Firmness in an infant mattress is a specified property rather than a comfort preference. The reasoning concerns what happens if a baby's face ends up against the surface.
A conforming surface can obstruct breathing
A soft mattress deforms around a head that rests on it, forming a depression. If a baby's face turns into that depression, the surface can obstruct the airway.
Soft materials also trap exhaled air rather than allowing it to disperse, which can lead to rebreathing of carbon dioxide in the pocket formed around the face.
Young infants have limited ability to lift and turn their heads clear. The margin available to correct the position is small, which is why the surface property matters.
The measurement is indentation, not feel
Standards specify how far a defined weight may sink into the surface over a set area. The test produces a number rather than an impression.
This matters because perceived firmness is unreliable. An adult pressing a mattress with a hand applies a different load over a different area than an infant's head does.
Firmness is also measured after simulated use in many standards, since materials soften with compression over time. The initial figure alone is not sufficient.
Fit is a separate requirement
Gaps between the mattress and the cot frame create spaces in which a limb or head can become trapped. Standards therefore specify maximum permitted gaps as well as dimensions.
Mattresses bought separately from cots are the common source of mismatch, particularly where either is second-hand or of non-standard size.
A mattress that fits when new can shrink slightly with use and cleaning, so the fit is worth rechecking rather than assuming it is fixed.
Waterproofing and ventilation pull in opposite directions
Waterproof covers are practical and hygienic, and they also reduce air movement through the surface, which is relevant to the rebreathing question.
Manufacturers address this through ventilated cover designs and breathable layers. Whether these produce a measurable difference is not settled, and marketing frequently outruns the evidence.
Adding extra layers on top of a compliant mattress undoes the specification. A soft topper reintroduces exactly the conforming surface the standard was written to prevent.
Second-hand mattresses are treated differently from other gear
Guidance on reusing cot mattresses is more cautious than for most equipment, partly because firmness degrades invisibly and partly because hygiene history cannot be verified.
Where a mattress is reused, the usual advice concerns condition, fit, firmness and a fully intact waterproof cover rather than age alone.
Safe sleep guidance covers surface, position, bedding and room sharing together, and current recommendations from a health service or health visitor are the reference point rather than any single product feature.