Atmospheric pressure, the weight of the air column above us, constantly presses down on everything on Earth’s surface, though we rarely perceive its presence. Changes in this invisible force, whether decreasing at high altitudes or increasing in deep waters, can profoundly impact the human body. Understanding these effects is important for those who travel to varying elevations or participate in activities like diving.
Understanding Atmospheric Pressure
Atmospheric pressure is the force exerted by the air above a given point, commonly measured in units like pounds per square inch (psi), atmospheres (atm), or kilopascals (kPa). At sea level, average pressure is approximately 14.7 psi, 1 atm, or 101.325 kPa. This pressure changes with altitude: it decreases as elevation increases due to less air above, and rapidly increases when descending below sea level, such as when diving.
Minor fluctuations also occur due to weather systems, with high-pressure fronts indicating stable weather and low-pressure fronts associated with changing conditions. Air density, which contributes to pressure, is highest at lower altitudes because gravity pulls most air molecules closer to the Earth’s surface. This results in lower pressure and less oxygen per breath at higher altitudes.
How Decreased Pressure Affects the Body
When atmospheric pressure decreases, such as during ascent to high altitudes, gases within the body expand. This expansion occurs in enclosed cavities like the ears, sinuses, and digestive tract, leading to discomfort or pain, often called barotrauma. For example, gas in the gastrointestinal tract can expand, causing bloating and abdominal pain (High-Altitude Flatus Expulsion or HAFE). If not equalized, expanding gas in the middle ear can cause the eardrum to bulge outwards, resulting in pain, muffled hearing, and in severe cases, rupture.
Reduced oxygen availability, known as hypoxia, is another consequence. At higher altitudes, while the percentage of oxygen in the air remains constant, lower atmospheric pressure means fewer oxygen molecules per breath. This drops oxygen saturation in the blood, making it harder for the body to get sufficient oxygen to its tissues. Symptoms of acute mountain sickness (AMS) include headache, nausea, fatigue, dizziness, and shortness of breath. In severe instances, fluid can accumulate in the lungs (High Altitude Pulmonary Edema or HAPE) or the brain (High Altitude Cerebral Edema or HACE), which can be life-threatening.
How Increased Pressure Affects the Body
Conversely, increased atmospheric pressure, typically experienced during diving, compresses gases within the body. As a diver descends, pressure increases, compressing air in spaces like the ears and sinuses, requiring equalization to prevent discomfort or injury. Without proper equalization, the eardrum can bulge inward, and in extreme cases, rupture.
Increased pressure also leads to greater absorption of gases, particularly nitrogen, into the blood and body tissues. While harmless at depth, if a diver ascends too quickly, these dissolved gases form bubbles within the body, leading to decompression sickness (DCS), commonly known as “the bends.” Symptoms of DCS range from joint pain and skin rashes to serious neurological issues, paralysis, or even death. Slow ascent rates and decompression stops during diving are crucial to allow these dissolved gases to be released.
Increased nitrogen pressure at depth can also lead to nitrogen narcosis, or “rapture of the deep.” This condition, noticeable around 100 feet (30 meters) of depth, affects the brain similarly to alcohol intoxication, causing impaired judgment, difficulty concentrating, and sometimes euphoria or anxiety. These effects can compromise a diver’s safety.
The Body’s Responses and Prevention
The human body adapts to changes in atmospheric pressure, particularly decreased pressure at high altitudes. This process, known as acclimatization, involves physiological adjustments over days to weeks. The body responds to lower oxygen by increasing breathing rate and depth, and over time, producing more red blood cells to enhance oxygen transport. The kidneys also balance blood pH during this adaptation.
To prevent altitude sickness, gradual ascent allows the body time to acclimatize to each new elevation. Staying hydrated and avoiding alcohol and excessive exertion during initial days at altitude also helps. Certain medications can be used preventatively for those susceptible. If symptoms develop, stopping ascent and descending to a lower altitude are important.
For increased pressure environments like diving, prevention involves adhering to safe diving practices. This includes proper equalization techniques during descent to manage gas compression. To prevent decompression sickness, divers must follow dive tables or computer guidelines that dictate safe ascent rates and necessary decompression stops, allowing nitrogen to off-gas. Avoiding diving with congestion or illness is also advised, as it can hinder equalization. Recognizing symptoms of decompression sickness or severe altitude sickness necessitates immediate medical attention to prevent serious complications.