There aren't any !
It is hard to go wrong with a quality Australian-made Memory foam mattress overlay/topper. Reason is that most toppers are around 5 cm thick (two inches) and even if the foam feels a little firm to start with, it will soften due to its temperature-sensitive properties when it comes into contact with your body's natural warmth.
At 5 cm thick, mattress toppers are specifically designed to fit snugly under your bottom sheet. It needs to be said that due to its thinness it obviously doesn't have the 'body' of a normal mattress and is therefore subject to greater download forces from the body, and there may be a problem over time with the tear strength (fracturing) of the foam. This will be more evident under the shoulders, buttocks and feet region where the load bearing and movement are greater, but you should get at least a minimum few years out of your topper, and if you turn it around and over often during its lifecycle and change the load regions then you will increase its life time.
SUMMARY:
Mattress toppers in Memory foam are a fantastic addition to your sleep needs. They diffuse and cushion your body, and obviate the need for you to upgrade to an expensive mattress which, it must be said, may still not do the job the salesperson has said it will do!!


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