Citing mercury levels found in canned tuna, a coalition of environmental and health care groups is urging that pregnant women avoid eating canned tuna and that preschool kids eat no more than one tuna sandwich a week. The Environmental Working Group and Health Care Without Harm bought 27 samples of tuna in grocery stores and discovered mercury levels they say could harm developing children. Though federal guidelines were used to gauge risk, the FDA questioned the findings in the coalition’s report.