The air surrounding Earth is a vast, invisible envelope of mixed gases that makes life on the planet possible. This gaseous mixture, retained by gravity, acts as a protective buffer, shielding the surface from solar radiation and moderating temperature extremes. Understanding the composition of the air we inhale is fundamental to appreciating the complex chemical and biological processes it supports.
The Primary Recipe
The composition of dry air is remarkably consistent across the globe, with two gases dominating the mixture by volume. Nitrogen gas ($\text{N}_2$) makes up the largest fraction, accounting for approximately 78.08% of the atmosphere. Following nitrogen is oxygen ($\text{O}_2$), the gas most often associated with breathing, which constitutes about 20.95% of the air.
The remaining fraction is largely composed of the noble gas argon (Ar), which registers a concentration of about 0.93% by volume. Carbon dioxide ($\text{CO}_2$) is the most significant variable component among the major constituents, currently hovering around 0.04%. Despite its small share, this gas plays a disproportionate role in biological processes and atmospheric heat retention.
How the Body Uses Air
Breathing begins with the physical movement of air into the lungs, where the primary purpose is to facilitate gas exchange. This process is driven by the principle of partial pressure, which is the pressure exerted by a single gas within a mixture of gases. Inhaled air travels through the bronchial tubes until it reaches the microscopic air sacs called alveoli, which are surrounded by a dense network of capillaries.
Oxygen from the air moves across the thin alveolar and capillary walls into the bloodstream because its partial pressure is higher in the alveoli than in the deoxygenated blood arriving from the body. Once in the blood, the oxygen quickly attaches to hemoglobin within red blood cells for transport to tissues throughout the body. Meanwhile, the waste product carbon dioxide, which has a higher partial pressure in the returning blood than in the alveoli, diffuses in the opposite direction into the air sacs.
The majority of the air we inhale, which is nitrogen, remains physiologically inert during this exchange. Nitrogen does not readily dissolve into the blood or react with hemoglobin under normal atmospheric pressure, so it is simply exhaled.
The Invisible Ingredients
Beyond the major components, air contains a range of trace elements present in trace amounts. Among these are the noble gases, such as neon, helium, and krypton, which are chemically unreactive and exist in fractions of a percent. Neon, for instance, is found at levels of about 18 parts per million, a concentration that remains relatively constant across the lower atmosphere.
The most variable atmospheric component is water vapor, the gaseous form of water, which can range from nearly zero up to about 4% of the air’s volume. This variability is highly dependent on temperature and location, playing a significant role in weather patterns and humidity. Water vapor also influences breathing comfort, as it is responsible for humidifying the air we draw into our lungs.
Air Quality and Impurities
The air we breathe often contains various contaminants that degrade its quality and pose risks to health. These impurities, known as air pollutants, include microscopic particulate matter ($\text{PM}_{2.5}$ and $\text{PM}_{10}$), which are tiny solid or liquid particles suspended in the air. When inhaled, the smallest of these particles can penetrate deep into the lungs and even enter the bloodstream, contributing to respiratory and cardiovascular problems.
Other significant impurities include ground-level ozone ($\text{O}_3$), a gas that forms when pollutants from vehicles and industry react chemically in sunlight. Breathing ground-level ozone can irritate the airways and trigger asthma attacks or other lung diseases. Carbon monoxide ($\text{CO}$), a colorless, odorless gas produced by incomplete combustion, interferes with the blood’s ability to transport oxygen. Monitoring and regulating the levels of these contaminants is necessary to protect public health.