Toxic Health Threat Warning Act of 2019 This bill requires the U.S. Army Corps of Engineers, before releasing water from flood risk management projects, to determine whether the water is contaminated with cyanobacteria based on tests conducted by the Corps, another federal agency, or the state in which the project is located. If the water is contaminated, the Corps must notify the public and affected governments of the contamination, planned releases, and the potential effects on human health.