It’s important to note, however, that “firm” does not necessarily mean hard. Traditional crib mattresses are rock-hard because, in order to prevent a baby's head from sinking in and to avoid suffocation risk, they are encased in non-breathable plastic.https://www.biloban.com/collections/crib-mattress-sheet

By making the Biloban crib sheet completely breathable, we were able to make it much more comfortable as well — without any risk of suffocation.
https://www.biloban.com/collections/crib-mattress-sheet

To be sure you have a firm crib mattress, push down on it and see if it immediately springs back into place when you take your hand away.https://www.biloban.com/collections/crib-mattress-sheet