Which pictogram serves to identify highly toxic substances in use?

The Skull and Crossbones pictogram is specifically designed to indicate the presence of substances that are highly toxic. This symbol alerts individuals to the serious health risks posed by the material, especially in cases where exposure could lead to severe health effects or even death. In the context of hazard communication, it effectively communicates the necessity for caution when handling or working in proximity to these highly hazardous materials.

Other pictograms serve different purposes; for instance, the Corrosive pictogram signals that a substance can cause destruction of living tissue or severe corrosion of materials, while the Flame pictogram indicates flammability or the possibility of combustion. The Environment pictogram is used for aquatic toxicity, marking substances that pose risks to the environment, but it does not denote human toxicity directly. Therefore, the Skull and Crossbones is the most appropriate symbol for identifying highly toxic substances.