Dr. Robert L. Brent has been studying environmental toxicology for nearly half a century. A distinguished professor at Thomas Jefferson Medical College in Philadelphia, he specializes in the effects of environmental factors like radiation, drugs and chemicals on the developing embryo and child. But Dr. Brent, who is also the head of a birth defects research laboratory at the Alfred I. duPont Hospital for Children in Wilmington, Del., said he often found himself defending the safety of such environmental agents in the face of misinformation that ignites the fears of parents and causes confusion. Too often, Dr. Brent says, many...