Is it true that fresh air heats up faster than stale air? If so, why?

Simon Lee School of earth and environmental sciences, University of St Andrews, UK Air usually seems “stale” if it contains elevated levels of carbon dioxide (from human respiration) and odour, neither of which affects how quickly it heats up. However, stale indoor air is often (though not always) more humid than fresh, outside air. Humid air has a slightly higher heat capacity than dry air, so it requires marginally more energy to warm. In practice, though, the difference is too small to notice, especially as the difference in humidity between indoor and outdoor air is often modest. Advertisement Ventilation can improve air circulation, helping heat from a radiator spread more evenly. If condensation has formed in the room, much of the heat supplied initially goes into evaporating this moisture, rather than raising the air temperature. Ventilating with drier outside air helps the room dry more quickly, so subsequent heating becomes more effective. “When soils are dry, less of the sun’s energy is used to evaporate water, leaving more available to heat the ground and air” A similar process occurs during heatwaves: when soils are dry, less of the sun’s energy is used to evaporate water, leaving more available to heat the ground and air. Robin Maguire Hobart, Tasmania, Australia We might define stale air as air that has been extensively breathed by people. In that case, it contains a higher proportion of carbon dioxide and water vapour than fresh air. We can see what this means by comparing the molar specific heat capacities of the relevant molecules at constant pressure – that is, the amount of energy required to increase the temperature of a particular volume by 1 unit. Nitrogen and oxygen (which make up over 99 per cent of fresh, dry air) each have a specific heat capacity of a little over 29 joules per kelvin per mole. The equivalents for carbon dioxide and water vapour (at normal temperature and pressure) are higher, around 37 and 35 J/mol.K respectively. So, the overall heat capacity of stale air is rather higher than for fresh air and therefore fresh air will indeed heat up more quickly than stale air, assuming a fixed rate of energy input. However, the effect is likely to be small. If the overall quality of some fresh air was degraded to the equivalent of a typical exhaled breath (which sounds fairly stale to me), this would result in the air composition changing by the addition of about 5 per cent CO₂ and perhaps a couple of per cent water vapour (depending on the initial humidity). This would imply an increase in overall heat capacity of only 1 to 2 per cent. Specific heat capacity increases slightly with temperature, more so for more complex molecules – CO₂ relative to nitrogen, for example. As a result, the difference in heat capacity between fresh and stale air will increase slightly as temperature rises. If the ambient temperature changed from 0 to 30°C (32 to 86°F), the heat capacity differential would increase by about one-tenth – not enough to affect matters greatly. To answer this question – or ask a new one – email [email protected].