Some neighbors, students and activists are saying that Boston University’s recently approved biosafety laboratory is too dangerous for the city. The school plans to start construction of the National Emerging Infectious Diseases Laboratory despite a pending lawsuit by those opposed to the building. The Department of Health and Human Services found any threat to Boston residents and the environment to be “negligible” thanks in part to elaborate safety systems and devices to ensure materials such as anthrax and the Ebola virus are well-contained. BU will be the second in the nation to have a building designated “Biosafety Level 4.”