A skull and crossbones on a yellow background is a warning symbol that a material is what?

A skull and crossbones or death's head is a symbol consisting of a human skull and two long bones crossed together under or behind the skull. The design originates in the Late Middle Ages as a symbol of death and especially as a 'memento mori' on tombstones.

In modern contexts, it is generally used as a hazard symbol that warns of danger, usually in regard to poisonous substances, such as deadly chemicals.

In 1829, New York State required the labeling of all containers of poisonous substances. The skull and crossbones symbol appears to have been used for that purpose since the 1850s.

In the 1870s poison manufacturers around the world began using bright cobalt bottles with a variety of raised bumps and designs (to enable easy recognition in the dark) to indicate poison, but by the 1880s the skull and cross bones had become ubiquitous, and the brightly coloured bottles lost their association.