Incidentally, the reason your nose runs in chilly weather is the same reason your bathroom windows run with water in chilly weather. In the case of your nose, warm air from your lungs meets cold air coming into the nostrils and condenses, resulting in a drip.
The lungs are also wonderfully good at cleaning. According to one estimate, the average urban dweller inhales some twenty billion foreign particles every day—dust, industrial pollutants, pollen, fungal spores, whatever is adrift on the day’s air. A lot of this stuff can make you very ill, but it doesn’t, by and large, because your body is normally adept at challenging intruders. If an invading particle is big or especially irritating, you will almost certainly cough or sneeze it straight back out again (often in the process making it someone else’s problem). If it is too small to provoke such a vehement response, it will in all likelihood be trapped in the mucus that lines your nasal passages or caught by the bronchi, or tubules, in your lungs. These tiny airways are lined with millions and millions of hairlike cilia that act like paddles (but beating furiously at sixteen times a second), and they swat the invaders back into the throat, where they are diverted to the stomach and dissolved by hydrochloric acid. If any invaders manage to get past these waving hordes, they will encounter little devouring machines called alveolar macrophages, which gobble them up. Despite all this, occasionally some pathogens get through and make you sick. That’s the way life is, of course.
Only recently has it been discovered that sneezes are a much more drenching experience than anyone thought. A team led by Professor Lydia Bourouiba of MIT, as reported by Nature, studied sneezes more closely than anyone had ever chosen to before and found that sneeze droplets can travel up to eight meters and drift in suspension in the air for ten minutes before gently settling onto nearby surfaces. Through ultra-slow-motion filming, they also discovered that a sneeze isn’t a bolus of droplets, as had always been thought, but more like a sheet—a kind of liquid Saran Wrap—that breaks over nearby surfaces, providing further evidence, if any were needed, that you don’t want to be too close to a sneezing person. An interesting theory is that weather and temperature may influence how the droplets in a sneeze coalesce, which could explain why flu and colds are more common in cold weather, but that still doesn’t explain why infectious droplets are more infectious to us when we pick them up by touch rather than when we breathe (or kiss) them in. The formal name for the act of sneezing, by the way, is sternutation, though some authorities in their lighter moments refer to a sneeze as an autosomal dominant compelling helio-ophthalmic outburst, which makes the acronym ACHOO (sort of).
Altogether the lungs weigh about 2.4 pounds, and they take up more space in your chest than you probably realize. They jut up as high as your neck and bottom out at about the breastbone. We tend to think of them as inflating and deflating independently, like bellows, but in fact they are greatly assisted by one of the least appreciated muscles in the body: the diaphragm. The diaphragm is a mammalian invention and it is a good one. By pulling down on the lungs from below, it helps them to work more powerfully. The increased respiratory efficiency that the diaphragm brings enables us to get more oxygen to our muscles, which helped us to become strong, and to our brains, which helped us to become smart. Efficiency is also assisted by a slight differential in air pressure between the outside world and the space around your lungs, known as the pleural cavity. Air pressure in the chest is less than atmospheric pressure, which helps to keep the lungs inflated. If air gets into the chest, because of a puncture wound, say, the differential vanishes and the lungs collapse to only about a third of their normal size.
Breathing is one of the few autonomic functions that you can control intentionally, though only up to a point. You can shut your eyes for as long as you wish, but you cannot shut off your breathing for long before the autonomic system reasserts itself and compels you to breathe. Interestingly, the discomfort you feel when you hold your breath for too long is caused not by the depletion of oxygen but by a buildup of carbon dioxide. That’s why the first thing you do when you stop holding your breath is blow out. You would think that the most urgent need would be to get fresh air in rather than stale air out, but no. The body so abhors CO2 that you must expel it before gulping in replenishment.
Humans are pretty poor at holding their breath—indeed, are inefficient breathers altogether. Our lungs can hold about six quarts of air, but normally we breathe in only about half a quart at a time, so there is plenty of scope for improvement. The very longest any human being has voluntarily held his breath was twenty-four minutes and three seconds by Aleix Segura Vendrell of Spain, who did it in a pool in Barcelona in February 2016, but that was after breathing pure oxygen for some time beforehand and then lying motionless in the water to reduce energy demand to a minimum. Compared with most aquatic mammals, this is really poor. Some seals can stay underwater for two hours. Most of us can’t last much more than a minute, if that. Even the famous lady pearl divers of Japan, known as the ama, don’t stay underwater for more than about two minutes normally (though they do make a hundred or more dives a day).
All in all, it takes a lot of lung to keep you going. If you are an averagely sized adult, you will have roughly twenty square feet of skin, but about a thousand square feet of lung tissue containing about fifteen hundred miles of airways. Packing such a lot of breathing apparatus into the modest space of your chest is a nifty solution to the very considerable problem of how to get a lot of oxygen efficiently to billions of cells. Without that intricate packaging, we might have to be like kelp—hundreds of feet long but with all the cells very near the surface to facilitate oxygen exchange.
Considering how complex an operation respiration is, it is not surprising that the lungs can cause us a lot of problems. What is perhaps surprising is how little we sometimes understand the causes of these problems, and of no condition is that more true than asthma.
II
IF YOU HAD to nominate someone to be a poster figure for asthma, you could do worse than the great French novelist Marcel Proust (1871–1922). But then you could nominate Proust as a poster figure for a great many medical conditions because he had a superabundance of them. He suffered from insomnia, indigestion, backaches, headaches, fatigue, dizziness, and crushing ennui. More than anything else, however, he was a slave to asthma. He had his first attack at nine and passed a wretched life thereafter. With his suffering came an acute germ phobia. Before opening his mail, he would have his assistant place it in a sealed box and expose it to formaldehyde vapors for two hours. Wherever he was in the world, he sent his mother detailed daily reports on his sleep, lung function, mental composure, and bowel movements. He was, as you will gather, somewhat preoccupied with his health.
Though some of his concerns were perhaps a touch hypochondriacal, the asthma was real enough. Desperate to find a cure, he submitted to countless (and pointless) enemas; took infusions of morphine, opium, caffeine, amyl, trional, valerian, and atropine; smoked medicated cigarettes; inhaled drafts of creosote and chloroform; underwent more than a hundred painful nasal cauterizations; adopted a milk diet; had the gas to his house cut off; and lived as much of his life as he could in the fresh air of spa towns and mountain resorts. Nothing worked. He died of pneumonia, his lungs worn out, in the autumn of 1922 aged just fifty-one.