1. Introduction

A dust mask is a moulded disposable mask made of paper pad (Goh et al., 2020). Fine dust has been recognized as an object that causes inconvenience in life, such as decreased vision, sedimentation of the lungs, and respiratory discomfort due to the natural phenomenon of yellow dust blowing from the Gobi desert in China (Lee and Kim, 2020)

In the past, fine dust has been recognized as a short-term problem that is solved by wearing a mask despite adversely affecting human health. However, as the economic level improved, interest in health aspects increased and the health of the respiratory sector, which was a major problem for lung patients and smokers, also attracted public attention. Particulate matter (PM) contributes to an increased risk of respiratory and cardiovascular illnesses, cancer, and preterm birth complications (Kang, 2015;Park et al., 2019). So today, the danger and severity of fine dust is emerging. PM as fine dust is a complex mixture of small solid particles and liquid droplets that are suspended in the air and vary in size, shape, surface area, chemical composition, solubility and origin (Pacitto et al., 2018;Kim et al., 2015).

Fine dust causes harmful problems such as worsening of cardiovascular and respiratory diseases and increased risk of lung cancer. Fine dust has been reported to decrease national GDP by 0.0052% and 0.005% in added value and 0.0058% in employment, with a monthly increase of 1%, leading to extensive social and economic losses (Rhee et al., 2018;Seo, 2015;Zhang and Choi, 2018). In other words, the fine dust problem is expected to cause serious damage to individuals and countries in the future. The population structure is expected to emerge as a major health problem in the future society by increasing the incidence of respiratory diseases among the elderly who are relatively unhealthy and increasing the risk of chronic respiratory diseases among newly born infants (Bae, 2016;Myong, 2016).

As a solution to reduce this fine problem, the government has implemented measures to reduce fine dust, such as restricting old diesel vehicles, promoting fine dust reduction campaigns in daily life, operating optimal dust prevention facilities for each industry, and reducing the uptime of dust-induced industrial facilities. Nevertheless, it is observed that the days of high concentration of fine dust exceed the environmental standard and increase every year. Therefore, in order to protect the hygiene and health of the individual, Korea is blocking the fine dust by wearing a mask in the spring (Han and Kim, 2015).

The mask that blocks fine dust is divided into two categories, ‘health mask’ and industrial ‘dust mask’, to manage performance standards. In the case of health masks, three categories of dust collection efficiency, face inhalation resistance, and leakage rate are classified according to the test performance criteria according to the KF grade of the Guidelines for Health Mask Standards set by the Korea Food and Drug Administration. In the case of industrial dust mask, 14 items including dust collection efficiency, face intake / exhaust resistance, leakage rate, field of view, strength, and elongation rate are selected according to the dust mask performance standards in an annexed list 4 of the ‘Protective Equipment Safety Certification Notice’ specified by the Ministry of Employment and Labor as the test performance standard (Han et al. 2004).

Health masks have a looser management standard than dust masks, so you only need to pass the quality assessment once.

In particular, in the case of an overseas study that analyzed the efficiency of a mask sold in Beijing in 2018, some of the masks on the market were up to 29% in the transmittance and 68% in the leak rate., and these masks that are not available are sold together (John et al., 2018;Patel et al., 2016).

Therefore, this study aims to verify the reliability of mask performance analysis by performing performance evaluation on masks that block fine dust marketed in Korea. In addition, through the research and development of masks, we would like to contribute to securing people’s right to health from fine dust by providing basic data on the development of masks of fine dust harmless to human body.

2. Research methodology

The performance standard of the face mask is based on the filter that filters air, and the US, NIOSH grades N, R, P are representative guidelines. In addition, based on the standards set in each country such as European FFP or domestic KF grade, various test items such as collection rate, respiratory resistance, flame resistance, and liquid penetration rate are defined. Among the health masks marketed in Korea, three categories of dust collection efficiency, face inhalation resistance, and leakage rate are tested according to the KF grade, “Guidelines for Health Mask Standards” issued by the Ministry of Food and Drug Safety. Based on this criterion, this study performed performance evaluation on two categories of dust masking efficiency and facial inhalation resistance for commercially available mask species.

2.1 Experimental analysis mask

The masks for experimental analysis are shown in Fig. 1, and were selected based on the market share of masks by company as of May 2017. The criteria for each subject were three KF80, three KF94, one KF99 and one N99 mask, and each product was opened and used immediately before the experiment after storage.

2.2 Aerosol for experimental analysis

The collection efficiency test used various particles such as corn oil, paraffin oil, silica gel, saccharin, NaCl aerosol and dioctyl-phthalate.

According to the analysis of the fine dust component of the Ministry of Environment, the fine dust in Korea is somewhat different depending on the season and weather conditions. It is reported that the fine dust is composed of aerosols containing an average of 55% or more of ions, and 20% to 30% of carbon and metals.

Therefore, in this study, sodium chloride aerosol with high similarity to the characteristics of dust generated in Korea was selected as analytical particles.

2.3 Experimental analysis test equipment

The face part of the sample was placed in an automatic filter inspection device (SFP Service Type 1300S), and sodium chloride aerosol was passed through the face part at a flow rate of 95 L per minute, and the concentration before and after the face part was measured. The measured value at this time was taken as the average value obtained in 15 ± 3 seconds, measured within 3 minutes after the start of the test.

2.4 Test method for dust collection efficiency of mask

In the mask dust collection efficiency experiment, five masks of each mask type were used as test samples, and the average value of each result was evaluated.

2.5. Experimental method of face intake resistance of mask

In the face intake resistance test, five masks of each mask type were used as test specimens, and the face intake resistance was evaluated as an average of the results.

The experiment measured the water column or the differential pressure (Pa) when the face part of the test specimen was put on the test iron and passed at a continuous flow rate of 30 L per minute.

3. Evaluation method and experiment result

3.1 Evaluation method

Evaluation criteria of the mask are shown in Table 1, and the overall experimental conditions were maintained at a humidity of 45% to 55% and a temperature of 25.5°C. According to the KF grade of the Guidelines for Health Mask Standards issued by the Ministry of Food and Drug Safety, two categories of dust collection efficiency and facial inhalation resistance were evaluated. The device as shown in fig. 5 is a device that measures the intake resistance and differential pressure of a mask, which is measured when breathing in while wearing a mask using a manometer. The measurement method is to measure the pressure difference caused by the resistance of the mask when the mask is mounted on a standard head model and then completely sealed with a tape around the mask, and then sucked at a flow rate of 30 L/ min.

3.2 Dust collection efficiency

Fig. 6 is an experiment showing the efficiency of dust collection for the mask under test. According to the Korea Food and Drug Administration's dust collection efficiency standards, the KF 80 mask is 80% or higher, the KF94 mask is 94% or higher, and the KF99 mask is 99% or higher.

In the dust collection efficiency test, all the products in the test masks were found to meet the standard value for each product grade. KF80 showed an average of 94.00% in three types, KF94 showed an average of 98.81%, KF99 was 99.74%, and N99 was 99.32%, showing better performance than the 90% collection efficiency of the mask. In addition, the higher the KF grade, the higher the dust collection efficiency, and the dust collection efficiency of each mask of each experiment showed a constant value without significant change.

3.3 Facial Intake Resistance

Fig. 7 is an experiment showing the efficiency of facial intake resistance for the mask under test. According to the Korea Food and Drug Administration’s facial intake resistance efficiency standards, the KF 80 mask is 60 Pa or less, the KF94 mask is 70 Pa or less, and the KF99 mask is 100 Pa or less.

The facial intake resistance was different for each product grade, but most products were measured with an average value between 20 Pa and 30 Pa. The three average values of KF80 are 24 Pa, and the three average values of KF94 are 19.70 Pa.

The three average values of KF99 was 32.20 Pa and the three average values of N99 was 27.20 Pa. In particular, the average value of the E mask and the H mask was measured at a lower pressure of less than 10 Pa, indicating that the facial intake resistance was lower than that of the other masks.

The product used in this experiment is not a defective product because it satisfies the facial inhalation resistance standard.

4. Review and discussion

Examination of face adhesion or inspection of leak rate is a matter of choice for test subjects. This is because it determines how different face groups can be selected. The leakage test conducted in Korea is to be specified by measuring the face dimension, so the total leakage rate (TIL) or inspection of the leakage rate (inward leakage) must be investigated. A leak rate test is an experiment that must be inspected as a prerequisite for the mask manufacturer's permission to sell the mask. (Han and Young, 2000).

The types of masks permitted by the Ministry of Food and Drug Safety are health masks (prevention of yellow dust and anti-prevention masks) and surgical masks. Health masks must be approved for each item by measuring the degree of blocking foreign substances that are inhaled from the outside through tests such as dust collection efficiency test, facial intake resistance test, and leak rate test. For example, the KF 80 mask has a leak rate of 25% or less, the KF94 mask has a leak rate of 11% or less, and the KF99 mask has a leak rate of 5% or less. In addition, the facial inhalation resistivity should be less than or equal to 60 Pa in KF80, less than or equal to 70 Pa in KF94, and less than or equal to 100 Pa in KF99.

Looking at the leak rate inspection method, 10 test subjects were selected as female or male (with clean beards and without beards or sideburns), and the test subjects determined the leak rate by grade according to the test results of 10 test subj ects once.

The fit test only examines the leakage path through the mask and facial skin. However, the total leak rate test is characterized by including all three cases of leakage through the filter itself, leakage through the exhaust valve, and leakage through the mask and facial skin (Han, 2004).

However, the leak rate test may have differences according to face dimensions, differences between men and women, and differences between ages and manufacturers, so detailed guidelines are needed.

The closeness test conducted in the United States, according to the LASL (Hack et al., 1973) report, recommends a test panel with a certain facial size.

This is not a legal mandate but only a recommendation. Nevertheless, many studies still use or apply LASL panels (Coffey et al., 2002).

In this study, the measure of H mask showed a large variation in inspiration resistance depending on whether the test iron was in close contact (3.4 Pa~44 Pa). As mentioned in the study by Coffey et al. (2002), facial adhesion varies with facial dimensioning, and it is difficult to express it as a standardized value. According to the above results, it was confirmed that the masks distributed in Korea under the conditions and situations in which the face contact was secured were guaranteed to block fine dust.

5. Conclusion

As a result of this study, it was found that all the face masks of the test subjects fully satisfied each product grade standard, unlike the similar research made in China. Since the KF80 grade does not show a big difference in the collection efficiency of each product, the purchase of a higher grade mask does not seem to have a significant difference in daily use.

Even with cheap products, the particle size distribution ranges from 0.04 μm to 1.0 μm and the average particle size is about 0.6 μm, and the difference in col-lection efficiency is around 5%.

As pointed out in the past research cases, domestic masks that are distributed to the private sector continue to pass the performance standard only once before launch. In March 2019, some mask manufacturers uncovered an unauthorized increase in manufacturing facilities without permission from the Ministry of Food and Drug Safety.

This is because, depending on the manufacturer's operating method, items such as facial intake resistance, etc., may be distributed under masks that do not meet the product grade.

The dust collection efficiency of the masks in this study showed reliable results, but it is necessary to supplement the system in a way that the masks satisfying the standard value can be continuously produced and supplied in the future.

In addition, it is necessary to continue the research to reduce the efficiency decrease according to the facial dimension selection in relation to the facial face adhesion. The use of a mask does not necessarily guarantee the prevention of disease infection as Covid- 19 or harmful dust intrusion into the body (Goh et al., 2020).

The community-wide benefits are likely to be greatest when face masks are used in conjunction with other non-pharmaceutical practices (such as social-distancing), and when adoption is nearly universal (nationwide) and compliance is high (Eikenberry et al., 2020).

Lastly, we hope that even after this research, we hope to ensure reliability through individual-led inspections and investigations, and we need to supplement the criteria for maximum and minimum values through further studies on facial inhalation resistance. Since the price is not a good indicator of the best performance, it is better to focus more in characteristics of the quality of the filter as a filtration typology, number of different filter layers and how the mask fits to the human face (Pacitto et al., 2018).