The air you breathe is roughly 78% nitrogen and 21% oxygen, with the remaining 1% split among argon, carbon dioxide, water vapor, and a handful of trace gases. That ratio has held steady for hundreds of millions of years, and it stays essentially the same whether you’re at sea level or on a mountaintop. What does change with altitude is the total pressure pushing those gases into your lungs, which is why high elevations feel so different even though the air’s recipe is identical.
The Three Main Gases
Nitrogen makes up 78.084% of dry air by volume. It’s the dominant gas in every breath you take, yet your body does almost nothing with it. Nitrogen passes in and out of your lungs without being absorbed in any meaningful amount. Its main role is structural: it dilutes oxygen to a concentration your cells can use safely. Pure oxygen, breathed for extended periods, actually damages lung tissue.
Oxygen accounts for 20.946%. This is the gas your body is after. Red blood cells pick it up in the lungs and deliver it to every tissue, where cells use it to convert food into energy. Even a small drop in available oxygen, from roughly 21% down to about 16%, causes noticeable symptoms like impaired judgment and faster breathing.
Argon rounds out the top three at 0.934%, just under 1%. It’s a noble gas, meaning it resists bonding with other elements, and for a long time scientists assumed it had zero biological activity. More recent lab research has complicated that picture. In animal studies, breathing high concentrations of argon (around 50 to 70%) appeared to reduce brain damage after oxygen deprivation, suggesting the gas interacts with living tissue in ways that aren’t fully understood. At the trace amount in normal air, though, argon is effectively inert. You inhale it and exhale it unchanged.
Carbon Dioxide and Trace Gases
Carbon dioxide gets outsized attention because of its role in climate, but it’s actually a tiny fraction of the atmosphere. As of December 2025, the global average concentration sat at 427 parts per million (ppm), or about 0.043%. That number has been climbing steadily from a pre-industrial baseline of around 280 ppm. Small as the percentage is, carbon dioxide is central to your body’s breathing reflex. Your brain monitors CO2 levels in the blood, not oxygen levels, to decide when to trigger another breath.
Beyond carbon dioxide, the air contains traces of neon, helium, methane, krypton, hydrogen, and xenon, all at concentrations measured in parts per million or less. Ozone also appears at ground level, typically between 20 and 45 parts per billion in midlatitude regions, varying with location, elevation, and nearby pollution sources. None of these trace gases play a direct role in normal breathing, but some of them, particularly methane and ozone, matter enormously for atmospheric chemistry and air quality.
Water Vapor: The Wild Card
Every percentage listed above describes “dry” air, meaning air with the water vapor removed. In reality, water vapor is always present, and its concentration swings wildly depending on temperature and geography. In cold polar regions, the air holds almost no moisture. In the tropics, water vapor can reach about 30 grams per kilogram of air, roughly 3 to 4% of the atmosphere by mass. That makes water vapor the single most variable component of the air you breathe.
Temperature is the main driver. Warm air can hold dramatically more moisture than cold air, which is why a July afternoon in Houston feels so different from a January morning in Denver even though both places share the same baseline gas mixture. Relative humidity describes how close the air is to its moisture-carrying capacity at a given temperature, so 50% humidity on a hot day contains far more actual water than 50% humidity on a cold day.
What Changes at Higher Altitudes
A common misconception is that air “thins out” by losing oxygen at high elevations. The ratio of gases stays the same at 5,000 meters as it is at the beach. What drops is atmospheric pressure, the force pushing all those gas molecules together. At sea level, atmospheric pressure is about 760 mmHg, which gives oxygen a partial pressure of roughly 160 mmHg. At the summit of Mount Everest, total pressure falls to around 260 mmHg, and the oxygen available for your lungs to absorb drops to about 55 mmHg. That’s roughly a third of the sea-level value.
Your body compensates by breathing faster and deeper, a response that pushes more carbon dioxide out of the blood and raises the relative concentration of oxygen in your lungs. Over days and weeks at altitude, your body produces more red blood cells to carry whatever oxygen is available. But in the short term, the lower pressure means less oxygen reaches your tissues, which is why altitude sickness can set in above about 2,500 meters.
What Happens to Air Inside Your Lungs
The air you inhale isn’t the same as the air that reaches the deepest part of your lungs. By the time a breath travels down your windpipe and into the tiny air sacs called alveoli, it has been warmed to body temperature and saturated with water vapor. That water vapor takes up space, reducing the partial pressure of oxygen from about 160 mmHg in the outside air to roughly 100 mmHg in the alveoli at sea level.
Meanwhile, carbon dioxide is constantly flowing in the opposite direction. Your blood carries CO2 produced by working cells back to the lungs, where it diffuses into the alveoli and gets exhaled. The exhaled breath you release contains about 100 times more carbon dioxide than the air you just inhaled and slightly less oxygen. This two-way exchange, oxygen in and carbon dioxide out, happens across a membrane thinner than a single cell, and a complete cycle takes less than a second.
Particles and Pollutants in the Mix
Air isn’t just gases. It carries a constantly shifting load of solid particles and liquid droplets collectively called particulate matter. Dust, pollen, sea salt, soot, smoke, and tiny drops of acid all float in the air at varying concentrations. Some of these particles are large enough to see, like visible smoke or dust clouds. Others are so small they penetrate deep into lung tissue when inhaled.
Indoors, the picture changes further. Volatile organic compounds (VOCs), chemicals that evaporate from paints, cleaning products, furniture, and building materials, are consistently found at concentrations two to five times higher inside homes than outside. In some cases, indoor levels reach up to ten times outdoor levels. Formaldehyde, benzene, and methylene chloride are among the most common. These aren’t part of the atmosphere’s natural recipe, but they’re very much part of the air most people actually breathe day to day, since the average person spends the majority of their time indoors.