The limewater test is a qualitative, not quantitative, method for detecting the presence of carbon dioxide. While it effectively indicates the presence of carbon dioxide by turning cloudy, it cannot precisely measure the exact concentration of carbon dioxide in the air. The test relies on a chemical reaction where carbon dioxide reacts with limewater to form calcium carbonate, which clouds the solution. However, the degree of cloudiness is not a reliable measure of the exact concentration of CO₂. This is because the reaction's visibility can be influenced by factors such as the volume of air passed through the limewater, the concentration of the limewater solution, and the duration of exposure. For precise quantitative analysis of carbon dioxide concentrations, more sophisticated methods are required, such as gas chromatography or infrared gas analysis. These techniques can accurately measure gas concentrations, providing exact data necessary for detailed scientific analysis.

The lower oxygen concentration in expired air compared to inspired air is a result of the process of oxygen uptake by the body's tissues. When air is inhaled, it contains approximately 21% oxygen, the standard concentration in the Earth's atmosphere. As this air passes through the lungs, oxygen is absorbed by the blood. The oxygen molecules diffuse across the alveolar membrane into the bloodstream, where they bind to haemoglobin in red blood cells and are transported throughout the body. This oxygen is then used by cells for metabolic processes, particularly cellular respiration, which generates the energy required for various bodily functions. Consequently, when air is exhaled, the oxygen concentration is reduced to about 16% as a significant portion of the oxygen has been absorbed and utilised. This decrease in oxygen concentration is indicative of the body's metabolic activity and highlights the efficiency of the respiratory system in oxygenating the blood.

The moisture content in expired air is noticeably higher than that in inspired air. This difference is primarily due to the respiratory process. As air is inhaled, it passes through the nasal passages, trachea, and bronchi, where it is warmed and humidified before reaching the lungs. The lining of the respiratory tract secretes mucus and moisture, which adds water vapour to the inspired air. When the air is exhaled, it carries this added moisture, leading to a higher water vapour content in expired air. The body's natural process of warming and humidifying air is essential for protecting lung tissue and facilitating efficient gas exchange. Additionally, this moisture helps in maintaining the internal temperature and hydration levels of the body, playing a vital role in homeostasis. The difference in water vapour content between inspired and expired air is therefore a direct result of the respiratory system's conditioning of the air.

Limewater, a solution of calcium hydroxide, reacts specifically with carbon dioxide to form calcium carbonate, which is insoluble and causes the solution to turn cloudy. In inspired air, the concentration of carbon dioxide is quite low, typically around 0.04%, reflecting the average CO₂ content in the Earth's atmosphere. This minimal concentration is not enough to cause a significant reaction with limewater, resulting in no noticeable cloudiness. In contrast, expired air contains a significantly higher concentration of carbon dioxide, approximately 4%, which is a direct result of the metabolic processes of cellular respiration within the body. When this CO₂-rich expired air is bubbled through limewater, the higher concentration of carbon dioxide reacts more extensively with the calcium hydroxide, leading to the formation of enough calcium carbonate to turn the limewater visibly cloudy. This experiment visually demonstrates the increase in carbon dioxide concentration in the air after it has been used for respiration.

Understanding the differences in air composition before and after breathing is crucial in biology for several reasons. Firstly, it provides insights into the efficiency and functioning of the respiratory system, particularly the lungs' ability to facilitate gas exchange. This knowledge is vital for understanding how oxygen is supplied to the body and how carbon dioxide, a metabolic waste product, is removed. Secondly, the differences in air composition reflect the metabolic activities of the body, specifically cellular respiration. The decrease in oxygen and increase in carbon dioxide in expired air are direct consequences of these metabolic processes. Lastly, this understanding has clinical relevance, as abnormalities in the composition of expired air can indicate respiratory disorders or metabolic dysfunctions. For example, a lower-than-expected increase in carbon dioxide in expired air could signal an issue with lung function. Therefore, the study of air composition changes is fundamental in respiratory physiology, health sciences, and understanding the body's overall metabolic processes.