The problem with this line of thinking is that we have been incorrectly ingrained to believe that a firm surface is the most important thing to look for when considering a safe crib mattress in regards to SIDS (more correctly known as crib death). The fear is that a baby will bury his face into a mattress that will conform to his mouth and nose, suffocating him. First of all, this is not SIDS or crib death, but suffocation. The two are different and classified differently- most of the time. Unfortunately, because it is recommended that a "firm mattress" be used, people assume that suffocation is a part of SIDS and fear that babies all over the world have been suffocating on their mattresses. This is not so.
The important part that is missed so often is the toxic gas theory for crib death, which is growing in acceptance. Crib mattresses are not dangerous because they are too soft or too firm. They are dangerous because they are made from chemicals that are emitting deadly, toxic nerve gases that babies are breathing in. The Back to Sleep Campaign helped prevent many deaths when babies placed on their backs were kept from breathing in these dense gases that hover just on the surface of the mattress. Sadly, most doctors and SIDS organizations either don't know or don't accept the real reason why the Back to Sleep Campaign has been successful! You can learn more about the toxic gas theory and it's development and cover-up in the book, Cot Death Cover-up? by Jim Sprott.
So what is a parent to do? First, find a comfortable mattress. Make that a priority. Then, wrap it with a BabeSafe mattress cover. The most important thing is that no matter what mattress you select, that you wrap it with this gas impermeable cover. This practice has been followed for 12 years in New Zealand and all over the world with ZERO deaths. Because many parents find that their babies will only sleep on their tummies (just like mine), a BabeSafe mattress cover is especially important. When the baby's mattress is wrapped, it is safe with regards to crib death.
Now that you have the mattress, you've got to get it covered! Do not use typical mattress pads made for crib mattresses. Use only what you know (don't trust the label) to be 100% cotton. A bath towel fits perfectly on a crib mattress and works well for this purpose. This 100% cotton "underblanket" serves only one purpose: comfort for the baby. On top of this, place your baby's fitted sheet, which can be as much as 50% poly/ 50% cotton, but try to use 100% cotton. DO NOT USE 100% POLYESTER (SUCH AS FLEECE).
Now for the blanket, which is another misconception. It is okay to use blankets in the baby's bed. Use 100% cotton blankets for this purpose. (Wool is okay, too, but we prefer cotton.) Do not use blankets made from polyester, acrylic, or that is padded, quilted, or filled. A simple cotton flannel or knitted blanket is sufficient as well as safe. (Again, many parents have been misled into thinking babies are suffocating on blankets when in fact, synthetic blankets emit the same toxic nerve gases once they become soiled.)
Many people ask if it's okay to use a previously used mattress. While an unwrapped used mattress is more dangerous than a new mattress, a mattress wrapped in a BabeSafe mattress cover is perfectly safe, whether used or new. (Remember that as soon as a baby sleeps on the mattress it is "used.") It also doesn't matter if you select a foam or innerspring mattress. If it is covered, it's covered. You might find foam to be more comfortable, but make sure you also get a BabeSafe mattress cover!!