It is not very effective against the danger of phosphine, as this gas is only slightly more dense than air. Any phosphine generated in a baby's cot is likely to be present in the air which a baby breathes, even if the baby is sleeping face up. Face-up sleeping does not eliminate the risk of cot death. If a baby's mattress is correctly wrapped and the correct bedding used, sleeping position is irrelevant to cot death prevention. Smoking was very common in Britain in the 1930s and 1940s, yet cot death was virtually non-existent.
Smoking is prevalent in present day Russia, yet cot death is rare in that region.
In fact, they are simply socio-economic parallels. Put another way, smoking is more common among poorer people - and so is cot death. But it does not follow that smoking is therefore a cot death risk factor. However, it is indisputable that along with having a higher rate of smoking, poorer people are also more likely to re-use mattresses. And it is also indisputable that cot death occurs much more frequently on re-used mattresses.