Hi Alice. I work for the EPA's radon program. Radon is indeed the second leading cause of lung cancer at 20,000 deaths a year. But it is easy to test for and relatively inexpensive to get your how fixed if you have high levels. You can get a do-it-yourself kit at a local hardware store or by calling 1-800-SOS-RADON. Or you can hire a testing professional. If you result is high a certified mitigator can install a system to lower the levels in your home. Although Maryland doesn't have a state certification program there are two private certification organizations with online directories: http://www.radongas.org, http://www.nrsb.org you may consult.
Also, it should be noted the national map is a resource for the states not the public. Plenty of houses in Zone 3 (lower potential) will still test high. So everyone should test regardless of where they live. Good luck!