Respiratory resistance test of N95 mask

Face masks or respirators are important components of personal protective equipment for medical personnel and workers in atmospherically hostile environment. This is especially true for healthcare workers who need to interact with patients inflicted by transmitted diseases such as the severe acute respiratory syndrome outbreak that occurred in March 2003 (Seto et al., 2003). Many national and international health agencies also recommended the use of face masks or respirators for the recent influenza A (H1N1) pandemic (Cowling et al., 2010). These healthcare workers have to wear the respirators for up to 12 h and this may induce physiological stress on them (Farquharson and Baguley, 2003). In another reported study by Lim et al. (2006), of 212 healthcare workers who participated in the survey, 37.3% reported headaches when they wore the respirators. Farmers wearing face mask while spraying pesticides in warm environments were also reported to experience heat stress on the body due to increased temperature and humidity within the face mask (Hayashi and Tokura, 2004).
Many reported studies were done on the effectiveness of various respirators but very few of them focused on the discomfort level of their use. There were reported studies on the effects of wearing N95 respirators and surgical face masks on thermal stress and subjective sensations of the wearer (Li et al., 2005). Although comfort level is subjective, there are few measurements that can be used to correlate with this comfort level. For example, a drier and cooler microclimate leads to better comfort (Li et al., 2005). A higher expiratory and inspiratory resistance reduces the ease of breathing and thus causes discomfort. A lower airflow volume will mean that the wearer may need to inhale harder to get the same amount of fresh air required. In a recent study by Roberge et al. (2010) using an automatic and metabolic simulator as a human surrogate, inhalation and exhalation resistances were found to increase by 0.43 and 0.23 mm of H2O pressure and it was concluded that increased exhaled moisture due to the wearing of respirators would not add significantly to the breathing resistance.
In this study, we conducted a series of experiments using typical equipment such as rhinomanometry and spirometer to assess objectively the increased breathing resistance with the use of N95 respirators on actual human subjects instead of simulators or human surrogates. The effect on air exchange volume due to the wearing of respirators will also be examined.
Fourteen Asian healthy human subjects (seven males and seven females) aged from 18 to 25 years participated in this study. They were all nasal symptom-free and had not taken any medication for at least 1 month before entering the study. Routine rhinological examination was performed to exclude recent infection and nasal deformity (e.g. obvious septal deviation). Before the experiment, the subjects were briefed on the nature, purpose, methods and risk of the study. They had the right to question any part of the procedure and to withdraw from the experiment at any point of the experiment. All subjects involved were on a voluntary basis and the study was approved by the institutional review board of the National University of Singapore.
In the experiment, we used the N95 (3M 8210) respirators (3M Korea Limited, Seoul, Korea). A fit test was performed during the screening visit (1 week prior) in order to ensure that each subject had his/her own personal respirator with an adequate size and was well fitted without leaking air. The subjects were briefed on the manufacturer’s instructions of wearing the respirator before the start of the experiment.
The measurement of nasal airway resistance was performed using a rhinomanometer NR6-2 (GM Instruments, Glasgow, UK). Nasal airflow was collected by a mask, which must form an airtight seal around the face and was then measured by a pneumotachograph. Resistance is calculated as a total pressure/flow (Pascal seconds per cubic meter). This technique is known as the posterior method and it does not interfere with the nasal passages. The original mask of the rhinomanometer is designed with air cushion to allow the user’s face to be fitted nicely. This is to prevent any leakages, which will affect the readings. However, this original mask will be unable to serve its function when the user also wears an N95 respirator as the original mask for the equipment will be too small to cover the face with N95 respirator. Therefore, we designed a modified full face mask modified from the face mask of a typical facial sauna steamer so as to provide proper fitting even when the user wore the N95 respirator. Holes of appropriate sizes were drilled at the bottom of the acrylic ‘Face mask’ to attach the tubes with sealant from the rhinomanometer. The appropriate thickness of cushion strip was attached onto the edges to serve the function of the air cushion from the original mask, and a good seal was achieved by the restraining rubber band over the head, reinforced by negative pressure during inhalation. The remaining gaps at the bottom of the Face mask were covered up by plasticine. The original mask and the modified full face face mask for the rhinomanometer are shown in Fig. 1.
A rhinospirometer (GM Rhinospirometer PC Linked model NV1) (GM Instruments) is an apparatus used to measure the volume of air inspired and expired through the nose. The spirometer records the amount of air that is breathed in and out over a specified period of time. Similarly, the modified full face mask was used to replace the spirometer’s nose tube to measure the total airflow from the user instead of individual readings from the left and right nostrils. The study subjects were required to breathe through the nose only for a period of 30 s with and without the N95 respirators. The nose tube from the spirometer was connected to the drilled hole at the bottom of the modified full face mask for the spirometer. The conductor of the experiment ensured that there were no leakages around the full face mask before carrying out the recording of results. The volume of air involved in the exchange was then noted down for that period.
In the posterior rhinomanometer test, a small tube was placed in the mouth long enough to sit on the tongue and with the lips closed round the tube. If the soft palate of the subject is relaxed, the pressure measured by this tube would be the same as the pressure driving the airflow through the nose. Measurement of the total nasal resistance was performed by following the standard technique that has been published by the International Standardization Committee (Clement, 1984).
Prior to measurement being taken, the subjects were asked to relax in a quiet condition for ?30 min. Then, they were asked to breathe in a relaxed manner through the nose and to avoid rapid maneuvers. Figure 2 shows a human subject wearing an N95 respirator during the tests.
All measurements (rhinomanometry and rhinospirometry) were recorded by a mean value of at least three consecutive tests with stable results. A test was performed comparing the standard mask to the modified face mask The results showed that the average percentage differences between these two measurements were 10.15 and 12.28% during inspiration and expiration.