The M256 series Chemical Agent Detector Kit (including both the M256 and M256A1) is an essential element in chemical warfare agent defensive measures. It is used to detect hazardous concentrations of blood, nerve, and blister agents. The M256 replaced earlier generation kits, which were more difficult to use under battlefield conditions.

In a combat situation, each person is responsible for observing his or her surroundings for signs of chemical warfare agent. Warning signs vary depending on the chemical warfare agent used and the delivery method. Warning signs include bombs, rockets, or artillery shells that do not detonate with the characteristic force of high explosive; spraying from low flying enemy or unidentified aircraft; animals with no apparent or observable cause of death; dead insects, especially those on animal carcasses; and individuals who are experiencing or demonstrating symptoms consistent with exposure to a chemical warfare agent. Two persons normally conduct a test with the M256 – one person reads the instructions that come with each kit while the second person performs the action. However, an unassisted individual can perform tests using the M256 when necessary.

The M256 is used to test for the presence of hazardous concentrations of blood, nerve, and blister agents in vapor, and nerve and blister agents in liquid form. The M256 has two primary test components – the vapor-sampler and M8 detection paper. The vapor-sampler is similar to a miniature chemistry set. All materials (solids and liquids) required to test vapors and aerosols are contained within the vapor-sampler’s body. M8 detection paper is similar to litmus (pH) paper. The difference is that M8 paper is specifically designed to react to nerve agents and blister agents in liquid form. The detection kit relies on a reaction between the chemicals and enzyme used in the vapor-sampler, or those impregnated within M8 paper, and a chemical warfare agent to produce a uniquely colored response. The colored response is compared to color charts to determine whether a chemical warfare agent is present. It takes 15 to 20 minutes to detect the presence of a blood agent, nerve agent, or blister agent using the M256 test kit, which includes a 10-minute period when the vapor-sampler is exposed to the gas or vapor in question. Test results, positive or negative, are then used as a factor, along with the aforementioned signs and the test results received from other chemical warfare agent detection equipment, to determine if personnel can remove their protective equipment.

In some instances, the M256 kit can produce a false positive or a false negative test result. A false positive detection indicates the presence of a hazardous concentration of a chemical warfare agent when no agent is present. A false negative detection indicates an agent is not present when one actually exists. Of the two types, a false negative detection is more serious because it can lead to an exposure to a chemical warfare agent that might otherwise be preventable. Extensive testing with known chemicals and compounds normally found on a battlefield indicate that a false negative detection, although possible, is unlikely. However, a false positive detection can occur if an outdated kit is used, if a kit is inadvertently exposed to some battlefield pollutants, or if the kit is used improperly.


The M256 Chemical Agent Detection Kit is one of several types of alarms and detection equipment the US military uses to detect the presence of chemical warfare agents (CWAs). The US inventory of detection equipment runs the gamut from highly technical automatic sensors and forward-looking infrared devices to manually operated detectors such as the M256 detection kit.[2] Today, a variety of CWA detection equipment is used throughout the world. Some countries use identical equipment. For example, both the US and Canadian armed forces use the M256 detection kit. Other countries, e.g., the United Kingdom and Germany, use CWA detectors of their own design, but capable of achieving the same results as the M256 detection kit.

Throughout much of recorded history, toxic chemicals or substances have been used as weapons of war. An early use of chemical warfare (toxic smoke) occurred in 423 BC, during the Greek Peloponnesian War.[3] World War I ushered in the modern era of chemical warfare and served as the catalyst for the United States’ chemical weapons program. In 1969, the US stopped the development and manufacture of CWAs.  Today, the US continues to develop CWA protection and CWA detection equipment.

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Figure 1.  M18 Chemical Agent Detector Kit

In the 1950s, the US began developing CWA detection and warning systems. The M18 Chemical Agent Detector kit (Figure 1) and the M19 detection kit (Figure 2) are earlier generation detection kits. Although these kits significantly improved a soldier’s ability to detect the presence of CWAs, it soon became apparent that they were difficult to use under battlefield conditions. Each kit contained breakable components and the procedures required to conduct the various tests were difficult to accomplish while wearing protective gear. To overcome these problems, the US developed the M256 series chemical agent detector (Figure 3).[4]

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Figure 2.  M19 Sampling and Analyzing Kit

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Figure 3.  M256 Chemical Agent Detection Kit

Since these chemical detectors are mentioned in several case narratives, this information paper provides basic information to familiarize the reader of our case narratives with the characteristics, use, and operation of the M256 series chemical agent detector.



A.   Background

Among the many tasks and responsibilities that each combatant has on the battlefield is to observe their surroundings for signs of CWA. Warning signs vary depending on the chemical agent used and the delivery method. Automatic sensors such as the M8A1 Automatic Chemical Agent Alarm[5] can help, but there are other more conventional warning signs. These warning signs include bombs, rockets, or artillery shells that do not detonate with the characteristic force of high explosive; spraying from low flying enemy or unidentified aircraft; the sighting of dead animals with no apparent or observable cause of death; dead insects, especially those on animal carcasses; and individuals who are experiencing or demonstrating symptoms consistent with exposure to a CWA. If the presence of CWA is suspected, individuals in the area are alerted so that they can don their protective equipment. Once everyone has suited up, the environment must be tested to determine whether a CWA is present and, if one is present, determine its type. One way to accomplish this task is to use the M256 Chemical Agent Detector kit.

The M256 kit is a manually operated chemical warfare agent detector. It is used to determine whether it is safe to remove the protective mask following a chemical warfare agent attack, or as a confirmatory test after a chemical agent alarm has sounded. The M256 is not an alarm; it is a tool used after soldiers have received other warnings about the possible presence of chemical warfare agents, and have responded by putting on their chemical protective clothing.   Proper use of the M256 enables commanders to determine if CWAs are present, or if they can remove some protective clothing.[6]   The M256 detects the presence of hazardous concentrations[7] of blood, nerve, and blister agents in both vapor and liquid form.[8] During the Gulf War, there were two versions of the M256 series chemical agent detector in use – the M256 and the M256A1. To reduce confusion, the term "M256" is used in this paper to identify both the M256 kit and the M256A1 kit. Information pertaining to only one version (M256 or M256A1) will be identified as such.

The US Army began using the M256 kit in 1978.[9] However, the kit could not detect the nerve agent VX[10] at a concentration level that was low enough to prevent injury to personnel. Research and development to correct this deficiency resulted in the development of the M256A1, which is identical in appearance to the original M256 and has the same operator’s manual. The only difference between the two versions is the sensitivity of the nerve agent vapor detection. The M256A1’s nerve agent test is ten times more sensitive than the M256’s nerve agent test. This means that the M256A1 can detect nerve agents such as VX at a concentration level that is below the concentration level that would injure personnel.[11] The first delivery of M256A1 kits occurred in August 1987.[12]

Before the Gulf War, the M256’s shelf life was five years. The sudden deployment to the Gulf quickly depleted locally available supplies of M256 kits. This combined with the fact that many kits were approaching the end of their shelf life or had already exceeded their shelf life, caused some shortages of unexpired kits. In response, the Army Material Command (AMC) advised "that all M256 and M256A1 kits that have reached or exceeded their expiration date may be used until replacement kits are received."[13] At the same time, they advised that a positive result for nerve agent should be verified by conducting a second test using a vapor-sampler with a different lot number than the first or by using an alternate detection system such as an M8A1 chemical agent alarm or Chemical Agent Monitor (CAM). The advisory only affected the nerve agent test and not the remaining tests (blood agent test, blister agent test and lewisite test) because those tests are not affected by the age of the kit. This action was a temporary measure taken to bridge the gap until additional kits arrived.[14]

B.  M256 Kit Description

The M256 kit can easily be carried by one person. It is contained in a high-impact plastic carrying case that measures seven inches high, five inches wide, and three inches in depth. The entire kit weighs 1.2 pounds. The kit can operate in temperatures ranging from minus 25 degrees Fahrenheit (-32 degrees Celsius) to 120 degrees Fahrenheit (49 degrees Celsius).[15] Aside from its carrying case, the M256 kit has three basic components: vapor-samplers, M8 chemical agent detector paper, and a set of operational instruction cards (Figure 3).[16] These components are designed so that an individual in full Mission Oriented Protective Posture (MOPP)[17] (Figure 4) can perform the appropriate tests.[18]

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Figure 4.  Sampling with M256 Kit while wearing MOPP gear

1.  M256 Vapor-Sampler

As its name implies, the M256 vapor-sampler (Figures 5A and 5B) is used to test vapors for the presence of nerve agents, blood agents, and blister agents. It is the primary testing device in the M256 kit. When issued, each new M256 kit contains 12 of these miniature chemistry sets individually sealed in a plastic laminated foil envelope. To minimize the chance of error, two persons should conduct a vapor-sampler test; one person reads the instructions and the second person performs the action. When necessary, one person can successfully operate the vapor-sampler. An individual vapor-sampler is used once and discarded. Because each vapor-sampler contains about 2.6 milligrams[19] of mercuric cyanide, used samplers must be handled as hazardous waste[20] and disposed of in accordance with the applicable service’s regulation regarding hazardous waste disposal.

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Figure 5A.  Vapor-Sampler (back view)

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Figure 5B.  Vapor-Sampler (front view)

The vapor-sampler’s plastic body holds the test components and connects the components of each individualized test to each other. Each sampler body has several glass ampoules, one hinged heater assembly, three test spots, one hinged protective strip, and a lewisite-detecting tablet with rubbing tab.

The glass ampoules contain various chemical solutions, called reagents that react in predictable ways to the vapor forms of nerve, blood, and blister agents. The glass ampoules are used in the individual chemical agent tests in which two heater ampoules are used with the heater assembly in the blister agent tests. The ampoules are visible from the back of the vapor-sampler and are connected to their appropriate test spot by a channel molded into the plastic vapor-sampler body. Ampoules are crushed between gloved fingers to release the reagents contained within.

There are three test spots, one for each of three CWA tests. Each test spot is positioned on the sampler body to minimize the chance of cross-contamination between test spots and ensure both sides of each spot are exposed to the suspect vapor. To assist the operator in identifying the test being conducted, each test spot is shaped differently.

The name of each test (nerve, blood, and blister) is also printed above each test spot. Each test spot is made from a different material selected for its strength, sensitivity, functionality, and reliability. The nerve agent test spot is star shaped and made from filter paper. The blood agent test spot is round and made from a glass fiber filter. Lastly, the blister agent test spot is square and is made of chromatographic media .[21]

The remaining components—the hinged heater assembly, hinged protective strip, and a lewisite rubbing tablet—are connected to the sampler body by rivets and a tab. The vapor-sampler’s heater is used with the blister agent test spot. The heater improves the evaporation of the blister agent test reagents, which helps the test achieve its required sensitivity.[22] It generates heat through a chemical reaction from chemicals in the heater’s ampoules. It is riveted to the sampler body so that the operator can swing the heater over the blister agent test spot to conduct the test and swing it back out of the way to observe the test results. The protective strip is also riveted to the sampler body. It is positioned over the blood and nerve agent test spots to help protect them from accidental contamination.[23] Finally, the lewisite detection tablet is covered by a protective plastic pull-tab that must be opened before use.[24]

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