What does standard NATO markers with a blue background and red lettering signify?

Standard NATO markers with a blue background and red lettering signify biological hazards. These markers are part of a unified system used by NATO forces to identify various types of hazards in the field, ensuring that all personnel can quickly understand the risks they may encounter.

The blue color typically represents a health hazard, while the red lettering indicates a form of alert or warning associated with contamination or possible exposure to biological agents. This clear visual communication is vital for maintaining safety and facilitating rapid responses to potential threats in a combat or operational environment.