Air Monitoring the “Old-School” Way — Still Relevant After All These Years

By Glenn Clapp CSP, EFO, CHMM, CFPS

There is no doubt that modern technology has enhanced our air monitoring capabilities not only in the number of substances we can monitor but also in the accuracy of the air monitoring that we perform. Even though we have made amazing gains in the air monitoring that we use we should not turn our backs on some of the “tried and true” old-school methods used in the past that are sure to work for us in tight situations. By old-school, we do not mean going back to the days of using canaries and fire damps as was done in mining operations to detect hazardous atmospheres, but rather methods such as power Hydrogen (pH) paper, oxygen sensors to determine the concentration of oxygen-displacing substances, metal oxide sensors, sulfur sticks for ammonia detection, and colorimetric tubes.    

If we look back to our initial hazmat technician training we will remember that we monitor for radiological substances first and then for corrosivity after that, underscoring the need for detecting corrosive environments that can attack the meter sensors, respiratory protection, and chemical protective clothing that our very lives depend on. We can use digital pH meters to determine the corrosivity of a liquid, however, pH paper is often the most effective means of monitoring corrosive environments. pH paper is inexpensive and can be obtained in various ranges of detection that show changes in pH through color changes on the paper. We can wet pH paper down with distilled water to detect corrosive vapors as well as use it in direct contact with liquids to determine if a substance is an acid or base. We need to also lead with pH paper taped to the end of a dowel as we enter possible hot zones and should tape pH paper to our hazmat suits (both wetted down with distilled water) to visually indicate corrosive environments. In addition to allowing us to determine the pH of a substance, pH paper also can serve as a rough estimator of vapor pressures. The further away from a substance the pH paper exhibits a color change, the higher the vapor pressure of the substance.

The second old-school method of air monitoring that we will discuss is the use of an oxygen sensor for non-traditional purposes. The standard four gas meter setup includes sensors to detect carbon monoxide, hydrogen sulfide, flammability, and of course oxygen. If we encounter a substance that displaces the oxygen we can use our oxygen sensor to determine the approximate concentration of the substance. As our atmosphere contains 20.9% oxygen in the air at sea level, we can state that oxygen comprises approximately one-fifth of air. If an oxygen-displacing substance is present in the atmosphere we can determine the difference between the oxygen sensor reading and the normal 20.9%. We then multiply the result by five as oxygen is one-fifth of the atmosphere. That result can then be converted to parts per million (ppm) through the conversion factor of one percent concentration being equal to 10,000 ppm to approximate the concentration of the oxygen-displacing substance. For example, let us say that our oxygen sensor is reading 20.8%. We subtract that value from 20.9%, yielding 0.1%. We then multiply that value by five to obtain 0.5%. Multiplying the 0.5% by 10,000 ppm (the conversion factor stated above) then lets us determine that the approximate concentration of the oxygen-displacing substance is 5,000 ppm. We need to remember that the result is just an approximation; however, as an estimation, it can be useful and can additionally serve to back up readings from other sensors.

The third tried and true method of air monitoring technology that is often overlooked is the metal oxide sensor. In the past manufacturers produced metal oxide sensors for hazmat responders that were so sensitive in detecting small concentrations of substances that they could be used as “sniffers” to detect the source of a small leak or release and some could even be used to measure the concentration of substances. Such sensors operate by detecting a change in the resistance of metal oxide due to adsorbed target gases reducing the oxygen present on the metal oxide and therefore causing a drop in resistance. In our present-day age, there are no manufacturers producing metal oxide sensors specifically for hazmat responders, however, there are manufacturers that produce metal oxide sensor monitoring equipment for the natural gas and propane industries that we can use to pinpoint leaks of those substances but not to determine exact concentrations. 

A truly old-school method of air monitoring that we can still employ in present-day times is utilized to detect releases of ammonia. Refrigeration technicians have been using sulfur sticks to detect small ammonia leaks in refrigeration systems for many years as a qualitative method that gives us a yes/no indication of whether ammonia is present. Sulfur sticks are sticks of material containing sulfur that are lit so that they smolder during use. As ammonia is encountered the sulfur reacts with it and produces a very visible white smoke. Sulfur sticks are only to be used in detecting small releases of ammonia as they can serve as an ignition source that may ignite concentrations of ammonia within its flammable range.

We will close our discussion on the old-time air monitoring methods that still hold their value with colorimetric tubes. Colorimetric tubes are glass tubes sealed at either end with a reagent inside that reacts to the presence of a specified substance. The ends of the tube are then broken off and the tube is inserted into a pump that is used to manually draw air through the tube. A directional arrow on the tube indicates the correct positioning of the pump and a series of pump strokes are performed according to the number of pump strokes indicated on the tube. Colorimetric tubes are qualitative in that they indicate a color change of the reagent if the substance is present, and they are also quantitative in that the extent of the color change on a graduated scale indicates the approximate concentration of the substance.  

As colorimetric tubes exhibit up to a plus or minus 25% margin of error in the quantitative realm, results are only to be viewed as an approximation and can best be used as a check and balance to other air monitoring methods. The main advantages of colorimetric tube systems are that they are simple, have very few moving parts, are very low maintenance, and operate independently of any power source.  

The basic principles of air monitoring also apply to the five old-school methods discussed above. We should always monitor slowly and methodically to allow proper capture of the atmosphere and the substance we are monitoring for. It is also imperative that we know the vapor density of the substance we are monitoring so that we can concentrate our monitoring efforts at the appropriate level of the atmosphere while still monitoring all levels overall. In addition, we need to store and properly maintain our air monitoring equipment so that it is ready for use and so that we can maintain our trust in that equipment.

In summation, we should not discount our old school methods of air monitoring that have displayed their value over time. While we may have more technologically advanced air monitoring equipment at our disposal, we should maintain our ability to use the time-tested methods in our air monitoring efforts as tools in our toolbox that we can select from. It is our duty as hazardous materials responders to pass the value of the aforementioned air monitoring methods along to the new generations of hazmat responders so that their value is known and the institutional knowledge of their use is not forgotten. As always, stay safe out there, and be sure to visit the North Carolina Association of Hazardous Materials Responders website at

Glenn Clapp is a past president of the North Carolina Association of Hazardous Materials Responders and has over 24 years of fire service and emergency management experience. He is currently an Improvement Specialist with the Industry Expansion Solutions Division of North Carolina State University and is a volunteer firefighter with the Fairview Fire Department. He is also a Technician-Level Hazmat Instructor, an Executive Fire Officer, a Certified Hazardous Materials Manager and a Certified Fire Protection Specialist. 

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