What to look for in a crib mattress

So you are going to buy a mattress for your crib do you know what to look for other than the size?

Your baby will probably spend their first 2500 hours in the crib,  well worth scrutinising the features and benefits of the mattress you are about to purchase

Lets examine the crib mattress (you are about to buy) on these basic points:-  Safety, Comfort, Support, Durability,  Waterproof Protection, Hygiene (how easy to keep clean) Breathability, Toxicity of Chemicals Used. Environmentally Friendly, Temperature Control.

Safety
The mattress should conform to the new safety standard BS EN1130 :201 this  covers only a few basic points. fire regulations, quality of components not very taxing really.
Size is important should not have more than a 3 cm gap between the mattress and the side of the crib if you push the mattress over to the side or end. 
Comfort and Support
The crib mattress must be firm, When the baby lies on it, it should not make a depression. The initial lie (first 1.5 cm) can be welcoming but the mattress base should be firm to very firm.
Durable
your baby will be out of a crib mattress  as soon as they can sit up,  approximately 6 months.  but your crib will last many years certainly if you have more children or you lend to a friend . If you but a quality mattress you can keep hygienic you do not have to keep filling up landfill with used mattress you may use it again and again.( see new baby new mattress)

Waterproof Protection, Hygiene, Breathability
 You have a choice where to put the waterproof protection on a mattress:-
!) Over the top of the mattress cover and under the sheet
2) under the mattress cover and over the base be it natural or foam 
if you put it over the cover when you