Radon gas is a naturally occuring substance from the types of materials and soils beneath the home. The best thing you can do to avoid the problem is to maintain excellent ventilation in your crawlspace. In other words, if there is a breeze blowing outside, you should be able to feel it down there. Your crawlspace should not smell overly musty if you are ventilating it well. Second, having a layer of 6 mil plastic sheeting, overlapping 12 inches at the seams helps reduce moisture in the crawlspace, as well as reducing the amount of radon gas escaping into the airspace beneath your home.
The issues you solve with ventillation and plastic sheeting (dryrot and other pest wood damage) are more important, in my book than the radon gas, but it is a convenient side benefit. Radon gas is a carcinogen, and is blamed as the second leading cause of lung cancer. You can purchase radon gas testing kits at your local big box store (home depot, etc).
Have fun, and do search the net for "radon gas" if you are still curious