The natural vs synthetic debate is an old one and fuelled as much by emotion and conflicting belief systems as it is by facts. It is also the source of a great deal of misinformation and the "greenwashing" that is so common in the mattress industry. This article is certainly not meant to be the "final statement" in this argument and there will be more detailed investigations published here over time however this should at least help to dispel some of the misinformation that you will be exposed to in the name of "going green".
Latex: Latex comes in a natural and synthetic version (this is outside of how it's made because any manufacturing method (Dunlop or Talalay) can use natural, synthetic, or a blend of latex rubber). Chemically they are a very similar molecule however natural latex also includes other ingredients in small quantities which contribute to its qualities and in general is a superior form of rubber in the properties that are most valued in latex. SBR (Styrene Butadiene Rubber or synthetic latex) does have some properties however that can be superior to NR (Natural Rubber latex) in some of its formulations which is its ability to resist heat aging, abrasion resistance, and softening (which in some combinations can reduce body impressions). It is also less expensive which is one of the reasons for its more common use. For this reason, a blended latex, often found in Talalay latex especially, may be more durable than the purely natural version of Talalay, particularly in the less dense very soft ILD ranges. In Dunlop latex or with Talalay that is not very soft, this is not as big an issue as they are both denser and very long lasting without any added SBR. Natural latex has many qualities which are superior to SBR, including its elasticity, and a natural version or good quality blended version would normally be preferable to SBR latex which does not have the same desirable latex qualities. In general terms with Talalay either a blend or natural version is a good choice. With Dunlop, since 100% natural is less expensive than Talalay, I would tend towards 100% natural unless there are other factors, especially cost, which are most important to you and in this case I would lean towards higher natural rubber content in a Dunlop blend.
As far as toxicity goes, both blended and natural latex are very safe and while they may sometimes have a smell which ranges from "vanilla" to "rubbery", this generally dissipates quickly and is not harmful. Most quality latex foams have been tested for toxicity and outgassing by reputable testing organizations and are completely safe ... even for babies. Your choice between NR and SBR at least can be based on your personal preferences rather than which one is "safer". If your preferences are towards completely natural, then both Talalay and Dunlop come in natural versions (they do not use SBR in their production) and Dunlop is also available in a certified organic version. There is no certified organic Talalay.
Memory Foam: All memory foam is synthetic and made in a similar way to polyurethane foam with some "rather nasty" chemicals added to it. Most have a tendency to not smell very good when they are initially made and the worst of them can be rather bad and last for a long time. Some memory foams can also give off toxic fumes which can be quite harmful and if the foam itself breaks down into dust and these are breathed in, this can also be a source of concern over time (in other words don't wait to the last possible moment to replace them). If you are one of those who loves the feel of memory foam, you will need to give up on your hopes of having a "green" or "natural" mattress (in spite of many false claims to the contrary in the industry which are completely misleading). If you do purchase a mattress with memory foam, make sure it has been certified for any offgassing or toxicity by a reputable testing organization such as Oeko-Tex or CertiPur. For safety reasons as well as durability considerations, I would also tend to avoid memory foams made by unknown manufacturers.
Polyurethane: These too are completely chemical in nature and even though some of them are partially made from polyols (one of the foam ingredients) which are derived from plants. Some of these are more highly processed than others canstill have an environmental impact in their manufacturing in different ways (some like soy which is a GMO crop and is responsible for some of the destruction of the rainforest) more so than others (like castor oil which requires less processing). There is no such thing as a natural polyurethane foam. While they do not usually have the same issues with toxicity and outgassing as memory foam, this is still a potential in their manufacturing and I would also make sure that if your mattress has polyfoam in it that the source is North American or has been tested by a reputable testing organization such as CertiPur as well.
Natural fibers: These in their best versions are high quality materials that are very breathable, natural, and durable. There are however some materials that are claimed as being "natural" that are anything but. An example of this is the "bamboo blends" or other viscose materials that are appearing more commonly in mattress ticking these days. While there is no disputing they are both comfortable, durable, and effective, this is not the issue in this article as they can sometimes be environmentally harmful in their production and are certainly not "natural". They are typically classified as "artificial" meaning they are neither natural or synthetic. Organic and/or more natural versions of cotton, wool, linen, silk and other well known fibers are all good choices for those who wish to be more green or organic in their lifestyle, want the better performance of most natural fabrics, or who are just more comfortable with materials which are more natural and in many cases more friendly to the environment.
Innersprings: These are made of steel ... often recycled steel ... and while this may not come out of the earth as steel, it is certainly not toxic or harmful. The two mattresses in North America that have been certified as completely organic are both made of innersprings and natural fibers.
Synthetic fibers and non woven materials: There is no disputing the benefits of certain artificial or synthetic fabrics in our society however these too are not "natural". Some of them like rayon or viscose are derived from plants or trees or other cellulosic materials and so could be more accurately called artificial. Others such as polyester are forms of plastic and are made from petrochemicals and will tend to be lower cost but there can also be a tradeoff in terms of performance (such as temperature regulation) and durability. In some cases there can be a range of "destructive effects" in their production even though they themselves may not be toxic. Again the choice of these materials and the benefits they may have should be based on personal preferences rather than the black and white "good or bad" arguments that are so common in the industry.
Fire Retardent methods: I have had quite a number of discussions with many manufacturers and others in the industry regarding this and it is certainly another area of controversy. Some of this controversy revolves around the issue of whether the new fire retardancy laws enacted in 2007 were really necessary to replace those already in existence and add a significant cost to the production of a mattress and introduce some very toxic chemicals into the equation. There are many who believe that the health costs of the introduction of these new laws are larger than the number of fire victims it was designed to protect (approximately 2000 per year). In any case ... here is my best assessment of good choices in fire retardant materials. Bear in mind too that with a custom built mattress and with a doctor's prescription, you are still able to buy a mattress without any fire retardancy layers.
Compressed wool: This is a very popular method of "natural" compliance and if done right there is no doubt it can be an effective method of complying with the fire code. Wool and other natural fibers are very breathable and provide an excellent microclimate and temperature control but the downside of thick compressed wool used as a fire barrier is that it can significantly add to the firmness of a softer comfort layer and reduce its ability to form a cradle.
Inherent silica/viscose: This is another good method of complying with the law and uses materials that are "less natural" but non toxic ... even though they may not be completely green (viscose is an artificial fiber as discussed earlier). This is the preference of many who prefer a non quilted ticking as it does not interfere as much with the feel of a mattress and is the "most natural" of the alternatives. Milliken and Visil are two common brands although there are many others.
Boric acid: A third alternative that is considered safe by many is Borax or Boric acid added to a "sock" material. There is quite a controversy about this as it is also used as roach killer but it is also a fact that there are many things that will kill insects that are not so harmful to humans (such as diatomaceous earth). In fact Borax kills insects by dissolving the waxy coating on their shell which then causes them to dehydrate rather than killing them through direct toxicity. Having said all that, it is also not the safest material and can be toxic to humans in large quantities so fabrics impregnated with borax or boric acid would certainly be my third choice ... even though I do not believe it justifies the outcry or claims of high toxicity connected to the label of "roach killer".
Since these 3 and the 4th option of no fire retardant layer at all that is available with a doctor's prescription is enough for most people to purchase a completely safe and even organic mattress, I would avoid all other fire retardancy methods and chemicals in a mattress.
There are other materials not covered here that are used in mattresses however these will give you some general information regarding the "Natural vs Synthetic" controversy and hopefully will also help you to navigate through the quicksand of the many misleading and sometimes completely untrue natural, green, and organic claims.