The skull and crossbones means poison
The toxic symbol is the skull and crossbones ☠︎. On medicine bottles and household chemicals it has meant "poison" for well over a century, and it is still the most widely understood warning that a substance can kill or seriously harm you if swallowed, inhaled or touched.
Under the GHS, the United Nations' Globally Harmonized System for labeling chemicals, a black skull and crossbones inside a red-bordered diamond is the pictogram for acute toxicity: products that can be fatal or toxic in small amounts. Long-term health hazards use a different pictogram, a person with a starburst on the chest.
In the US, many families also know Mr. Yuk, a green scowling face created in Pittsburgh as a poison sticker for children, partly because kids linked the skull and crossbones with pirates rather than danger.
Toxic, biohazard or radioactive?
These signs are easy to mix up. ☠︎ warns about poison and toxic chemicals. ☣︎ is the biohazard symbol for dangerous living things such as bacteria, viruses and medical waste. ☢︎ marks radiation. ⚠︎ is the general warning sign for any hazard, and ⚡︎ means high voltage.
If you are making a real safety label, use the official printed sign required where you live. The characters here are perfect for documents, chats and quick notes, but they do not replace a compliant label.
Toxic in slang, and how to type ☠︎
Online, toxic usually describes people, not chemicals: a toxic relationship, a toxic ex or a toxic teammate who flames everyone in chat. People reply with ☠︎, 🤢, 🐍 or a row of red flags 🚩🚩🚩. For a skull used as art or a goth look rather than a warning, see skull symbols.
To type ☠︎, search "skull" in your phone's emoji keyboard, press Windows + . on Windows or Control + Command + Space on a Mac. In Microsoft Word, type 2620 and press Alt + X.