Disposable filtering facepiece respirators (FFRs) are the most frequently used respirators in U.S. private industry and healthcare, and the N95 class of FFR (N95 FFR) is the single most selected version [1]. FFRs are classified as negative pressure respirators because inhalation against the resistance of the filter media creates pressure within the FFR deadspace (V_D) that is negative with respect to ambient air pressure. U.S. Occupational Safety and Health Administration (OSHA)-mandated respiratory protection programs require the use of National Institute for Occupational Safety and Health (NIOSH)-certified FFRs. NIOSH certification limits for FFR inspiratory and expiratory filter resistance are 35 and 25 mm H₂O pressure, respectively (tested at a constant airflow rate of 85 L∙min^-1) [2]. These values are considerably less than the 60 - 140 mm H₂O pressure that has been identified previously as being “noticeable but well tolerated” during the hardest work experienced by a worker wearing a half facepiece, elastomeric, negative pressure, air-purifying respirator [3]. Other comparable regulatory design requirements exist elsewhere (e.g., European Standard EN:149 indicates inhalation resistance limits of 7.1 and 24.4 mm H₂O pressure, respectively, at 30 and 95 liters-per-minute of continuous airflow and exhalation resistance limits of 30.5 mm H₂O pressure at 160 liters-per-minute of airflow). Many current models of N95 FFRs have demonstrated filter resistances of <25 mm H₂O pressure during non-human testing at low-moderate work rates [4–6]. Jones [7] reported that, for subjects at rest and during 5 minute periods of light, moderate and high work rates, in-line pressure transducer measurements of FFR filter resistance were proportional to the work rate and ranged from 0-20 mm H₂O pressure. The impact of filter resistance on airway pressures was investigated by Lee and Wang [8] who used a modified rhinomanometry method to demonstrate average nasal inspiratory and expiratory pressure increases of 0.32 Pa/cm³/sec (32 mm H₂O/L/s) airflow when wearing an N95 FFR, an approximate doubling of baseline (no N95 FFR) nasal airway resistance [9]. Despite these relatively low resistances reported, complaints of breathing difficulty with N95 FFRs are nonetheless commonly expressed by a sizeable proportion of users in the healthcare industry [10–12]. The greater the resistance, the more energy expenditure is required for ventilation and the greater the perception of discomfort that can lead to decreased compliance with wear and, thus, compromise protection. All of this begs the question of the lowest feasible filter resistance that can be designed into N95 FFRs to maximize discernible (subjective) breathing comfort and minimize (objective) physiological strain while still maintaining adequate filtration and fit characteristics. The current study, part of NIOSH’s Project B.R.E.A.T.H.E. (Better Respiratory Equipment using Advanced Technologies for Healthcare Employees) [13], was carried out by the National Personal Protective Technology Laboratory (NPPTL) of NIOSH to investigate the subjective and physiological responses to wearing N95 FFRs with filter resistances lower than most standard N95 FFRs currently on the market. This data could be important for respirator manufacturers, standards development organizations, researchers, respiratory protection program managers and end users.

Ten healthy, non-smoking subjects (7 men, 3 women) participated in this study, the majority of whom (8/10) were experienced N95 FFR users. Demographic mean values (standard deviations) were: age 24.5 years (3.8), height 179.0 cm (11.0), weight 75.3 kg (12.4), and body mass index 23.4 kg/m^-2 (2.9). On the day of testing, subjects underwent a screening history and physical examination by a licensed physician. Subjects were dressed in athletic shorts or pants, tee shirts and athletic shoes during exercise testing. The study was approved by the NIOSH Human Subjects Review Board, and all subjects provided oral and written informed consent.

Identical appearing, cup-shaped, prototype FFR samples from 3M Company (St. Paul, MN, US) were supplied to NPPTL in nominal filter resistances of 3mm, 6mm and 9mm H₂O pressure drop at 85 L/min of constant airflow (hereafter referred to as prototype respirators [PR] 3, 6, and 9, respectively) achieved through modifications of the filter media. Prior to study trials, pressure drop (i.e., pressure differential of airflow across the respirator filter) of the PRs was verified at NPPTL with the TSI 8130 automated filter tester (TSI, Shoreview, MN, USA) that is utilized for NIOSH respirator certification testing. Results indicated pressure drops of 3.6 mm, 6.5 mm and 9.3 mm H₂O pressure for PR 3, PR6, and PR9, respectively, at 85 L/min constant airflow.. Subjects underwent standard OSHA respirator quantitative fit testing for each of the PRs with the TSI Portacount® Pro Respirator Fit Tester Model 8038 that counts the number of airborne particles in order to determine the ratio of ambient particles to within-respirator particles. The standard OSHA respirator quantitative fit test includes seven measured repetitive exercises of one minute duration each (i.e., normal breathing, deep breathing, head turn from side to side, bending head up and down, talking, bending over, normal breathing) and one 15 second exercise (grimace). Passing a fit test required a fit factor of ≥100, equivalent to a passing score on an OSHA quantitative respirator fit test and indicating that ≤1 in 100 particles was entering the FFR wearer’s breathing zone [14].

Subjects were instrumented with various sensors for continuous physiological monitoring during the exercise trials. The respiratory rate (RR) was monitored with the Zephyr Bioharness™ (Zephyr Technology Corporation, Annapolis, MD, US), an elasticized chest strap utilizing an proprietary embedded capacitive sensor to evaluate chest expansion and contraction [15,16]. The heart rate (HR), oxygen saturation (SpO₂) and transcutaneous carbon dioxide (tcPCO₂) were continuously monitored with the Tosca 500™ (Radiometer, Copenhagen, DM), an earlobe-mounted combination pulse oximeter and heated, Severinghaus-type CO₂ sensor [17,18]. Tympanic membrane (ear) temperatures (T_tymp) were measured immediately pre-and-post exercise trials with a Welch/Allyn Pro 4000 infrared tympanic thermometer (Braun GmbH, Kronberg, FRG). PR deadspace temperature (TV_D) and relative humidity (RHV_D) were continuously monitored with the iButton (Maxim, San Jose, CA, US), a small (16 mm x 6 mm) wireless sensor that was adhesively attached to the inner surface of the PRs midway between the respirator center and the edge of the right upper quadrant of the PR [19,20].

The study was carried out in a temperature and humidity controlled exercise physiology laboratory with mean (standard deviation) ambient conditions of: temperature 20.9 °C (1.3), relative humidity 23.5% (5.6), and barometric pressure 29.0 mm Hg (0.17). PRs were weighed pre-and-post testing on a calibrated Accu 6201 digital scale (Fisher Scientific, Pittsburgh, PA, US) to determine moisture retention. Immediately prior to each exercise study, measurement of some baseline pulmonary variables (i.e., tidal volume [V_T], exhalation minute ventilation [V_E], volume of carbon dioxide production [VCO₂], volume of oxygen utilization [VO₂]) was carried out utilizing a noseclip and tight-fitting mouthpiece connected to a Vmax Spectra respiratory analysis system (SensorMedics, Yorba Linda, CA, US) over a 5-minute period at sedentary (seated) activity. Subjects then donned a PR (PRs 3 and 9 were randomized; PR6 was not randomized because it was received from the manufacturer at a later date) according to the manufacturer’s instruction, performed positive and negative pressure user seal checks to assess the seal of the PR to the face [14], and walked on a treadmill at a low-moderate work rate (5.6 km/h) for 1 hour. Immediately upon completion of the exercise, and without interruption, the subject removed his/her PR and continued walking at the same rate on the treadmill for an additional 5 minutes while repeating measurements of the same pulmonary variables to approximate the impact, if any, of PR wear on pulmonary parameters. During the study trials, subjective measurements were taken immediately prior to the exercise period and then every 20 minutes using visual analog numerical scales for exertion (Borg Perceived Exertion Scale [a 15-grade scale ranging from “no exertion at all” to “maximal exertion]) [21], thermal comfort (Frank Comfort Scale [a 10-point scale ranging from “the coldest you have ever been” to “the hottest you have ever been”]) [22], and three 7-point respiratory scales for perceived inspiratory effort and perceived expiratory effort (ranging from “not noticeable” to “intolerable”) and overall breathing discomfort (ranging from “no discomfort” to “intolerable discomfort”) [23]. There was a minimum respite of 30 minutes between any consecutive exercise tests, during which subjects were seated and allowed to drink bottled water or a sports drink ad lib.

All subjects passed each individual component of the respirator quantitative fit test for each of the PRs. There were no significant differences between the three PRs with respect to the physiological variables HR (F=0.37, p=0.63), RR (F=1.97, p=0.18), SpO₂ (F=2.59, p=0.12), tcPCO₂ (F=1.11, p=0.34), T_tymp (F=0.97, p=0.39) and pulmonary variables VO₂ (F=0.78, p=0.47), VCO₂ (F=0.47, p=0.63), V_T (F=0.89, p=0.42), and V_E (F=0.45, p=0.64) (Table 1).

There were no significant differences between the three PRs in TV_D (F=0.05, p=0.94) and RHV_D (F=0.001, p=0.99) (Table 2), and mean moisture retention was ≤0.1 gram over one hour. No significant differences were noted between the three PRs in subjective scores for exertion (F=1.33, p=0.28), thermal comfort (F=1.08, p=0.33), inspiratory effort (F=0.008, p=0.96), expiratory effort (F=0.98, p=0.36), and overall breathing comfort (F=4.17, p=0.06) (Table 3).

Time had a significant effect on tcPCO₂ (F=3.57, p=0.02), inspiratory effort (F= 15.65, p=0.001), expiratory effort (F= 13.96, p=0.002), overall breathing comfort (F=9.47, p=<0.005), T_tymp (F=11.43, p=0.008), and all other physiological (HR [F= 30.19, p<0.01], RR [F=58.14, p<0.01]) and pulmonary function variables (VO₂ [F=245.97, p<0.01], VCO₂ [F=387.46, p<0.01] V_T [F=83.62, p<0.01], and V_E [F=125.51, p<0.01], save for SpO₂ (F=1.39, p=0.27), TV_D (F=0.05, p=0.94), and RHV_D (F=0.01, p=0.99). There was also a significant time effect on all subjective scores; exertion (F=48.75, p<0.01), thermal comfort (F=32.59, p<0.01), inspiratory effort (F=15.65, p<0.05), expiratory effort (F=13.96, p<0.05), and overall breathing comfort (F=9.47, p<0.05).

Cup-shaped FFR PRs with filter resistances of 3 mm, 6 mm and 9 mm H₂O pressure (measured at a constant airflow rate of 85 L/min), do not have significantly different impacts on physiological and subjective responses when worn for one hour at a low-moderate work rate (5.6 km/h). The level of detection of airway inspiratory resistance is 6 - 7.6 mm H₂O/L/sec [36–40], so that cup-shaped FFRs with filter resistances in this range might not be perceived as significantly more comfortable than standard low-resistance FFRs with minimally higher filter resistance; however, given the small sample size of the current study (n=10), additional testing is warranted. Although many users of modern, low-resistance FFRs continue to complain of breathing difficulty [10–12], this may be related to numerous other inputs (e.g., warmth, anxiety/claustrophobia, alteration of breathing pattern from nasal to oronasal, etc.) such that efforts to decrease cup-shaped FFR filter resistance below 9 mm (measured at a constant airflow rate of 85 L/min) may prove of little benefit in terms of FFR breathing comfort and tolerance at low-moderate work rates.