Moisture Permeability- The “paradox” of barrier properties and comfort of medical protective clothing

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Now, the war against the COVID-19 has reached the critical stage. 42000 medical staff have come to Hubei from all provinces and cities brought hope to the land of Jingchu. Medical protective clothing is their “battle armor”. In hospitals and laboratories with the most dangerous epidemic situation, these “medical practitioners” wear thick protective clothing and fight on the front line of the epidemic.

Dr. Zhou experienced protective clothing for the first time

“A lot of people told me that I might have suffocation, vomiting and other uncomfortable reactions after wearing protective clothing. I didn’t believe it at first.” As the first group of medical staff supporting Wuhan Jinyintan hospital, Dr. Zhou Changjun had never wore protective clothing before supporting Wuhan. Before he came to Jinyintan hospital to take over the ICU, he experienced the feeling of wearing protective clothing for the first time: damp and hot, suffocating and hard to breath, his clothes were constantly wet by sweat, then dried by the body, and then wet again,it’s like a sauna. “The first feeling coming out of the quarantine ward I feltlike paralyzed. I just wanted to lie on the ground and my clothes were all wet.”

In the medical frontline, facial indentation has become a standard match for medical staff, while soaking in sweat has become their normal work status.

Moisture Permeability- The medical protective clothing “paradox” of barrier properties and comfort

According to the definition of the national standard GB 19082-2009 technical requirements for single-use protective clothing for medical use, medical protective clothing is a professional clothing for medical personnel who contacts potentially infectious patients’ blood, body fluids, secretions, particles in the air, etc. to provide barrier and protection. It can be said that “barrier properties” is the key index of medical protective clothing, such as water resistance, synthetic blood penetration resistance, surface moisture resistance, filtration effect (barrier to non-oil particles), etc.

Compared with these indexes, there is one index which is a little special, namely “moisture permeability” – it represents the permeability of protective clothing to water vapor. In short, it is to evaluate the ability of protective clothing to dredge the sweat vapor emitted by human body. The greater the moisture permeability of protective clothing is, the problem of suffocation and sweat difficult to discharge can be greatly alleviated, which is more comfortable for the medical staff to wear.

To some extent, barrier function and breathability are contradictory. The improvement of barrier ability of protective clothing usually sacrifices part of breathability to achieve the balance the barrier function, which is one of the goals of enterprise research and development at present, and also the original intention of the standard GB 19082-2009. Therefore, in the standard, the requirements for moisture permeability of disposable medical protective clothing materials are clearly specified: not less than 2500g / (M2 · 24h), and the test method is also provided.

Desiccant method vs. water method. The test method difference between national standard and foreign standard

As mentioned above, GB 19082-2009 specifies the test method for moisture permeability of medical disposable protective clothing materials — refer to GB/T 12704-1991 test method for moisture permeability of textiles — part onedesiccant method. The principle is to place the moisture permeable cup filled with desiccant and sealed with fabric sample in the sealed environment with specified temperature and humidity, and calculate the moisture permeability, moisture transmission rate and moisture permeability coefficient of the sample according to the change of the mass of the test cup within a certain period of time. During the test, it is necessary to weigh the mass of the test cup at intervals. In particular, after weighing, slightly shake the desiccant in the cup to mix it up, so as not to weaken the drying effect due to long-term use of the upper desiccant. At the same time, it should be noted that the total moisture absorption increment of the desiccant should not exceed 10% to avoid affecting the accuracy of the test.

The above is the situation of China national standards, at present, the international standard of medical protective clothing is NFPA（National Fire Protection Association）1999：2018<Standard on Protective Clothing and Ensembles for Emergency Medical Operations>,it stipulates the performance requirements and test methods of emergency medical protective clothing; AAMI（The Association for Advancement of Medical Instrumentation）PB-70:2012< Liquid Barrier Performance and Classification of Protective Apparel and Drapes Intended for Use in Health Care Facilities>, which specifies the liquid barrier performance and classification requirements for disposable and reusable protective clothing. Among them, only NFPA 1999:2018 “moisture vapor transmission rate” stipulates “moisture permeability”.

NFPA1999 stipulates that“Garment materials …… shall have a moisture vapor transmission rate of 650g/m2·24hr or greater.”The test method refers to Procedure B-Water methodin ASTM E96<Standard Test Medicals for Water Vapor Transmission of Materials>, test under condition of 23℃.  Fill the test cup with distilled water,calculate the transmission rate of water vapor permeates through the sample into the controlled environment atmosphere by weighing the test cup periodically. GB T 12704.2-2009 part two also adopts this test method.

It can be seen that the difference between desiccant method and water method is that, in desiccant methodinner side of the sample is dry while the other side is wet, in water method, the outer side is of low humidity while the other side is high. The water vapor pressure difference between the two methods can be considered the same, but the difference is the humidity on two sides of the sample. In terms of the selection of test methods, it should be determined according to specific product standards. ASTM E 96 also recommends that the two test methods should not be expected to obtain the same test results, and the test method which is morefits for the service conditions should be selected.

Select of test condition

In addition, according to the author’s testing experience and related literature, most of the fabric’s moisture permeability increases with the increase of temperature; when the temperature is fixed, the fabric’s moisture permeability basically decreases with the increase of relative humidity. Therefore, the sample moisture permeability under a certain test condition can not represent the moisture permeability measured under different test conditions!

Although GB 19082-2009 stipulates the technical requirements for medical protective clothing, the test condition is not specified. The author also consulted the test method standard GB / T 12704.1, which provides three test conditions: A, 38 ℃, 90% RH; B, 23 ℃, 50% RH; C, 20 ℃, 65% RH. It is suggested that group a test condition should be preferred in the standard, which is suitable for laboratory test and research because of its high relative humidity and rapid infiltration speed. Considering the practical application environment of protective clothing, it is suggested that a group of tests under 38 ℃, 50% RH test conditions should be performed as well by enterprises, so as to evaluate the moisture permeability of protective clothing materials more comprehensively.

How is the “moisture permeability” of current medical protective clothing

According to the test experience and available literature, the main materials and structure of medical protective clothing materials have the moisture permeability between 500g / (M2 · 24h) and 7000g / (M2 · 24h), mostof the moisture permeability isfrom 1000 g / (M2 · 24h) to 3000 g / (M2 · 24h). At present, while expanding production capacity to solve the lack of virusprevention materials such as medical protective clothing, professional research institutions and enterprises have started to consider tailoring protective clothing for medical staff from the perspective of “comfort”. For example, the temperature and humidity control technology of protective clothing developed by Huazhong University of science and technology uses the air circulation technology inside the protective clothing to dehumidify and regulate the temperature, so as to keep the protective clothing dry and improve the comfort.

Now we are winning the war against the novel coronavirus. We praise the hard work of the front-line “soldiers in white”, we also dedicate to satisfying their needs for comfort. Although the indentations on their faces and wet clothes mark their vows of “regardless of pay, life and death”, we hope to see their reassuring smile to save lives and injuries in a comfortable and safe environment through our own powerful support!