Reading accounts of historic spreads of infectious diseases through human travel reminds me of the modern phenomena that almost every time that I embark on airplane travel I end up getting sick as a result of it. I was curious to see if this has just been my misfortune, or if it is a common occurrence so I did a little bit of research on the topic and will attempt to put my findings in context here.
First, let me distinguish the fatal diseases endemic to 15th century sea travel from the far-from-fatal annoyances that I experience from air travel. Besides the dangers of the water, ancient sea travel was made more perilous by disease caused by unsuitable living conditions. Poor diets, poor sanitation, and close living quarters all made ships rampant with diseases. Often only half of the crew would return from long sea voyages because of the high mortality rate caused by disease on board ships. Furthermore, upon arrival to foreign lands, many European explorers were struck by local diseases (such as malaria) and because they lacked any immunity to the disease the outcome was often death. Quite differently, the relatively short duration of modern airplane travel and my ability to bring food on board means that my diet is only minimally impacted by the flight. Also, sanitation seems to be no worse on airplanes than in other settings of my life. Advancements in modern medicine allow me to receive vaccines before I travel so that I can be immunized to foreign diseases. The sickness that I experience from air travel is limited to common flu and cold symptoms such as an upset stomach, head pain and fever, sinus congestion and the likes.
A couple factors can explain the prevalence of these symptoms during and immediately after air travel. One factor has to do with the circumstances of the time that I usually travel by airplane. Often during the months of summer vacation I have more time to travel leisurely by way of car, bus, or train and since my travel is purely recreational, my budget confines me to such ground travel. Contrastingly, the academic calendar allows less time for travel during breaks for winter holidays, so in order to arrive at my destination in time I must rely on an airplane. Also, road conditions are worse in the winter months making ground travel less appealing. Furthermore, since family obligations are typically the reasons that make me travel hundreds of miles during these times, my parents are usually willing to cover the additional travel expenses inherent in air travel.
Anyways, I travel by airplane most frequently in the cold, winter months. There are some studies that indicate that human immune systems may have a degree of seasonality built into them, making humans more susceptible to disease during the colder winter season. Although no such study has been conducted directly on humans, mammalian seasonality is well established in the biologist intellectual community. The theory suggests that the human immune system is more vulnerable in cold temperatures because the body must exert more energy than otherwise to keep the core temperature near 98.6 degrees Fahrenheit. In simple terms, this means that the body has less energy to devote to fighting invading disease germs.
Also, many diseases have lytic cycles, meaning that they may reside within the body in a latent stage and until triggered by stress, fatigue, or adrenaline. There is a possibility that pressure to finish academic work before traveling home may induce enough stress and cause enough shortage of sleep to trigger the symptomatic stage of a disease already inside my body. Alternatively, excitement about going home may arouse enough adrenaline within me to similarly trigger a latent virus.
Winter months are further conducive to the spread of disease because people tend to hang out indoors more, where diseases are more easily incubated. Especially at holiday gatherings, people are crowded together indoors so diseases can spread easily between them. Additionally, since these holidays usually bring people together who don't see each other very often, they are exposed to diseases to which they probably don't have immunity against.
These seasonal factors may provide an explanation as to how external, situational components can play a role in making me sick at the times that I travel by air. Several properties about the airplane itself also may be to blame for making me sick. Similarly to what was already mentioned, when people are crowded closely together it is easier for diseases to spread between them and airplane cabins happen to be one of the most crowded biospaces that I ever occupy. Not only are seats packed closely together, but low ceilings also combine to make the space available per person very small.
Insufficient air ventilation may also aid in the spread of diseases on board airplanes. In the mid 1980's the airlines, in an attempt to boost fuel economy, redesigned airplane ventilation systems so that instead of releasing 100 percent fresh air they used half fresh air and half recirculated air. Most airplanes' ventilation systems are now equipped with high efficiency particulate air filters, but these filters do not completely reduce exposure to infectious particles and since they are not required many air passengers are breathing unfiltered air.
Even the fresh air that enters circulation within an airplane may also pose problems for passengers' health. At high altitudes, where airplanes are for most of their flights, the gas ozone is more abundant than at ground level. Ozone is an air pollutant that acts as a respiratory irritant and can cause chest tightness, coughing, and shortness of breath. Repeated exposure to ozone can lead to permanent lung scarring and loss of lung function. At cruising altitudes, ozone is 60 times more abundant in the air than the recommended maximum human intake. Some airplanes have installed catalytic converters to turn ozone molecules into oxygen, but again since this is not required passengers cannot be assured that breathing airplane air will not be hazardous to their health.
Additionally, ultra violet radiation is stronger at higher altitudes causing air travelers to be exposed to 265 times the radiation dose they receive on the ground. Studies have shown that flight crews have breast cancer at twice the rate as the population at large, and skin cancer rates as high as 15 times the general populace. Although these effects are somewhat disconcerting, they mostly explain long term, residual effects of air travel and cannot actually be applied to describe the circumstances of the immediate sickness I have felt from traveling by airplane.
As I've already mentioned, a person is more likely to get sick when in close proximity with many other people and when that person's bodily systems are stressed. Inflight stresses can include lowered barometric and oxygen pressure, noise and vibration from turbulence, erratic temperatures, low humidity, jet lag, and cramped seating. Most airplane cabins are pressurized to an altitude of 6,000 to 8,000 ft. which, for most people, means that there is less oxygen in the air. At sea level a human's blood becomes 97% saturated with oxygen, but at this lowered atmospheric pressure a person's blood is only 90% saturated. The body compensates by increasing the heart rate and respiratory rate so that it can circulate the necessary oxygen content throughout the bloodstream. Quicker breathing and an accelerated heart rate may worry some passengers and cause them to become frantic about the state of their health. The lower atmospheric pressure of the cabin also causes gases within human body cavities to expand as much as 25%. This can cause pain in the abdomen, middle ear, and sinuses, which if foreign to an air passenger may also cause them distress.
Lastly, but certainly not of least importance, is the fact that the air within airplane cabins has a very low humidity. This is because the fresh air drawn in from high altitudes is almost completely devoid of moisture. In fact, it is drier than the air of any desert. As a result, passengers may experience a drying effect of their airway passages, the cornea (particularly under contact lenses), and the skin. This also makes it easier for someone to experience dehydration if they are not being careful enough about their water intake. The dry cabin air also makes passengers' nasal membranes dry and more vulnerable to airborne infection. This is because the mucus in a person's nose is one of their first lines of defense against invading germs and when their nasal passages are drier, the mucus is more penetrable. Also, when a person's sinuses get dry they get more irritated which produces more tears in the epithelial layer which makes it easier for germs to slip into the body.
To summarize, I've found that a person is more likely to get sick if their body is under a lot of stress and if they are in a crowded biospace with many other people. The typical time of year that I usually travel by airplane may explain some stresses on my body, but the atmosphere of an airplane certainly can explain a few others. Also, because airplanes bring many people into close quarters, they are likely to pass on their germs to each other. Since these people usually aren't from the same locality and don't usually interact with each other, they likely do not have immunity to germs that they are not familiar with and they are more likely to get sick from exposure to these germs. I think it is not just my misfortune, but that every air passenger faces a pretty significant risk of catching a minor disease when they choose to travel by air.
Sources:
Cipolla, Carlo M. "Epilogue", Guns Sails, and Empires: Technological Innovation and the Early Phases of European Expansion, 1400-1700.
Aerospace Medical Association, www.asma.org/publications
"Lysogeny", McGraw-Hill Encyclopedia of Science and Technology.
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