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There are various ways to classify masks; generally, we divide them into two main categories based on their structure and working principle: air-filtering masks and supply-air masks. The working principle of air-filtering masks, or simply filtering masks, is to allow air containing harmful substances to pass through the filter material in the mask before it is inhaled by a person ； A supply-air mask refers to a system that delivers clean air, free from harmful substances, to the wearer’s face through tubes and a mask, using power sources such as air compressors or compressed gas cylinders, so that the person can breathe. Filter masks are one of the most widely used types in our daily work. The following details the methods for selecting such masks as well as the conditions under which they should be used. The structure of a filtering mask should be divided into two main parts: one is the main body of the mask cover, which can be simply understood as the framework of the mask ； The other is the filter material section, which includes filter cotton for dust prevention as well as chemical filter cartridges for gas protection. Therefore, when it comes to the selection and use of filtering masks, some manufacturers’ products offer the convenience of allowing you to use the same mask frame; when protection against dust is needed in dusty working environments, you can simply attach the appropriate filtering cotton to it, and thus you end up wearing a dust-proof mask ； When it is necessary to provide protection against toxins in a toxic environment, the filter cotton can be replaced with a corresponding chemical filter cartridge, turning it back into a gas mask; or, depending on your work requirements, more combinations are available for you. We can see that air-filtering masks come in various types as well, such as half-face masks, which cover only the respiratory organs (mouth and nose) ； Comprehensive type, meaning the mask can cover the entire face including the eyes ； Electric air supply type, that is, driven by batteries and motors, it draws in air containing harmful substances, filters it through filter media, and then makes it available for human breathing. Among half-face masks, there are also those that require no maintenance; such masks are primarily used for dust protection, with the main body and the filtering material being integrated together, and the filtering material cannot be replaced ； Low-maintenance type: This type of mask does not have any replaceable parts; if it gets damaged, the entire mask must be replaced, which helps save labor costs and maintenance expenses ； It is of the general maintenance type, meaning that individual components of the mask can be replaced one by one when damaged, and regular maintenance is also necessary. So far we have learned about the various types of masks. Then, how should one choose one of them to meet the requirements of one’s working environment? Can simply picking any one of them suffice to meet the needs in different situations? To answer the above questions, it is first necessary to understand the appropriate conditions for using different types of masks, which is very important when choosing a mask. For example, air-filtering masks are only suitable in environments where the oxygen concentration is above 18%; otherwise, supply-air masks should be used. When the properties of harmful substances are unknown, the worst-case scenario should be considered, and equipment such as SCBA should be used. So, for filter masks, when the oxygen concentration is above 18%, can we just pick any mask and wear it? Of course not; we also need to understand an important concept, namely the mask’s Assigned Protection Factor (APF). For different types of masks, a coefficient has been assigned based on studies of their protective level when in use; this is known as the Assigned Protection Factor (APF), and it helps us choose the appropriate mask. Different ** or organizations specify different APF values for masks, but they mean the same thing. For example, according to the American ANSI Z88.2–1992 standard, the specified protection factor for air-filtering half-face masks is 10; this means that when wearing such a mask, it helps to reduce the concentration of harmful substances by a factor of 10 ； The same ； The APF value for air-filtered full-type models is 100, while that for full-type electric forced-air models is 1000, etc. The purpose of wearing masks for protection is to reduce the concentration levels of harmful substances in the working environment to acceptable exposure levels (regulations specify different allowable concentration values for various substances; reference can be made to the U.S. PEL values and TLV values, among others). Therefore, before choosing a mask, it is necessary to understand the actual exposure levels to harmful substances, what the allowable exposure levels for these substances are, and what the oxygen concentration levels are, among other things ； Then make the correct choice based on the specified protection factor of each mask. Of course, choosing the type of mask isn’t the end of everything; to enable it to function effectively ； There is still much to be done to effectively protect you from harmful substances. For example, it is also necessary to select appropriate filter materials ； Users should be trained to ensure that masks are worn correctly, and must keep wearing them whenever exposed to harmful substances, and so on. Furthermore, it is necessary to ensure a good fit between the mask and your face, and achieving this requires taking into account various factors such as purchasing high-quality masks that pass fit-testing, ensuring that workers wear them correctly, and choosing masks of the right size. Otherwise, even if the correct type of mask is used, it will not be possible to truly protect one from harmful substances. Remember, choosing the right type of protective mask is an important step for effective protection, as your health depends on it.