The main chemistry prep room at my institution (and also at the ambulance station) is labelled with the standard green compressed gas sign. However, there is a small '2' included within the diamond. None of the chemistry technicians, tutors or ambulance staff seem to know what it is referring to and I can't find any reference of it on the internet. Would anyone here happen to know?

Example of Compressed Gas (2) Hazard Sign

The related products of this online provider show that other hazards have different numbers but some also have two (e.g. flammable). Is there a standard that is being referred to here?


1 Answer 1


The 2 refers to DOT (Department of Transportation) hazard class 2. This reference describes the different DOT hazard classes.

As summarized in the comments:

Class 2 (compressed gas) is divided into three subclasses (2.1 - flammable, 2.2 - non-flammable, non-toxic and 2.3 - toxic). I imagine the diamonds for Class 2 don't contain the subclass number because there is a different diamond colour/symbology for each subclass (as opposed to for instance 5.1 (oxidising agent) and 5.2 (organic peroxide), which have the same symbol and colours).

  • 1
    $\begingroup$ If you look at the wiki article on dangerous goods, there are good explanations of many of the various hazmat signs we see daily and take for granted. If my edit is accepted, you'll notice a slight change in the wording, but the icon, sign color, shape, and safety number are all the same. This is because these have been standardized internationally, including the fact that your example must have a green background. Crazy, huh? $\endgroup$
    – Anthony
    May 7, 2012 at 10:54
  • $\begingroup$ Another great resource for searching and browsing data about various hazardous materials (more geared towards professionals) is ADR Dangerous Goods. $\endgroup$
    – Magnus
    Jun 16, 2017 at 11:48

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