How to use chemical gas masks correctly

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A gas mask is a type of personal respiratory protection device commonly used in tasks involving toxic substances. The chemical filter cartridges it contains are capable of removing harmful gases or vapors from the air, or reducing their concentration, thereby protecting the health of the user. According to our country’s standards, gas filter cartridges are generally classified into those designed to protect against organic gases, ammonia, mercury vapor, and acidic gases (such as chlorine, hydrogen chloride, and sulfur dioxide). It is very important to understand how the gas filter box works, as this helps people use it correctly in their tasks. The filter cartridge is filled with an adsorbent, which has a porous structure and is usually activated carbon, such as coal-based carbon or coconut shell carbon. The porous structure gives activated carbon a very large internal surface area; the surface area per gram of activated carbon can reach around 1,000 square meters, enabling it to have a high adsorption capacity for many gases and vapors. Gases are substances that exist in a gaseous state at normal temperature and pressure, such as chlorine and carbon monoxide. Vapors are gases produced by the evaporation of substances that exist in a liquid state at normal temperature and pressure, such as benzene and propylene. Adsorption is the process in which gas molecules are attracted to the surface of activated carbon, thereby being separated from the air. This attraction is relatively weak; it is a minor physical force, also known as physical adsorption. Its strength is closely related to the properties of organic vapors. To make the gas filtration cartridge more selective toward certain chemicals, activated carbon is impregnated with chemical reagents; as a result, specific chemicals can undergo chemical reactions with these reagents on the activated carbon. This type of adsorption is also known as chemical adsorption. Most canister filters rely on chemical adsorption (see Table 1). Table 1 Types of Gas Purification Canisters and Their Filtration Mechanisms
Type of Gas Purification Canister | Filtration Mechanism | Examples of Chemical Reagents
Organic vapors | Physical adsorption —— Ammonia | Chemical adsorption; nickel chloride, cobalt salts, copper salts, acids
Acidic gases | Chemical adsorption | Carbonates, phosphates, potassium hydroxide, copper salts
Mercury vapor | Chemical adsorption | Iodine, sulfur
Chemical adsorption is a type of chemical bond; it is much stronger than simple physical adsorption and is irreversible. As long as care is taken to prevent penetration during use and the gas purification canisters are replaced in a timely manner, no special problems usually arise. By \"penetration,\" it is meant that the activated carbon has become saturated with adsorbents and has lost its filtering capacity; if it is not replaced, harmful gases will be inhaled directly by the users. However, the situation is more complex for organic canisters. Firstly, there are a wide variety of organic vapors, and the same canister has different adsorption capacities for various substances, making it difficult to determine a uniform replacement time. Another significant issue is that, due to the weak physical adsorption forces, the adsorption process may reverse, leading to desorption. Desorption occurs naturally during the period when activated carbon is not in use, and it can also take place as a result of the selective adsorption by activated carbon of substances with greater affinity for it, thereby replacing the substances that have been adsorbed; such easily replaceable substances are usually referred to as volatile substances. In Europe, a boiling point of less than 65°C is used as a guiding criterion for distinguishing volatile substances; common examples include propanone (boiling point 56.5°C), dichloromethane (boiling point 39.8°C), gasoline (boiling point 40–200°C), petroleum ether (boiling point 40–80°C), diethyl ether (boiling point 34.6°C), and acetaldehyde (boiling point 20.8°C). For canisters used to filter such substances, during periods when they are not in use, even if there is no airflow within the canister, the substances that were previously adsorbed will desorb. The desorbed substances spread from the carbon layer at the inlet end of the canister toward the back, until they fill the entire canister. This phenomenon is known as migration. Factors that cause migration include: the volatility of the adsorbed substances – the higher the volatility, the more likely migration will occur ； —— Environmental humidity: if it is greater than 50%, migration is more likely to occur ； —— Original adsorption capacity of the gas filter cartridge ； —— Filter canister storage time ； —— Types of vapor. Desorption and migration pose problems for the reuse of organic gas canisters. Migration will cause the canister to puncture prematurely upon reuse; if the desorbed substances have spread throughout the canister, users will immediately inhale high concentrations of harmful substances, which is extremely dangerous. Desorption also occurs under another condition. For example, on the first day, the gas filter box was used to absorb a relatively volatile substance A; its usage time was shorter than the box’s lifespan, and the substance did not penetrate through it. On the second day, the gas filter box was used in a different environment to absorb another substance that is not easily volatile, substance B (workers who are on the move, such as repairmen, may be exposed to this substance). As a result, A was replaced by B; A penetrated more quickly and at a concentration higher than that of the surrounding environment, which is also extremely dangerous. This phenomenon also occurs when substances of types A and B are present together. Regrettably, current research is not yet able to provide a reliable way to classify the volatility of substances; it remains necessary to rely on existing knowledge and the experience gained by users through practice in order to determine which substances are highly volatile and which are relatively so. The following suggestions are provided for users of chemical gas cartridges: 1. For stable operations, draw on existing experience, and based on the types and concentrations of pollutants present at the work site, as well as factors such as the intensity of work, temperature, humidity, and the volatility of substances, establish a schedule for replacing the gas cartridges and adhere to it. It would be helpful if the canister manufacturer could provide recommendations on the service life of the canister based on site parameters. 2. If the boiling point of the adsorbed substance is below 65°C, it is recommended to replace the canister daily. 3. If it is believed that the adsorbed substance is volatile, and the canister is not used for a few days during its use, such as over the weekend, it is recommended to replace the canister. 4. Even if it is assumed that the adsorbed organic substances are not easily volatile, if the canister is not used for about two weeks during its operation, it is recommended to replace it. Penetration time of Bagu gas filter canisters (for reference only): Industrial gases (according to EN141). Test gas, test concentration (PPM), penetration time (minutes): A2 – Organic vapors, solvents, hydrocarbons; C6H12: 5000, >35. B2 – Inorganic vapors: H2S: 5000, >80; Cl2: 5000, >20; HCN: 5000, >25. E2 – Acidic vapors: SO2: 5000, >20. K1 – Amine derivatives: NH3: 1000, >50. NO and NO2: 2500, >20 each. CO2: 2500, >20