Many gases have toxic properties, which are often assessed using the LC50 (median lethal dose) measure. In the United States many of these gases have been assigned an NFPA 704 health rating of 4 (may be lethal) or 3 (may cause serious or permanent injury), and/or exposure limits (TLV, TWA or STEL) determined by the ACGIH professional association. Some, but by no means all, toxic gases are detectable by odor which can serve as a warning. Among the best known toxic gases are carbon monoxide, chlorine, nitrogen dioxide and phosgene.
Definition
References
List of highly toxic gases Wikipedia(Text) CC BY-SA