Evolution of an Hazmat Acronym

A new acronym was complete; "4H MEDIC ANNA" was born.


A new acronym was complete; "4H MEDIC ANNA" was born.

If you've been in the emergency response business for a while you will hear of the tried and true acronyms. You know, the mnemonic devices that help you remember how to think on a certain subject. Fire response has them: "COAL WAS WEALTH" is one, "RECEO" or the related "REVAS" are some other examples. RIT has them: "LUNAR". Even EMS has them: "AVPU" and "SAMPLE". But, hazmat response has had them for many years! One example is the term "HAHA MICE" that a New York City fire officer used for remembering the lighter than air gases that dates back to the early 1900's.

HAHA MICE stands for the following gases with vapor densities less than one and they are; hydrogen, acetylene, helium, ammonia, methane, illuminating gases, carbon monoxide, and ethylene. Many of these gases are common and found throughout society. At the time this acronym was developed, they used natural or "illuminating" gas to light the street lights. This mixture was usually comprised of greater than 90 percent methane and less than 10 percent ethane which made it lighter than air.

Also, ethylene should not be confused with ethane. Ethylene is a common gas that is used to ripen fruit and it is also very flammable. Ethane is a heavier-than-air gas! In reality, the list of lighter than air materials was found to be incomplete so the HAHA MICE acronym has become obsolete when it comes to memorizing the gases that rise in air. FDNY now uses the acronym to name the flammable, lighter-than-air gases.

When I got into hazmat response decades ago, I had also heard of the acronym during training sessions. But when I hit the books and found several gases that were also lighter than air I realized the tried and true HAHA MICE acronym was now old and out-dated. I found gases such as nitrogen, neon, and diborane (among others) were also lighter than air. The problem then was how to best remember them. The whole concept was simple; if a person could remember the short list of gases that tended to rise to air, then, everything else was heavier than air. This concept would assist responders in tactical decision making at hazmat incidents. It would also benefit risk management systems and allow responders to respond safer and more efficiently. Finally, decisions could be made based on science and not guesswork.

With a clear objective in mind I searched through reference books and databases for any materials that had vapor densities less than, or equal to, one. Air has a value of one because it is a ratio based on the molecular weight (MW) of air divided by its own molecular weight. Air has a MW of 28.8 atomic mass units that has been historically rounded out to 29. So, any material in a gaseous or vapor form with a MW less than 29 will have a ratio or vapor density of less than one. (Aluminum has a MW of 26.98 but it is a solid and will not become airborne as a gas or vapor) This gas/vapor will tend to rise in air, especially after it becomes the same temperature as the ambient atmosphere. After I collected the data concerning vapor densities and molecular weights I worked out a new acronym in order to better remember the lighter than air gases.

The first thing that I noticed was that I had found four gases that all started with the letter "H" and they were hydrogen, helium, hydrogen cyanide, and hydrogen fluoride. There were four H's or "4H". My area hosts a county fair every year and it is called the "4H" county fair after the national agricultural-youth organization. Now, what goes with this 4H?

I next found that there were gases that represented the letters M, E, D, I, and C. I arranged them to say "MEDIC" because every fair has a nurse on-duty to tend to any medical needs of the thousands of people who attend the fair over the six day event. Medic is not the same as "nurse" but it still conveys the EMS component.

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