Introduction
Human respiration depends on the continued availability of an atmosphere whose composition permits adequate oxygen uptake. The colloquial question of whether the surrounding medium qualifies as air invites empirical examination of its major and minor constituents at the scale of individual inhalation.
Background
Earlier studies have reported that dry air at sea level comprises approximately 78 % nitrogen, 21 % oxygen and 0.9 % argon by volume, with carbon dioxide at roughly 0.04 %. Local deviations arise from combustion, industrial processes and biological activity. These deviations remain small in aggregate yet may influence respiratory comfort and long-term health when sustained.
Methods
Air was sampled at 1.5 m above ground level at three urban sites during morning, midday and evening periods. Samples were drawn into evacuated stainless-steel canisters and analysed within four hours by gas chromatography with thermal-conductivity and flame-ionisation detection. Particulate matter was measured concurrently with optical counters calibrated to PM2.5. All procedures followed published laboratory protocols for trace-gas analysis.
Results
Nitrogen accounted for 77.8–78.4 % and oxygen for 20.7–21.1 % of sampled volume across all collections. Argon remained stable at 0.91–0.93 %. Carbon dioxide ranged from 420 to 480 ppm, exceeding the pre-industrial baseline by 15–30 %. PM2.5 concentrations averaged 8 µg m⁻³ with peaks near 25 µg m⁻³ during afternoon traffic periods.
Discussion
The measured proportions continue to satisfy the operational definition of air for respiratory purposes. Nevertheless, the observed increments in carbon dioxide and fine particulates suggest measurable anthropogenic influence at the breathing zone. These increments fall within ranges previously linked to small but detectable changes in minute ventilation among sensitive individuals.
Limitations
Sampling was restricted to a single temperate city and to three consecutive days. Seasonal, meteorological and topographic variability therefore remain unexamined. Extension to rural and indoor environments would be required before broader inferences can be drawn.