Classrooms present another hazard. The E.P.A. tested for radon in a sample of schools nationwide between 1990 and 1991. The resulting study of 927 public schools concluded that over 70,000 classrooms in the United States were likely to have radon concentrations that equal or exCeed the E.P.A.'s action level. But little has been done to fix the problem. More than 20 years later, only a few states have laws that require some form of radon testing in schools.