What Makes Foam have Memory
To give a piece of foam memory, you start with a synthetic polyurethane foam material and add certain types of chemicals which add an increase in weight or density to the foam. With the addition of these chemicals, a non-toxic, visco-elastic foam material is created. Visco-elastic can be synonymous with memory foam or some like to refer to this as "viscoelastic memory foam." However, there are many features along the way that will help determine what type of memory foam you end up with. To understand exactly what differences are found in each final product, you first have to start with understanding how to grade each foam.
What about organic or green memory foam?
One new trend in all bedding is to speak 
about organic memory foam or green memory foam.  While there are ways to 
make memory foam more environmentally friendly (Biogreen® 
Memory Foam is one such example), for memory foam to have the same type of 
visco-elastic feel and support as the original NASA developed formulation, it 
cannot be organic, soy based (usually 5 - 10 % soy can be added to memory foam), 
nor can it be all-natural.  However, the memory foam made in the United 
States has been shown to be safe and non-toxic and some manufacturers have gone 
one step further to reduce or eliminate V.O.C. off-gassing odors and other 
borderline chemical additives.
 
What to look for in a good quality memory foam mattress
Obviously it will be important to allow the foam to do what it
is best for:  Allowing for a better contouring of your body shape and
therefore less pressure placed on the areas of your body that press harder
against the bed.  However, don't just go by your weight, because if you are
tall, then your weight is dispersed over the bed more.
A good memory foam mattress will not need a box spring
underneath it to make the mattress feel good.  You can order a platform
base to increase the height of the bed, but remember that this mattress should
feel good if it was placed on an even, concrete surface.  A good way to 
test the surface of a box spring or box foundation is to sit or even stand on 
various areas and see if it compresses underneath you.  If you have an 
existing foundation that is weak, you can easily place two cut 3/4 to 1 inch 
thick plywood sheets over it to give it the necessary support.
Also, you may want to look at the warranty that a company
offers, because you can then worry less about spending extra money for a higher
weighted mattress and can attempt to get the correct mattress feel, but save 
on cost.  
Beware of Cheap Imitations!
Unfortunately, not all foam is created 
equal.  Many companies will put together the mattress in the U.S., but buy 
the foam overseas to save money.  With overseas foam, we cannot be sure of 
the quality or the composition, so we are very wary of recommending mattresses 
that use memory foam not produced in the United States, with the exception of 
Tempur-Pedic, who has the only high quality Swedish manufacturing facility we 
know of.  The 
rest of the overseas foam makers produce memory foam that seems to flatten out with 
compression, even though it may feel comfortable upon the first feel.  
Therefore, the memory foam made overseas may not last as long, be as safe to 
sleep on, and offer enough support.  Unfortunately, many companies state 
their beds are made in the U.S. because the mattress is put together in the 
U.S., even though they do not use U.S. produced foam.  For a list of 
companies that have produced documentation verifying that their mattresses are 
100% produced in the U.S., click here.
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