sb reader mom101 steered me to an interesting new zealand study regarding conventional mattress offgassing and SIDS. [they call it crib death over there, g'day mate]
according to the study mattresses release toxic gas when a common fungus reacts with the chemicals inside a mattress.
These heavier-than-air gases are concentrated in a thin layer on the baby's mattress or are diffused away and dissipated into the surrounding atmosphere. If a baby breathes or absorbs a lethal dose of the gases, the central nervous system shuts down, stopping breathing and then heart function. These gases can fatally poison a baby, without waking the sleeping baby and without any struggle by the baby.
i find this theory believable b/c one of the first things i intuitively wanted for our own bed where the sweetest boy would sleep was a good mattress cover to protect him and the mattress from each other.