Hi diane37,
Just to add to jefmoody’s great comments …
23 or 24 ILD is actually “soft” not “firm” or even “medium”. Of course the thickness of a layer will have just as much effect as the actual softness itself because with thinner comfort layers the upper part of the layer below it will also have an effect on the firmness or “feel” of the mattress. If there is also a soft polyfoam quilting layer over the latex then it will be softer than sleeping directly on the latex itself. You can read a little more about some of the “theory” behind different designs in the links in mattress firmness/comfort levels in post #2 here but in most cases theory is much too complex to use as the basis for a choice without specific reference points of actually testing a mattress with a similar design. Without specifically testing a mattress with a similar design … a more detailed conversation with the manufacturer is a much more meaningful and accurate way to predict how a mattress may feel for you based on the “averages” of others with a similar body type, sleeping style, and preferences than anything else.
For most people the upper layers contribute the most to the “feel” of a mattress when you go to sleep or initially lie on it and are also the most important part of durability but the deeper layers are just as important for the support/alignment of the mattress which is a big part of how you feel in the morning. Latex is also the most durable foam material so to the degree that any layer contributes to durability it would be the most durable option (you can read more about the many factors that are part of the durability of a mattress in post #4 here)
You can read more about some of the differences between a latex/polyfoam hybrid and an all latex mattress in post #2 here.
Depending on how someone defines “safe” … most of the foams that are made in North America and are CertiPur certified for harmful substances and VOC’s and would be considered safe by most people. For those who want to delve into issues concerning the relative “safety” of various materials and don’t mind doing research where there are no definitive or black and white answers to the question of “how safe is safe enough for me?” then post #2 here and the other posts and resources it links to can provide much more information (and perhaps more than you really want or need to know :)).
The major cause of allergies is dust mites and other allergens that can accumulate in your mattress (not the mattress materials) and you can read more about controlling dust mites in post #2 here
An all latex mattress generally or any “all foam” mattress generally does best on a firm non flexing foundation or platform bed rather than a box spring which flexes. There is more information about foundations and some good sources in the foundation thread and the posts it links to as well. Both Brooklyn bedding and Arizona Premium Mattress (and other foam mattress manufacturers as well) would normally suggest firm non flexing foundations or platforms with gaps between the support surfaces that are close enough together to prevent the materials in the base layer of the mattress from sinking into the gaps (with a few exceptions where a flexing foundation is recommended as specific part of the design of the sleeping system).
Phoenix