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Notes from the HAZMAT Refresher -- how much Benzene did you breathe today?

By Andrew Pollack on 11/01/2006 at 10:13 AM EDT

This month’s HAZMAT refresher course, taught by Al Nygren of Training Technologies International, gave us a valuable new perspective on how our metering equipment works and the mechanisms behind some of the dangers we face. The course was a change of pace from the usual HAZMAT classes which focus on the decontamination process and identification of specific hazards. It had a heavy emphasis on the chemistry behind many of the fuels and oxidizers we may run into at any time.

All that white board chemistry can be pretty dry, but Mr. Nygren came prepared for restless firefighters by bringing demonstrations. In one of these he made two points with a single example, by showing what adding a common oxidizer to an ordinary rag can do. In the experiment, Nygren produced a rag which he had soaked with a commonly used oxidizing agent in a 30% solution with water and allowed to dry. The rag was protected from the rain by a cardboard box, and placed on apron in front of Central Station. At the first touch of a flame, the rag burned up entirely in seconds – producing a very large flame. In a second or two the rag was entirely consumed.

The point of Mr. Nygren’s demonstration was the oxidizer increased the rate at which the rag burned, but it was the rag itself which was burning – not the chemical. The chemical itself is not a fuel. Further, because there was no fuel vapor, our meters would not pick up any danger at all.

Another demonstration was a bit more basic, but was fun in any case as it is always fun to play with Liquid Nitrogen. Nygren pulled what appeared to be an empty baloon from his flash of the supercold fluid, and place it on the table where it proceded to inflate as the air itself thawed from solid to vapor. He was making a point about how vapors can be produced by changes in both temperature and pressure.

Our meters are not perfect, and pointing out where they can leave us with danger was a key part of the class. Below are some really important things to consider when using our meters.

      • Our meters register Carbon Monoxide and Hydrogen Sulfide in PPM (parts per million), but register hydrocarbon fuels as a % of LEL (Lower Explosive Limit) by using a sensor to check for actual combustibility -- a sort of safe version of using a match to check for the presence of explosives.
      • It takes 10,000 parts per million to equal a 1% level of a gas in the atmosphere.
      • Gasoline has an LEL of about 1.5% in normal air* (though it various by manufacturer). Our meters begin to alarm when the amount of fuel in the air reaches 10% of the LEL for the fuel. In other words, for Gasoline, our meters alarm when the vapors reach about 0.15% of the atmosphere – which is 1500ppm.
      • At 9% of the LEL for Gasoline (0.135%), there will be 1350ppm of vapors in the air, and no alarm is yet sounding.
      • Gasoline contains up to 5% Benzene*, a known cancer causing agent. The STEL (Short Term Exposure Limit) for Benzene is 2.5ppm average over 15 minutes, and the TWA (Time Weighted Average) is 1ppm average over 8 hours.

If you put all this together, you see that at 9% of the LEL for Gasoline*, there can be as much as 67ppm of Benzene in the air and our meters will give no warning. Even 15 minutes exposure to this atmosphere is more than 250 times the allowable limit.

There are many other things to consider when working with our meters. One common issue we may very well run into is “Cross Sensitivity” between the sensors. For example, in an atmosphere containing 100ppm of Carbon Monoxide, our sensors can show 10ppm of Hydrogen Sulfide. Since we need to take action at even 1% Hydrogen Sulfide to find out where it is coming from, that can lead to a wild goose chase.

The biggest lesson overall from the class, is that we need to know the environment we’re working in. We need to identify the source of the hazard and understand the implications of that hazard beyond just what the meters pick up.

* Source: CITGO Gasoline MSDS available online at

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