2.2. Disinfection Treatments

2.2.1. Microwave Irradiation
Metal nose clips of respirators were eliminated, and the respirators were exposed to microwave irradiation with 750 W power (MR-280M, LG Electronics, Seoul, Korea); for 1 min, the outer side was directed to the irradiation, then, for 1 min, the inner side was directed to the irradiation. All treated samples were stored for 12 h under ambient temperature before the filtration test.

2.2.2. Oven-Dry
Respirators were dried at 90 °C for 1 h in a drying oven (Withlab Co., Ltd., Gyeonggi-do, Korea).

2.2.3. UV Irradiation
UV rays were radiated from 16.5 cm away from the tray using a UV sterilizer (KRS-A1, KARIS, Gyeonggi-do, Korea). The wavelength of 253.7 nm UV ray was irradiated through the inner area of the sterilizer in 42 cm × 32 cm × 32 cm. The power consumption of UV light bulb was 10 W and the irradiation was conducted for 1 h for inner side and another 1 h for outer side of respirators.

2.2.4. Chlorinated Disinfectant Immersion
Chlorine-based disinfectant (Yuhan Corporation, Seoul, Korea), formulated with sodium hypochlorite (NaClO) (5.5%), sodium hydroxide (NaOH) (0.3%), and water, was used for this treatment. The chlorine disinfectant was diluted in tap water to 5% (v/v), which makes the final sodium hypochlorite concentration in water to be 0.275%. The respirator samples were soaked in the solution for 10 min, then rinsed in tap water for 3 min two times, and dried for 24 h under an ambient condition.

2.2.5. Ethanol Immersion
The respirator samples were immersed in an aq. ethanol (EtOH) solution of 70% (v/v) for 10 min, then dried for at least 24 h.

2.2.6. Isopropanol Immersion
Isopropanol (IPA) is commonly known as a discharging agent for electret media [36]. Respirators were immersed in IPA liquid (≥99.9%) for 10 min, and dried.

2.2.7. Laundering
Laundering of respirators was done with and without detergent. For water-laundering without detergent, each respirator was put in a stainless can of Terg-O-Tometer (T-O-T, Yasuda Seiki Seisakusho, Tokyo, Japan) with 1 L of tap water. The laundering with water (without detergent) was conducted with agitation speed of 90 rpm at 24 °C for 10 min, and then 3 min for two more times. For detergent-laundering, a detergent (Actz power gel, Pigeon, Seoul, Korea) composed of anionic surfactant was used. Respirators were laundered in T-O-T with 1 L of 0.1 wt% aq. detergent solution at 24 °C and 90 rpm for 10 min, then the samples were rinsed with 1 L of tap water for 3 min four times to thoroughly remove the detergent residue [37].