# Their last breath is in your next one Breathe out and watch your breath travel around the Earth on the real winds. Then find out how many molecules from the breath of someone you have lost enter you with every breath. ## How many molecules are in one breath? What you just let out is about half a litre of air. That's not much, but it is crowded: roughly **1.2 × 1022** molecules. Written out in full, the number has 23 digits. Counting one a second would take you more than 25,000 times the age of the Universe. ## How many breaths fit in the atmosphere? Now think of all the air on Earth, from the ground to where the sky runs out. Cut it into breaths like yours and you get roughly **9.0 × 1021** breaths. Look at the two numbers: molecules in a breath, breaths in the atmosphere. They are almost the same size. That coincidence is the whole story that follows. after one hour ## Where does your breath go? Every point on the globe is a piece of your breath. In the first hour it doesn't go far: it mixes into the air above the town and the country around it. Start somewhere else after three days The wind catches it. At mid-latitudes the air aloft flows mostly from west to east, so the cloud stretches into a trail that crosses whole countries. after three weeks The trail has gone round the Earth. Your breath is now a thin ring around the hemisphere, roughly along your line of latitude. after three months The ring has widened from the tropics towards the pole. The equator, though, is a kind of threshold: the winds of the two hemispheres mix with each other slowly. after one year Some of it has crossed the equator. Measurements of sulphur hexafluoride, a gas tracked by ground stations, put the exchange time between the hemispheres at 1.1–1.4 years. after two years ## How long until your breath is everywhere? About two years. Your breath is now spread, almost evenly, through the whole lower atmosphere, where we breathe. Anyone, anywhere, draws in on average **one molecule** of it with every breath, counting only the nitrogen and argon, which are not lost. The two nearly equal numbers above do the sum: a share of one in 9.0 × 1021, multiplied by 1.2 × 1022, gives 1.4; three-quarters of that lasts, so about one. ## And the breath of those who are gone? Think of someone who is no longer here. You don't have to tell us who. Just tell us when they died and where they lived. The year they died Where they lived How old they were ## How many molecules of their last breath are in yours? On average, **one**. Not in every breath: sometimes none, sometimes two or three. **Nearly two in three** of your breaths hold at least one molecule of their last. Try it. Each press is one breath of yours. Breathe in ## And from their whole life? The last breath is only one. A person breathes about 21,600 times a day, around 7.9 million times a year. Over an 80-year life, that is more than 630 million breaths. So each breath of yours holds about **640 million** molecules that once passed through their lungs (at 15 breaths a minute; probably fewer, because a child's breaths are smaller). Nitrogen and argon, about three-quarters of the air you breathe out, are not used up: a nitrogen molecule lasts 13 million years on average. No molecule carries any mark of them. But they are there. ## How much of the air has been breathed before? The same sum, for all of the roughly 117 billion people ever born. All their breaths, added up, come to more than 1019. The answer is the reverse of what you might expect: of all the air on Earth, only about **one molecule in 800** has ever been through a human lung. Even on the most generous assumptions for nitrogen and argon, it is at most one in 550. Almost all the air you draw in has never been breathed by any human. And yet a breath holds so many molecules that even that small share means that every breath of yours carries more than **1019** molecules once breathed by people who lived before you, back to the first humans. ## Where will the breath you took today go? The breath you let out at the start is above you now. In two years it will be all through the lower atmosphere. A child born in Sydney in 2027 will breathe it all their life: on average one of its molecules with each breath. So will that child's children. Breathe in slowly, once. If the person you thought of died more than two years ago, about one molecule of that air was in their last breath. ## Method and sources A resting adult breath is about 0.5 litres. At body temperature and sea-level pressure that is {{n_breath_exact}} molecules (ideal gas law); a breath of room air at 20 °C holds {{n_in}} molecules of dry air. The mass of dry air in the atmosphere is 5.1352 × 1018 kg (Trenberth and Smith, 2005); at a molar mass of 28.96 g/mol that gives {{n_atm}} molecules. We count only nitrogen and argon, 74% of exhaled, saturated air; the rest (oxygen, carbon dioxide and water vapour) is taken up and given back faster by plants, the ocean and rain. A nitrogen molecule lasts about 13 million years on average before life or lightning splits it (Jacob, 1999, ch. 6), and argon, a noble gas, has no chemical way out. For a recent death, leaving oxygen out makes the result too small, if anything, by up to a third. The denominator is the whole atmosphere, although air reaches the stratosphere only after 5–10 years; that too makes the figures for the first years too small, if anything. The mean number of molecules from one fully mixed breath that land in one breath of yours is {{lambda}}. Molecules caught at random follow a Poisson distribution, so the chance of catching at least one is {{p_any_exact}}. The breathing rate is 15 breaths a minute, the middle of the usual 12–20 for a resting adult. The number of years lived by all humans (1.4–2.0 × 1012) comes from the model on Of everyone ever born, which number are you?, built on PRB estimates. Children breathe smaller volumes, and the same molecule can be breathed more than once, so even "one in {{ever_one_in}}" is more likely too high than too low. The bound of {{ever_one_in_hi}} uses the highest estimate of years lived and 20 breaths a minute. The path of the breath on the globe is a model, not a forecast. The points ride the monthly mean wind from the NCEP/NCAR reanalysis (1991–2020 average), averaged by air mass between 1,000 and 100 hPa (nearly all the air below the stratosphere), plus a random step for the eddies a monthly mean smooths away. We chose the size of that step (K = 1.0 × 106 m²/s) so that the model's exchange time between hemispheres is about 1.2 years, within the values measured with sulphur hexafluoride (1.1–1.4 years; Yang et al., 2019). For someone who died less than two years ago, the result comes from the same model: how much of their breath is now in a 15-degree band around your latitude, compared with how much would be there if it had spread evenly. We don't know the exact month, so we count from the middle of the chosen year (for the current year, from halfway through the part that has passed). The idea is not new: the physicist James Jeans did the same sum in 1940, with "Julius Caesar's last breath". Sam Kean wrote a book around it in 2017. - Trenberth, K. E. and Smith, L. (2005). The mass of the atmosphere: a constraint on global analyses. Journal of Climate 18(6), 864–875. https://doi.org/10.1175/JCLI-3299.1 - Jacob, D. J. (1999). Introduction to Atmospheric Chemistry, ch. 4 (atmospheric transport) and ch. 6 (nitrogen cycle). Princeton University Press. https://acmg.seas.harvard.edu/education-introduction-atmospheric-chemistry/ - Patra, P. K. et al. (2011). TransCom model simulations of CH4 and related species: linking transport, surface flux and chemical loss with CH4 variability in the troposphere and lower stratosphere. Atmospheric Chemistry and Physics 11, 12813–12837. https://doi.org/10.5194/acp-11-12813-2011 - Yang, H. et al. (2019). Evaluating simulations of interhemispheric transport: interhemispheric exchange time versus SF6 age. Geophysical Research Letters 46, 1113–1120. https://doi.org/10.1029/2018GL080960 - Kalnay, E. et al. (1996). The NCEP/NCAR 40-Year Reanalysis Project. Bulletin of the American Meteorological Society 77(3), 437–471. Data: NOAA PSL long-term monthly means. https://psl.noaa.gov/data/gridded/data.ncep.reanalysis.html - Physiology, Tidal Volume. StatPearls. https://www.ncbi.nlm.nih.gov/books/NBK482502/ - Tachypnea. StatPearls (normal resting rate: 12–20 breaths a minute). https://europepmc.org/article/MED/31082106 - Quantify impacted scope of human expired air under different head postures and varying exhalation rates. Building and Environment 46 (2011): composition of exhaled air. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7127751/ - Population Reference Bureau (2022). How Many People Have Ever Lived on Earth? https://www.prb.org/articles/how-many-people-have-ever-lived-on-earth/ - Jeans, J. (1940). An Introduction to the Kinetic Theory of Gases, ch. I, §17, p. 32. Cambridge University Press. https://archive.org/details/introductiontoth000487mbp - Kean, S. (2017). Caesar's Last Breath: Decoding the Secrets of the Air Around Us. Little, Brown. https://www.hachettebookgroup.com/titles/sam-kean/caesars-last-breath/9780316381628/ - Natural Earth, 1:110m land (public domain). https://www.naturalearthdata.com/ Canonical: https://mariuscomper.uk/ultima-suflare/en/