Soft Infant Sleep Surfaces and Exhaled-Air Pooling: A Rebreathing Model That Converts Surface Firmness into a Carbon Dioxide Fraction at the Face

Public guidance on infant sleep is unambiguous that the surface should be firm, but the guidance is usually given as a rule without its mechanism, which makes it hard for a tired parent to judge the borderline objects — the soft topper, the adult bed, the cushion — that rules do not name. This paper models the mechanism directly: exhaled air pooling in the pocket that a yielding surface forms around a face-down infant's face. With stated pocket volumes of 120 millilitres on a soft surface against 25 on a firm one, the steady inspired carbon dioxide fraction reaches 4.4 percent on the soft surface, against 0.04 percent — effectively fresh air — on the firm surface, while inspired oxygen falls to 16.6 percent. Firmness, converted into pocket geometry, becomes a number a parent can reason with. This study is simulated, not measured.