The air around us
2017; Wiley; Volume: 28; Issue: 1 Linguagem: Inglês
10.1111/ina.12423
ISSN1600-0668
Autores Tópico(s)Environmental Education and Sustainability
ResumoIndoor AirVolume 28, Issue 1 p. 3-5 EDITORIALFree Access The air around us William W Nazaroff, William W Nazaroff Editor-in-Chief orcid.org/0000-0001-5645-3357 Search for more papers by this author William W Nazaroff, William W Nazaroff Editor-in-Chief orcid.org/0000-0001-5645-3357 Search for more papers by this author First published: 15 December 2017 https://doi.org/10.1111/ina.12423Citations: 9AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat At rest, an adult breathes 12–15 times per minute, inhaling and exhaling 0.4–0.5 L of air with each breath. The breathed air is altered in some respects: The exhaled air contains less oxygen, but more carbon dioxide and more water vapor than was inhaled. Breathing does not alter the dominant component of air, nitrogen. Comprising almost 80% of the inspired air, the inhaled molecules of N2 are effectively quantitatively exhaled. A half-liter of inhaled air has a mass of about half a gram. It comprises roughly 1022 molecules, a number that is easy to write (thanks to scientific notation), but almost incomprehensibly large. Consider this. One of the wealthiest people on earth is Bill Gates; he controls assets worth on the order of 100 billion US dollars. The total number of humans who ever lived on Earth is a similar number, about 100 billion souls. If each of those 100 billion people was endowed with 100 billion dollars, the corresponding total wealth would be 1022 dollars, numerically comparable to the molecules inhaled with each breath. Or, ponder this. The geological age of Earth is 4.5 billion years. There are about 32 million seconds in a year. Earth's age in seconds is ~ 1017, a number 100,000 times smaller than the number of air molecules that you inhale with each breath as you read this editorial! The number 1022 is remarkable for another reason. Its square, 1044, corresponds approximately to the total number of molecules in the Earth's atmosphere. Among the consequences: The 1022 molecules that we exhale with each breath constitute one part in 1022 of the entire atmosphere. Now, put aside for a moment the complexities associated with dispersion and mixing, and also neglect the finite lifetimes of individual molecules in the atmosphere. Assume that the molecules in an exhaled breath remain airborne indefinitely and also mix thoroughly throughout the atmosphere. Assembling the quantitative elements from the previous paragraphs, one can arrive at a remarkable conclusion: to a decent approximation, with each breath, we inhale one of the molecules exhaled in the last gasp of Caesar at the time of his assassination, 2000 years ago. This startling inference is the catalytic agent for the opening story in a new book, Sam Kean's Caesar's Last Breath: Decoding the Secrets of the Air Around Us (Little, Brown and Company, New York, 2017). Kean observes that there is nothing magical about Caesar's last breath or even Caesar. In fact, the math applies generally. To a good approximation, in each breath that we inhale, there is a molecule that was previously exhaled from every breath of every human who has ever lived. That mind-stretching finding speaks to one of the ways in which humans share this planet. We are interconnected with every breath we take, across time and across space. How many breaths have humans exhaled in aggregate? If we assume that the historical population-weighted mean life duration is ~50 years, then, at 15 breaths per minute, the total number of breaths per human life would be ~400 million and the cumulative total for all 100 billion historical humans would be ~ 4 × 1019 breaths. With 1022 molecules in each breath, and allowing that the overall estimate is uncertain, the implication is that something in the range of 1041_1042 air molecules (from among a total atmosphere of ~1044 molecules) have been inhaled and exhaled previously by a human being. In other words, 0.1–1% of the air molecules that we inhale with each breath has been previously breathed. (Here is a supporting technical comment: Because N2 makes up almost 80% of the atmosphere, because the lifetime of individual N2 molecules in the atmosphere is about 10 million years, and because the history of homo sapiens extends back a much shorter time, only about one million years, this analysis produces a reasonably accurate magnitude estimate of the quantity rebreathed, provided we neglect indoor environments. We will revisit this important detail at the end of the editorial.) Kean's book is impressive both in the scope of its contents and in the richness of the stories told. Caesar's Last Breath is organized in three major sections, respectively covering (i) the natural history of Earth's atmosphere, (ii) human discoveries and exploitation of the atmosphere's composition, and (iii) future frontiers. Selected molecules (mainly gaseous) constitute one set of characters in this book. Most chapters and brief "interludes" open with a ball-and-stick diagram of a molecule or two (22 in all) along with a numerical estimate of how many of those molecules are inhaled with each breath. Many of my favorite small molecules are featured, including O2, N2, CO2, H2O, and O3. Sharing the spotlight with specific molecules are human characters, whose contributions—scientific or otherwise—are highlighted while recounting elements of their personal histories. Altogether, as summarized in Table 1, the major portion of the book weaves together three primary elements: scientific and technological innovation, key chemical species, and principal historical players. Table 1. Some narratives recounted in Caesar's Last Breath Discovery/innovation Individual(s) Nitrogen fixation Fritz Haber, Carl Bosch Discovery of oxygen Carl Scheele, Joseph Priestly, Antoine-Laurent Lavoisier Anesthesia William Morton, Horace Wells, James Simpson Steam power Thomas Savery, Thomas Newcomen, James Watt Explosives (dynamite) Ascanio Sobrero, Alfred Nobel Buoyancy and ballooning Joseph-Michel Montgolfier, Jacque-Alexandre-César Charles, Anne-Jean Robert, Joseph-Louis Gay-Lussac, Jean-Baptiste Biot Argon and other noble gases William Ramsay, John William Strutt (Lord Rayleigh) Nuclear weapons and radioactive fallout Harry Daghlian, Louis Slotin Refrigeration Albert Einstein, Leo Szilard Weather control Irving Langmuir Weather prediction Lewis Fry Richardson Chaos theory Edward Lorenz One might notice, skimming the right column of Table 1, a heavy concentration of European male names. That impression is accurate: The stories in this book are almost all based either in Europe or in North America. There are no women named in Table 1. In the book's index, about 100 primary entries refer to people. Only six of these are women and all but one (Dixy Lee Ray as governor of Washington) are presented as embellishments of the stories of primary male figures. Caesar's Last Breath was fun to read. I found that I learned a considerable amount, so it was rewarding as well. Admirably, Kean embraced the effort to describe challenging technical material and to do so in a quantitative manner and (often) mechanistic manner. He has written this book for a nontechnical (and mainly US) audience, and that feature led to some understandable but unappealing decisions. For example, he avoided the use of scientific notation, leaving us with statements like this for carbon dioxide: "you inhale 500 quadrillion molecules every time you breathe." I had to use Google to be reminded that a quadrillion is 1015. (We will return later to whether the statement is quantitatively accurate.) Kean often uses traditional US units of measure, rather than the metric system. At times, he drifts too far toward a folksy conversational tone, even when exploring a profound topic, as in this sentence: "The hunt for life on other planets raises all sorts of highfalutin spiritual questions about human beings and our place in the cosmos." (Why did he feel the need to insert the informal adjective "highfalutin?" The sentence would have been stronger without it. But I quibble.) Although I was impressed with Kean's effort to be quantitative and mechanistic, using my "skeptical reader" glasses, I did find some errors, both conceptual and numerical. He incorrectly states (p. 274) that small particles remain airborne owing to the effect of buoyancy. In fact, it is their slow settling velocity resulting from the balance of drag and gravitational forces that allows them to remain aloft for extended periods; the contribution of buoyancy is negligible. He incorrectly states (p. 317) that chlorofluorocarbons (CFCs) collectively, "…account for one-quarter of human induced global warming." According to IPCC's assessment for 2011, total anthropogenic radiative forcing then was 2.29 W/m2, of which only 0.18 W/m2 (8%) was attributable to CFCs. In a footnote (pp. 352–353) regarding a whimsical (and theoretical, of course) comparison of the temperatures of hell and heaven, he misinterprets the Stefan-Boltzmann law, writing "Since the ambient temperature of a planet rises quickly with an increase of sunlight (it scales up according to the fourth power)…." In fact, all else being equal, a blackbody's temperature rises in proportion to the one-fourth power (not fourth power) of the incident radiation. The biggest shortcoming of Kean's book is that he fails to effectively address his subtitle. "The air around us" is not the pristine atmosphere, certainly not when considering the composition of the air we breathe. People mostly live in cities. Urban air is not equivalent to the clean troposphere. People mostly breathe indoor air. Indoor air is not equivalent to outdoor air. To prepare this editorial, I checked Kean's numbers, especially with regard to the quantities of molecules inhaled with each breath for each of the species considered. His analysis was quantitatively correct in the introduction, for Caesar's last breath. He also did well for most of the molecules that are well mixed throughout the atmosphere, so that the background troposphere concentrations reasonably approximate inhalation exposure concentrations. However, some of his reported numbers do not survive scrutiny. For example, with respect to sulfur dioxide, he states (p. 17) that the level is "currently 0.00001 parts per million in the air; you inhale 120 billion molecules every time you breathe." Urban air concentrations are much higher than the 10 ppt he is suggesting. The WHO guideline for the 24-h average SO2 concentration is 20 μg/m3, corresponding to 7.5 ppb, roughly 1000× larger than the level assumed by Kean to be in the air we breathe. Kean's estimate for the inhalation of water vapor molecules is even more severely inaccurate. He notes that the abundance of water vapor is variable "depending on landscape and weather." He writes (p. 158) that, "you inhale anywhere from a few billion to several quadrillion molecules every time you breathe." This statement is quite far from the quantitative truth. The upper bound of the range quoted would require that the water vapor in air be present at a level less than a part per million. (Five quadrillion divided by 1022 is 0.5 ppm.) Indoors, common conditions are temperature in the range 20–25°C and relative humidity (RH) in the range 30–70%. At sea level, for 20°C and 30% RH, the molar abundance of water vapor in air is 0.7%. With each breath, one would inhale ~7 × 1019 or 70,000 quadrillion water molecules. For 25°C and 70% RH, the number of water molecules inhaled with each breath would be about three times larger, ~2 × 1020. Carbon dioxide is prominently featured in two stories in Kean's book. First, he recounts its role in the Lake Nyos disaster, a natural eruption that precipitously killed about 1700 people in Cameroon in 1986 (pp. 43–48). Second, Kean considers CO2 as a prominent greenhouse gas (pp. 315–319). In assessing the number of CO2 molecules inhaled, Kean makes two mistakes. One appears to be a straightforward factor of ten calculation error. If one inhales CO2 at a molar abundance of 400 parts per million, then the 1022 total molecules inhaled would contain 4 × 1018 CO2 molecules. That quantity corresponds to 4000 quadrillion, almost 10× larger than the 500 quadrillion reported by Kean. For the most part, though, the air around us does not contain the atmospheric background of 400 ppm CO2. As is well known to readers of Indoor Air, because carbon dioxide is a prominent metabolic gas, indoor levels are commonly substantially elevated above the 400 ppm atmospheric background. Primarily owing to the balance between human metabolic emissions and removal by means of ventilation, indoor CO2 levels can often be 1000 ppm or even higher. At 1000 ppm, with each breath we would inhale ~1019 CO2 molecules, or 10,000 quadrillion, a value 20× larger than reported by Kean. Considering carbon dioxide brings us back to the matter of rebreathed air. In indoor environments with high occupant density, such as classrooms, it is common for the CO2 level to be maintained at about 1000 ppm using mechanical ventilation provided at an outdoor air rate of ~7.5 L/s per person.1 At steady state, the flow of air in and out of the occupants' lungs is of the order of 0.1 L/s per person (obtained by multiplying 12–15 breaths per minute by 0.4–0.5 L/breath and converting from a per-minute to a per-second rate). The ratio of 0.1/7.5 is on the order of 1%. In steady-state conditions, that ratio would represent a rebreathed fraction, that is the fraction of the molecules inhaled that were previously exhaled from the lungs of one of the current occupants.2 So, with each breath, what we share in terms of molecules previously exhaled by all of the earth's human inhabitants (0.1–1%) is often quantitatively matched, and sometimes even exceeded, by the number of molecules recently exhaled by the persons with whom we share occupancy of an indoor space. Being so fundamental to our lives and our well-being, the atmosphere and its components are worthy of our attention. Sam Kean has done a fine service with his new book recounting interesting and important stories about air. However, he has also stumbled in a way that is all too common, mistaking the composition of the atmosphere as a whole with that routinely encountered by humans. Mainly, "the air around us" is indoor air; secondarily, "the air around us" is urban air. To understand how our atmosphere affects the human condition, the stories of indoor air and of the urban atmosphere deserve as much attention as those of the global atmosphere. References 1Persily A, de Jonge L. Carbon dioxide generation rates for building occupants. Indoor Air. 2017; 27: 868- 879. 2Rudnick SN, Milton DK. Risk of indoor airborne infection transmission estimated from carbon dioxide concentration. Indoor Air. 2003; 13: 237- 245. Citing Literature Volume28, Issue1January 2018Pages 3-5 ReferencesRelatedInformation
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