Toxic Health Threat Warning Act of 2021 This bill requires the U.S. Army Corps of Engineers (USACE), before releasing water from a flood risk management project, to determine whether the water is contaminated with cyanobacteria (blue-green algae) based on tests conducted by the USACE, another federal agency, or the state in which the project is located. If the water is contaminated, the USACE must notify the public and affected governments of the contamination, its planned release, and the potential effects on human health.