All latex mattresses on the market (even organic ones) contain 5-10% of natural or synthetic materials. Why? Because it's impossible to make a latex mattress simply with the milk from the Hevea brasiliensis tree. Certain substances are necessary to process the rubber in order to transform the liquid milk into a solid mattress you can sleep on. They include foaming agents, curing agents, accelerators, gelling agents, antidegradants, sometimes fillers, and others.

The remaining 90% of the ingredients that compose the mattress are either: 100% natural rubber (can be organic or not), synthetic rubber (SBR) or a blend of both. However there will always be a percentage of natural and/or chemical ingredients in all cases.

So when a salesman tells you that their latex is 100% natural, what they really mean to say is that 90% of the ingredients that compose the mattress are 100% natural rubber, and the remaining 10% are other ingredients, that are either natural, synthetic or a mixture of both.

In the case of our Organic Dunlop latex here is the breakdown of the ingredients that compose it (note that all the ingredients are found in nature, so that the product can be certified organic)

1-Organic Latex Rubber 90-95%

2-Zinc Oxide 2-3%

3-Fatty Acid Soaps 1-2%

4-Sulfur 1-2%

5-Sodium 1-2%

In the case of Talalay latex the ingredients are not available. 

From Wikipedia:

In the marketing of products such as beds that include "natural" latex foams created with the Talalay process, these products are sometimes characterized as 'organic' or as completely natural. However this claim may not be completely true since unspecified curing agents are used in the Talalay process and Styrene Butadyne is commonly used in the blend.

Is there such a thing as 100% Organic Talalay latex?
No. The Talalay process was specifically designed to take advantage of the uniform chemical and physical properties of synthetic latex. The simplest way to prove this fact is that no Talalay latex has ever been certified organic under the GOLS standard. The reason for that is simple: the GOLS standard forbids the use of synthetic latex.

What about the latex that's advertised as "100% Natural"?
Most 100% Natural Latex advertised, is a blend of natural rubber and synthetic latex aka ”styrene butadiene rubber" (SBR) which are two petroleum based chemicals, and each listed as VOC's. To be sure your getting what you pay for, ask for the eco-INSTITUT certification, which tests for VOC's, including styrene and butadiene. If either of these compounds are found in the latex sample, even in fraction amounts, it will fail and not receive the certification. It's important to note that the chemical styrene is toxic to the lungs, liver, and brain. source