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Symbols whose documented purpose is to signal danger to whoever sees them — drawn from chemical labelling, biological and radiological safety, and automotive instrumentation, which are entirely separate regulatory traditions that happen to share this one communicative job.
A black skull and crossbones in a red-bordered diamond, the GHS symbol for acute toxicity.
A trefoil of overlapping circles, designed to be instantly recognisable as a warning for biological hazards.
A three-bladed trefoil, conventionally black on a yellow background, warning of ionising radiation hazards.
A red circle-and-exclamation-mark dashboard icon, often flanked by curved brackets, warning of a parking-brake or hydraulic brake-system fault.