Was looking at a website called
http://www.cotlife2000.com/
It has information on this topic, this paragraph is particulary important
"Proof that the risk of cot death rises from first to second babies in families; and from second to third babies; and from third to fourth and later babies; and that babies of solo parents are at higher risk again (a corollary of the fact that re-use of mattresses increases the risk of cot death):
Analysis of official British cot death statistics (Source: Office of National Statistics, United Kingdom)
The finding of the rising risk of cot death from one sibling to the next refutes every proposition that cot death has a medical cause. The more times an unwrapped mattress is used from one baby to the next, the greater is the risk of cot death.
This accounts for the higher cot death rate among lower socio-economic families, who are more likely to sleep babies on secondhand mattresses.
If a mattress contains any of the elements phosphorus, arsenic or antimony, and if certain household fungi have become established in the mattress during prior use, any generation of toxic gas commences sooner and in greater volume."