A material which is a gas at 20°C (68°F) or less and a pressure of 101.3 kPa (one atm) and which is known to be so toxic to humans as to pose a hazard to health during transportation; or in the absence of adequate data on human toxicity, is presumed to be toxic to humans because when tested on laboratory animals it has an LC50 value of not more than 5000 ppm.

Note: This label must bear the class number.

Also see:

What are the Federal Regulations for Hazardous Materials Labeling?