Yes, there is slightly less oxygen in humid air compared to dry air of the same volume at the same temperature and total pressure. Air is a mixture of gases, primarily nitrogen and oxygen, and humidity is the presence of water vapor within that mixture. When water vapor is introduced into a fixed volume of air, it occupies space that would otherwise be filled by other gas molecules, including oxygen. This physical phenomenon results in a minor reduction of oxygen molecules per breath.
Molecular Displacement
The reduction in oxygen molecules is governed by the fundamental laws of gas mixtures, specifically Dalton’s Law of Partial Pressures. This law states that the total pressure of a gas mixture equals the sum of the partial pressures of the individual gases. Since the total barometric pressure remains constant, when water vapor is added, its partial pressure contributes to the total. Consequently, the partial pressures of other gases, like nitrogen and oxygen, must decrease to maintain the constant total pressure.
The mechanism is also related to the molecular weight of the gases involved. Water vapor (\(\text{H}_2\text{O}\)) has a molar mass of approximately 18 grams per mole. Nitrogen (\(\text{N}_2\)) is about 28 grams per mole, and oxygen (\(\text{O}_2\)) is roughly 32 grams per mole. For a fixed volume of air at constant temperature and pressure, the total number of molecules must remain the same.
When lighter water molecules enter the air mixture, they effectively displace some of the heavier oxygen and nitrogen molecules. Because the incoming water molecules are lighter than the molecules they replace, humid air becomes less dense than dry air. This displacement results in a measurable decrease in the mass of oxygen within a specific volume of humid air.
Partial Pressure Versus Volumetric Percentage
Oxygen concentration is often cited as 20.95%, which is the volumetric percentage of oxygen molecules in dry air. When water vapor is present, the air is diluted, causing the volumetric percentage of oxygen in the total mixture to decrease slightly. However, this slight decrease in volumetric percentage is not the most relevant factor for human respiration.
The human body’s ability to absorb oxygen depends on the partial pressure of oxygen (\(P_{O_2}\)), which is the pressure exerted by only the oxygen molecules. \(P_{O_2}\) is calculated by multiplying the total atmospheric pressure by the fractional concentration of oxygen. When humidity is present, the water vapor pressure (\(P_{H_2O}\)) must be subtracted from the total pressure before calculating the partial pressures of the remaining gases.
At sea level, total atmospheric pressure is typically around 760 mmHg. Inside the alveoli of the lungs, air is warmed and saturated with water vapor at body temperature (\(37^\circ C\)). This gives the water vapor a fixed partial pressure of about 47 mmHg, which reduces the total pressure available for all other gases, including oxygen. This reduction in \(P_{O_2}\) is what ultimately drives oxygen into the bloodstream.
Why Humid Air Feels Harder to Breathe
The slight decrease in oxygen partial pressure due to humidity is negligible for a healthy person at sea level. The feeling of breathlessness in humid conditions is a physiological response related to the body’s inability to cool itself effectively. The body’s main cooling mechanism is the evaporation of sweat from the skin.
When the air is highly saturated with water vapor, the rate of sweat evaporation slows significantly. This inhibition of evaporative cooling leads to an increase in core body temperature, causing heat stress. To compensate, the body increases blood flow to the skin and the heart rate rises.
The body’s increased effort to regulate temperature also raises its metabolic rate, increasing the demand for oxygen. This higher demand, combined with lower oxygen availability and increased respiratory effort, creates the sensation of labored breathing. For individuals with pre-existing respiratory conditions, such as asthma or Chronic Obstructive Pulmonary Disease (COPD), high humidity can trigger airway inflammation and narrowing (bronchoconstriction). This physical narrowing is a much more significant cause of breathing difficulty than the minor molecular displacement of oxygen.