializes in capturing hydrogens instead. And fortunately for you, the production of energy by cellular respiration is slow and dispersed enough to warm you without immolating you.
To envision how this process works, it helps to use Albert Einstein's technique of conducting "thought experiments" to let your imagination follow some oxygen molecules down your throat and into your lungs on your next breath. About three-quarters of the air that you have just inhaled consists of nitrogen molecules, none of which will serve you apart from helping to keep your lungs inflated. Your target is the 21 percent of air that consists of oxygen molecules, but you can only get at them by harvesting the whole mess, like scooping up piles of mixed jelly beans and then picking out the colors you prefer.
As your chest expands and air presses into it, the gas squeezes through bronchiole ducts as thin as a human hair into hundreds of millions of bubble-like alveoli that compose the pink spongy interiors of your lungs. Their combined absorptive surface area approaches 750 square feet, roughly equivalent to one-third of a singles tennis court. From there most of the air molecules work their way into narrow spaces between the alveoli, where blood capillaries gather them up. On that microscopic scale your blood resembles water crammed with translucent blobs of crimson Jell-O that can squeeze past an alveolus within less than a second as your lungs pulsate. Those red blood cells are fast-moving vehicles that can carry oxygen through hundreds of miles of vessels to destinations all over your body.
Meanwhile, carbon dioxide molecules that recently formed inside your cells stream out of your blood and into the alveoli. In all that confusion most of your newly inhaled oxygen is simply blown right back out of your lungs. Although wasteful in one sense, such inefficiency can be a good thing. Residual oxygen allows mouth-to-mouth rescue breathing to revive an unconscious person rather than asphyxiating them with carbon dioxide.
A pint of your blood can carry roughly one-fifth of a pint of oxygen gas, almost enough to sustain you at rest for one minute. But the supply runs lower along the journey from your lungs to your cells and back. By the time venous blood makes it back to your alveoli, it contains so little oxygen compared with the air in your lungs that the imbalance automatically drives the diffusion of more oxygen into your blood.
The primary goal of an oxygen molecule in your body, if molecules can be said to have goals, is to be dismembered inside of you. But if you could accompany it on its fatal trip to one of your cells, you would have to do so as a purely metaphysical being because, naturally, you couldn't be made of unshrinkable atoms and still be yourself, any more than a brick building could shrink to the size of a standard brick and still be recognizable. Scaling the atomic realm up to match you would not work either, because everything around you in that fantasy world would then have to be moving unrealistically fast. An atom is about ten billion times smaller than you are, and an oxygen atom's trip from your heart to your hand would be millions of miles long on an equivalent human-size scale. The blood in the brachial artery of your arm covers this distance within a single second, so a person-size atom would have to travel faster than the speed of light to do the same, which Einstein's research on relativity showed to be impossible.
Even without the improbable logistics of shrinking and growing involved, the atomic realm is much stranger than the ones most of us are used to. Atoms are so mobile and the outer boundaries of their electron clouds so unstable that the atomic surfaces of objects are more indistinct than firm. In that world of the incredibly small there is no air to breathe, no sound to hear, and visible light cannot illuminate objects as it does in our much larger size range.
Nonetheless, here is an example of what you might imagine seeing and feeling in a thought experiment if a single skin cell were magically inflated ten million times into a living hill three hundred feet high so you could more easily see what happens to oxygen inside of it. At that magnification the sizes of the atoms making up the cell approach those of sand grains, and the body that you normally occupy would be large enough to lie down with your head in New York, your waistline atop the Pacific Ocean, and your feet in Australia.
You will now need to enter that hill by forcing your way in through the pliant, oily membrane. It is wet in there, so suspend disbelief even further and assume that you can still breathe despite the syrupy substance that fills the cell. The scene looks positively industrial. Structural protein cables as thick as your arm stretch in all directions, giving the cell its shape.
Just over there is your destination—a cylindrical bubble-like thing that is roughly the size of a tractor trailer truck. This is a mitochondrion, a living power plant that uses food as a fuel. Each of your cells may contain dozens to hundreds of these, and they can vary in shape from peas to noodles. It is within such mitochondria that your breath oxygen meets its doom.
Enzymes in the cell and the core of the mitochondrion smash food molecules into a rich stew of electrons, hydrogen ions, and CO2. The electrons are then fed to a series of proteins that lie embedded in a soft membrane surrounding the core, some of which will twitch, bend, or roll as the electrons pass through them. And as the molecular machinery churns, it also stores chemical energy that can power muscles and metabolism. In other situations it can help to generate body heat instead.
At the end of the line, each exhausted electron makes one last jump to clear the way for the ones coming up behind it. And here is the precise point from which your need for oxygenated air arises.
Oxygen uses those leaping electrons to tether the hydrogen ions from shattered food molecules to itself. In this transmutation of food and air, disparate components of your meals and breaths recombine to create H2O molecules, your own homemade metabolic water. A tenth of the fluid in your body, from the blood in your veins to the moist gleam in your eyes, is built from scratch this way with the aid of oxygen that you inhaled during the last few days. Air and water are therefore more closely related to one another than the alchemists ever imagined, as each can represent a reconfiguration of the atoms of the other.
This, then, is why you breathe. You stoke the machinery of your cells with air, carry the watery leftovers around for a time, and then release them back into your surroundings through sighs, sweat, tears, and more substantial wastes. In doing so you split a flame's oxidation of carbon and hydrogen into two separate processes, thereby creating situations in which only metaphorical fires of life will flicker. The atmosphere literally becomes a part of you every time you draw a breath, and part of you returns to it on every outgoing breath as well.
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Depending on the time of day and the season of the year, the air you walk through and pull into your lungs changes more than you might expect. This is just one of many discoveries by Ralph Keeling, a scientist at the Scripps Institution of Oceanography who tests the atmosphere the way a police officer might test your breath with a Breathalyzer.
For more than two decades Keeling has been measuring the oxygen content of air samples that are collected daily in Hawaii, Antarctica, and elsewhere, sealed into small containers, and shipped to his lab in La Jolla, California. Like traces of alcohol in someone's breath, slight changes in the composition of the atmosphere can tell a lot about what the world's combined masses of people, vegetation, and plankton are doing.
It is often said that forests are the "lungs of the planet" because they produce oxygen that we breathe, but the metaphor falls short in some respects. Lungs don't produce oxygen but instead consume it, and Keeling's work has shown that only about half of your oxygen comes from terrestrial plants. The rest is made by algae and cyanobacteria in lakes and oceans, with a small additional measure produced by the splitting of water vapor in the upper atmosphere by radiation from the sun and distant stars.
However, when combined with the carbon dioxide analyses that his late father, Charles David Keeling, launched at Mauna Loa Observatory in Hawaii in 1958, the long-term oxygen records do show an almost eerie resemblance to the readouts of a medical breath-monitoring device. Annual pulses of oxygen are mirrored by cyclic drops in CO2, and together these data open a unique window on the atomic connections between plants and the earth.
When the elder Keeling first began to study the air, he expected it to vary a great deal from place to place. To his surprise, however, much of the variability vanished when samples were collected with consistent methods at remote locations where the air is free of local influences from respiring forests and cities. The atmosphere mixes more thoroughly and rapidly than scientists had hitherto realized, and average CO2 concentrations in Hawaii are remarkably similar to those at the Scripps pier in La Jolla.
Equally noteworthy, however, were various kinds of rhythmic oscillations that appeared in the gas records. Every day the carbon dioxide concentrations dropped slightly, only to recover at night, and larger seasonal pulses occurred with dips in summer and peaks in winter. When Ralph Keeling began to measure oxygen to complement his father's work, his results showed similar patterns but in reverse. With these data you can watch the atmosphere respond to the breathing of countless plants and microbes as the earth spins on its axis and circles the sun.
The pacemaker of these pulses is sunlight. When dawn awakens California, the lawns and palm trees of La Jolla begin to pump oxygen into the air and pull carbon dioxide out of it, as does the Pacific plankton drifting offshore. When that portion of the world spins onward into the shadow of night again, the oxygen production shuts down, but the cellular CO2 factories, which need no solar power supply, keep running and quickly drive local carbon dioxide levels back up again while oxygen levels drop.
A similar pattern emerges in alternating hemispheres through the seasons, as well. When plants sprout and leaf out in spring, O2 rises rapidly and CO2 declines. Later in the year when photosynthesis slows and dead leaves begin to decay and release carbon dioxide, the opposite trends prevail. This pattern produces a sawtooth effect in the Keeling charts, and if you could hear the oscillations rather than see them they would sound like the breathing of a long-distance runner. The oxygen curve would go, "Puff hard in spring, take a long deep breath in winter, puff again," and so on.
Here again, however, the lungs-of-the-planet metaphor has limits. Not only do these lungs emit oxygen; they do it asymmetrically. When one hemisphere exhales the other one inhales, which would be interesting to watch if real lungs were to do so within a runner's chest.
The Keeling records clearly show that we affect the atmosphere, too, but in more disturbing ways. In early 2013 average concentrations of heat-trapping carbon dioxide reached 400 parts per million (ppm, or a ten-thousandth of a percent), having risen from an average closer to 312 ppm during the 1950s. Most of that change represents the burning of fossil fuels along with the decay and fires associated with deforestation. Unlike the photosynthesizers, these artificial "lungs" of the modern world consume O2 and release CO2 like our own, and they do it continuously on a massive scale.
While the long-term carbon dioxide record tilts upward along with global average temperatures, the oxygen trend points downward. According to the Scripps O2 Program Web site, oxygen concentrations at La Jolla dropped by 0.03 percent between 1992 and 2009. This, as Ralph Keeling said in an interview with the San Diego Union-Tribune, is the global "signature of combustion."
Should we now worry about running out of oxygen in addition to global warming? Not according to Keeling. In another Union-Tribune interview he explained that there is plenty of oxygen in the air, and the tiny percentage of loss of oxygen in itself isn't an issue. Rather, "the trend in oxygen helps us to understand … what's controlling the rise in CO2."
In other words, declining oxygen shows how closely tied we are to this planet, and how much we now affect the atomic world around us. The Scripps O2 Program reports that roughly a trillion metric tons of oxygen were consumed by fossil fuel combustion since the Industrial Revolution, but it still amounts to a mere tenth of a percent decline in the huge oxygen reservoir of the atmosphere. This is far too small to have any direct impact on human health, and seasonal oxygen levels can often drop by 10 percent or more over major cities without ill effects. So enormous is Earth's oxygen reservoir that the annual output of all the world's plants and plankton barely dents it. A million billion metric tons of oxygen molecules ride the air, representing about two thousand years of seasonal contributions. During cooler, darker seasons and at night when photosynthesis shuts down, life survives mainly on oxygen that was emitted long ago.
To bring this concept down to a more personal level, you could probably walk over and touch some of the kinds of oxygen sources that are helping to sustain you right now. A study published by the research forester David Nowak and his colleagues in Arboriculture & Urban Forestry suggests that an average acre of trees creates enough oxygen from year to year to keep eight people alive, although the exact relationship varies according to the species, age, and environmental settings of the trees. American urban trees as a whole produce a net total of 67 million tons of oxygen per year, enough to sustain two-thirds of the population of the United States. Of the United States cities listed in that report, the champion oxygen generator was lush and leafy Atlanta, Georgia, which could keep most of its residents perpetually breathing with the 95 thousand tons of oxygen that its own trees release each year. New York City and Washington, DC, came in second and third with 61 thousand and 34 thousand tons, respectively. Freehold, New Jersey, came in last with only 11 hundred tons.
There are many fine reasons to plant trees in cities, from temperature control to aesthetics, but oxygen production is not one of them. You can always find plenty of perfectly good oxygen to breathe in the most barren desert or, for that matter, in Freehold, New Jersey. And no matter where you live, you always have more than enough oxygen available during winter even though the local trees are leafless or dormant. Most of your oxygen molecules come from more distant places and times, and only a minute fraction of the oxygen in your next breath was produced during the past year.
Although it can be tempting to think that plants make oxygen gas for your benefit, it isn't so. What you breathe is merely the accidental crumb droppings of a feast that light harvesters prepare for themselves. Plants use oxygen like you do, and they consume almost as much of it as they produce. Any net production of oxygen by a forest or ocean is simply leakage, and much of what does escape is soon consumed in rust, decay, and fire. If tons of dead plants, animals, and microbes were not continuously buried in soils and ocean sediments before they could rot or burn, the atmospheric oxygen account would eventually balance out to zero and everyone would suffocate.
Your other oxygen sources can be found with the help of a fine-mesh net that you might pull through a patch of seawater. If the haul is mostly yellow-brownish, then you have probably harvested single-celled algae known as diatoms. Under a microscope they look like dabs of golden jelly encased in glistening glass shells that can look like snowflakes, needles, or designer hubcaps. Other kinds of photosynthetic plankton bear shells of creamy white chalk or resemble strands of green malachite beads. Plant-like relatives, the seaweeds, can resemble green cellophane, long sheets of soft brown rubber, or crinkly lettuce. Ralph Keeling has calculated that marine photosynthesizers such as these produce nearly thirty billion tons of oxygen per year in the Northern Hemisphere, while the larger southern oceans yield roughly half again as much.
Oxygen shortages are most likely to become a problem if you move too far upward through the sea of air. Gravity pulls gas molecules downward and crowds them more tightly together at ground level, and at sea level you operate under an average pressure of roughly fifteen pounds per square inch. The weight doesn't normally bother you, though, because the air is evenly distributed over your body and the pressure inside you matches that on the outside. It is more like floating in a swimming pool than like having three-quarters of a ton of water balloons piled atop your head and shoulders. Most of us don't even notice this ever-present pressure until our ears pop in an airplane or on a road trip through tall mountains.
Atop a mile-high peak, you still get 83 percent of the oxygen that you would at sea level, according to an air-pressure calculator posted online by altitude.org, but if you climb beyond eight thousand feet or so where relative oxygen abundances in the thinning air drop to 75 percent or less, altitude sickness could begin to present problems. People sometimes develop headaches in high-flying commercial airliners, and in some cases this may reflect mild hypoxia due to low cabin pressure, which can resemble conditions on a mountainside at a mile-and-a-half elevation.
People whose ancestors lived at high altitude for many centuries tend to carry genetic mutations that can help them to cope with low oxygen supplies. Many Tibetans breathe faster and take in more air per breath than the rest of us do, a genetic inheritance that helped their ancestors to make up for the shortage. Among many Andeans, large amounts of modified hemoglobin in the blood also extract more oxygen per breath.
But even for such specially equipped people, long-term existence on the world's tallest crags is impossible. We simply can't afford to stretch our atomic connections to photosynthetic life too far.
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Careful research has shown that we breathe in order to harvest oxygen gas from the air, that we convert it to water, and that this need for oxygen somehow links us to plants. But what, exactly, is that vital relationship between plants and people? In order to trace it in more detail, we must focus less on oxygen molecules than on oxygen atoms.
Consider the atomic connections between you and a potted plant. If you and your plant were to spend a few hours together in a well-lit room, then you might be able to play a game of catch with a single oxygen atom in different molecular forms. You couldn't use your breath CO2 for this whimsical thought experiment, though, because the plant would turn it into vegetable matter, and your main method of getting an oxygen atom back after releasing it in a CO2 molecule would be to eat the plant.
If instead you were to inhale an oxygen molecule that emerged from the plant, your cells could turn it into metabolic water. Now separated and "disguised" as H2O, one of the oxygen atoms from that original gas molecule might escape through your breath and drift back through the air to the plant in the form of water vapor. If that molecule were to enter a leaf it might be smashed in photosynthetic reactions that could spit the same oxygen atom back out at you again in yet another oxygen gas molecule.
Supported by many more scientific discoveries than Scheele, Priestley, and Lavoisier had access to, you can now use your well-informed imagination to follow the botanical side of this exchange even deeper into the cellular level. Plants turn water into oxygen with the help of green chloroplasts that somewhat resemble mitochondria in size and shape but are packed with layered membranes that resemble spinach lasagna. When sunlight strikes a leaf, some of it hits emerald-colored molecules that are anchored in the chloroplast membranes. Those sunstruck chlorophylls, in turn, fire electrons into molecular machines that help to drive the construction of sugars which can later become sap, stems, flowers, and seeds.
But there's a hitch. The chlorophylls must be reloaded after firing. In a leaf, water molecules are the handiest sources of those electrons, and chloroplasts excel at ripping water apart to get at them. Solo oxygen atoms that are left over from that demolition join to form gas molecules that may be consumed immediately in nearby mitochondria or released into the air, perhaps eventually to visit your lungs.
The broad patch of sunlight that slides across the face of the earth over the course of a day triggers great bursts of photosynthesis wherever it goes. If water molecules were larger, you might hear them exploding like firecrackers as solar-powered life smashes them and sprays molecular shrapnel into the air and oceans. The oxygen gas that emerges from this mayhem faces many possible fates, all of which eventually end in destruction whether it be within a fleck of rust, a flash of lightning, or a cell inside your fingertip.
But if oxygen molecules were sentient beings, they would surely take such things in stride. On a planet such as this which is so full of life, the demise of any given molecule is as inconsequential in the longer history of its component atoms as it is inevitable. By some estimates, today's rates of global oxygen production could theoretically smash every water molecule in the oceans within several million years, and in their book Plant Physiology, Hans Mohr and Peter Schopfer calculated that land plants alone have split the equivalent of Earth's entire supply of surface water about sixty times since the evolution of large forests four hundred million years ago.
From a geological perspective, then, much of the water and most of the oxygen gas that you use is relatively young, with its age more likely counted in mere centuries or millennia than in millions of years. Therefore, although it can be tempting to think that what you drink and breathe today could be the very same stuff that dinosaurs once used, most of it probably isn't.
On the other hand, the atoms from which those air and water molecules arise are much older, and they not only connect you to the atmosphere, oceans, and organisms of the earth—your atomic connections also extend into the depths of space and time as well.
The water that moistens your mouth and flows through your veins represents multiple generations of atoms: hydrogen from the birth of the universe and oxygen from the hearts of stars. Shortly after the Big Bang 13.8 billion years ago, your hydrogen atoms condensed amid clouds of subatomic particles. There was no oxygen at all then because oxygen atoms form within stars, and the first stars had not been born yet. The oxygen atoms that now reside within you are therefore younger than your hydrogen atoms, having formed millions or even billions of years later.
The first stars ignited when primordial hydrogen formed clumps that became large, dense, and hot enough to trigger nuclear fusion reactions. Similar reactions also power the sun, whose core temperatures are measured in tens of millions of degrees. In that painfully brilliant plasma, electrons dash away from their host nuclei, which then crash directly against one another without the protection of electron shells. So violent are the collisions that they overcome the mutual repulsions of positively charged protons and allow more powerful nuclear forces to click them together. These new clusters form the nuclei of helium atoms, which are fittingly named after Helios, the ancient Greek god of the sun. This gigantic solar fusion reactor releases so much energy that it drives the entire photosynthetic activity of the earth while also baking you on a beach towel from 93 million miles away.
Most of your oxygen atoms, however, were probably born within larger stars that fused their hydrogen nuclei into much larger elements. The nuclear ancestors of the elemental oxygen that gives your body water most of its mass were once identical to the older, smaller hydrogen atoms that now perch upon it. When the megastars matured, senesced, and died, your newborn oxygen atoms blew off into space like pollen from bright, flaming flowers. To look into the night sky is to survey distant gardens in which the elements of life are ripening, and your body is a composite harvest from those cosmic fields. Throughout history, people have spoken of the earth as our mother and the sun as our father, perhaps reflecting prevailing views of traditional sex roles. In an atomic sense, however, it would be more accurate to think of the earth and the sun as our siblings, because they both formed from the same star debris as the elements of life within us. Earth is indeed a kind of surrogate mother to us in that our bodies are derived from it, but we exist today only because our true celestial star mothers died long ago. May your next breath of fresh air be dedicated to their memory.
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From space, Earth resembles a floating blue bead, and if you keep that image in mind it will help to drive home a lesson that was arguably one of the most important contributions of the NASA space program. As abundant as atoms are on this planet, their numbers are finite. Watch a satellite video of the clouds that sweep across the face of the world, and you will see in an instant that the winds that carry them over one curved horizon may reappear on the opposite horizon: The truth of such seeming platitudes as "What goes around comes around" becomes obvious.
When viewed from a great distance the sky resembles a shockingly thin film, and most of its molecules are packed into a mere ten-mile slice of a total planetary diameter of nearly eight thousand miles. At sea level you might find more than ten trillion trillion atoms in a cubic yard of air, but just outside that vaporous skin is the relative vacuum of the solar system, in which fewer than a dozen atoms might enter each of your lungs should you try—and fail—to breathe out there. The next time you see a photo of the earth taken from space, try to convince yourself that a pollutant-spewing smokestack anywhere in the world doesn't unleash potentially harmful substances into the same precious air supply that keeps you and your loved ones alive.
Keeling and his colleague Stephen Shertz showed that oxygen gas emitted by plants and plankton mixes throughout each respective hemisphere within two months and spreads worldwide in a little more than a year. The sensitivity of the oxygen and carbon dioxide balance of the atmosphere to the activities of living things shows that recycling is not just a passing fad but a tradition that has always been practiced on the atomic level by all life on Earth. To live, rather than to merely exist like inanimate rock, is to borrow and repurpose the elements of the world around you, and then release them again.
The first time I encountered this line of thought during the early 1960s, I was reading a comic book when I should have been doing my homework. I don't recall the title or even the main point of the story, but one cartoon panel still stands out in my memory. In it a gray-haired inventor hunched over a workbench, and the caption read, more or less: "You are now breathing some of the air molecules that Leonardo da Vinci himself breathed during his lifetime."
Being a young kid, I didn't fully grasp the finer points of that story. I didn't even understand why Leonardo was the main character. Was it because he breathed more air than the rest of us? But many years later, having learned that Leonardo is merely one among several historical figures who are commonly used to illustrate breath recycling (including Julius Caesar, Jesus, Shakespeare, and Hitler), I still find it intriguing. So do many other people, judging from the large number of hits in a Web search of the subject.
You have to pick carefully through the facts and figures necessary to make such a calculation, but most sources end up reporting the presence of between one and fifteen breath atoms from the person in question within each of your own breaths. To begin with, a reasonable value for the total mass of the atmosphere is needed, such as the five million billion metric ton estimate that was published in 2005 by Kevin Trenberth and Lesley Smith of the National Center for Atmospheric Research in Boulder, Colorado. That represents something like 1044 air atoms (that is, unimaginably many), of which a fifth are oxygens.
If you also take into account gas exchange with the oceans, the volume of an average lung, the breathing rate of an average human, and so on, you're likely to come close to the range of one to fifteen Leonardo atoms per breath and reach the same basic conclusion: We all share the same air. But to make your calculation as accurate as possible, you must also select the right kind of gas to follow.
The one-to-fifteen estimate doesn't apply to oxygen gas, for example, because it is too unstable to linger in the air for very long. Cells, forest fires, lightning bolts, and space radiation are likely to smash it sooner or later, turning it into water and other compounds. In a crowded elevator you do share some oxygen molecules with your companions, as the waning fraction of unused air in your lungs passes from person to person. But on a planetary scale, the chance of your sharing an oxygen molecule with Leonardo or a more distant historical figure is closer to nil. Most of the oxygen that you breathe is too young for that. And although nitrogen gas is much more abundant and stable, it nevertheless cycles through food chains and can therefore leave and return to the atmosphere as well.
A better gas to illustrate the recycling of air would be one that isn't created or consumed by living things. The most widely cited choice is that of the American astronomer Harlow Shapley, who used argon as the subject of his calculations.
Argon is ubiquitous in the atmosphere, although it represents less than 1 percent of the total. That rarity is a good thing, because breathing high concentrations of argon is a bad idea. It is much heavier than most gases and therefore harder to force out of your lungs once it enters. Suffocation by argon in industrial accidents is more common than death by more overtly poisonous gases such as chlorine. But 1 percent is nothing to worry about, and it makes a convenient air tracer for Shapley's thought experiment, which yielded a high end estimate of fifteen argons per breath that are likely to have been inhaled previously by someone like Leonardo.
His 1967 essay, "Breathing the Future and the Past," followed the argon atoms in a single breath so vividly that I'll quote it here:
We shall call it Breath X. It quickly spreads. Its argon, exhaled this morning, by nightfall is all over the neighborhood. In a week it is distributed all over the country; in a month, it is in all places where winds blow and gases diffuse. By the end of the year … [it] will be smoothly distributed throughout all the free air of the earth. You will then be breathing some of those same atoms again.…
This rebreathing of the argon atoms of past breaths, your own and others', has some picturesque implications. The argon atoms associate us, by an airy bond, with the past and the future.… You contribute so many argon atoms to the atmospheric bank on which we all draw, that the first little gasp of every baby born on Earth a year ago contained argon atoms that you have since breathed. And it is a grim fact that you have also contributed a bit to the last gasp of the perishing.
Every saint and every sinner of earlier days, and every common man and common beast, have put argon atoms into the general atmospheric treasury.… Argon atoms are here from the conversations at the Last Supper, from the arguments of diplomats at Yalta, and from the recitations of the classic poets.… Our next breaths, yours and mine, will sample the snorts, sighs, bellows, shrieks, cheers, and spoken prayers of the prehistoric and historic past.
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Shapley was no poet or mystic, and his nerd credentials were impeccable. He studied astronomy at Princeton, helped to establish the National Science Foundation, served as director of the Harvard Observatory, and calculated the size of the Milky Way. But I'm not surprised that he waxed so eloquent in his essay, or the least bit shocked by the whiffs of philosophy and spirituality in it. Strong undercurrents of emotion often emerge amid the facts and figures that science uncovers, and when discoveries are truly profound they can release profound insights and feelings, as well.
As brilliant as he was, Henry Ford apparently failed to realize that he needed no test tube to capture the atomic essence of Edison's last breath. You can collect a sample of it anytime—along with samples from the last breaths of Caesar, Jesus, Shakespeare, Hitler, and Leonardo—and even with a few bits of air that carried your own first cries as a newborn.
It's easy to do, here on this sky-blue sphere of atoms. Just take a breath, if you please.
Copyright © 2014 by Curt Stager
