It is like organic food. Why is it good to eat organic food? Because it does not contain strange additives potentially harmful to your health that non-organic food may.
Filling: polyurethane foam
Most traditional mattresses are made of polyurethane foam. Polyurethane foam contains chemical additives and because it is flammable, fire retardants are also used to pass the legally required flammability tests to sell a mattress. Foam breaks down over time and the particles become airborne. Breathing them in may cause a lot of health issues such as irritation of mucous membranes, coughing, asthma-like reactions, weakness, fatigue etc. Here are two more great articles in this topic, one from Green Future and one from Sara Davenport in Reboot Health.
Cover: synthetic materials
Covers are often made of non-natural materials and may contain flame retardants or added chemicals. For example when you see the term "aloe vera cover" or "bamboo cover", it may well be that the material of that is about 50% synthetic and 50% bamboo (for example) and it is treated with some chemicals to pass the flammability tests.
1. Check for labels. GOTS and/or GOLS are perfect, Oeko 100 is not bad.
GOLS: GOLS (Global Organic Latex Standard) certifies that a latex mattress contains at least 95% organic latex. In reality this cannot be more than 98%-99% percent due to the way latex foam is manufactured. There are certain non-chemical additives that need to be used to transform the liquid latex into foam. Almost all of these additives vapour away and the end product is about 98-99% organic latex. GOLS also stands for social criteria.
GOTS: GOTS (Global Organic Textile Standard) certifies that at least 95% of the materials used are organic with restrictions on the other 5% prohibiting the outright use of certain substances such as chemical flame retardants and polyurethane (main ingredient of memory foam). GOTS also stands for social criteria.
Oeko Tex Standard 100: Oeko Tex 100 does not ensure that the mattress' fibers were produced organically but it does set limits for the emission of harmful chemicals such as formaldehyde and VOCs. It also bans the use of certain chemical flame retardants, colourants, and allergenic dyes. As stated above, this is a good certificate and health wise probably safe, however it doesn't ensure organic cultivation of the fibres. Due to the lack of organic cultivation process and social criteria, mattresses in Oeko Tex 100 covers tend to be a bit cheaper than GOTS and GOLS certified mattress.
2. Check if each layer in the mattress is certified organic.
It could very well be that only a few of the layers are natural/organic and the rest are not. Always worth checking. For example if you find a "bio" mattress that is too cheap, it is probably too good to be true.
3. Check how the mattress passes the flammability tests. The importance of BS7177, Low hazard test.
In the UK all mattresses sold to the public need to pass certain flammability tests. The name of the test is "BS 7177, Low hazard" test. It is called low hazard as the requirements are less strict than for a mattress used in hotels or hospitals ("medium hazard"). Manufacturers use different techniques in order to pass this test. Some use chemical fire retardants, some use wool as a natural fire retardant. Latex could be enhanced by adding a thin layer of graphite latex on top and bottom of the latex foam to pass this test. Graphite latex is normally 70% of latex and 30% of graphite. As for latex foam, this is the only natural way to be able to have organic latex and still pass the flammability tests. You can read more about the flammability test here.
4. Ask about the glue.
A lot of glue is obviously not healthy and can off gas. Some manufacturers don't use glue at all and you can read about adhesives used in mattresses here. The issue with that is that the layers may be misaligned. We use a Greenguard Gold certified, water based, chemical free glue called Simalfa to put the mattress layers together.
5. Look and smell - does it look and smell organic?
When you see the mattress just follow common sense. Organic, untreated cotton is ecru colour, this should be the colour of your mattress cover. Latex foam is white. There should be no smell and no off gassing.