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There are many different shapes of human nose according to the width of the nostrils, the distance between nostrils, the height of the nose, nose ridge length, nose protrusion, external area of the nose, and the area of the nostrils. Also, each race has different nasal morphologies which result in variations of nasal airflow patterns and nasal functions (Zhu et al., 2011). Therefore, the design of human nose is not simple. Moreover, no information of nasal hair density was reported. Thus, in the present study, the real human hairs (Hairdyrect/Japan Co.) that are commercially available for the purpose of increasing hair density were substituted. The commercial human hairs with the 95 μm thickness and 2 hairs 1 mm^-2 density were attached onto the inside of the artificial nostril designed in this study (see Fig. 1). The number of total nasal hair at both nostrils (expressed in terms of anatomy as the nasal valve area) estimated from the hair density and surface area of the designed nostril were 4,522. Unlike the reconstructed artificial nasal valves shown in Fig. 1, the actual nostril (i.e., nasal valve) is a highly complex structure. The internal nasal valve is the narrowest part of the airway in the middle third of the cavity. The larger external nasal valve is located in the alar wall (Becker and Becker, 2003). Thus, it is very difficult to reconstruct a more realistic anatomical structure. The reconstructed artificial nose (typically found in Asian (average the noses of adult male and female) morphologies (Becker and Becker, 2003)) was finally affixed to a part of the face model of the rescue training mannequins (see Fig. 1).

Fig. 1. 
The reconstructed artificial nasal valves (top) and its setting up to a part of face model (bottom).

An experimental setup was designed in a closed experimental room (W2.5×L3.0×H2.4 m) at the Fukuoka Women’s University in Japan. Fig. 2 shows the schematic diagram for laboratory experiment. The experimental setup consists of the reconstructed artificial nose, two optical particle counters (HHPC3+, HACH Co.), and an artificial respirator (ARF-900EII, Acoma Co.). In order to remove the dust material, the inner artificial nose was cleaned every time using a ultrasonicator before each experiment.

Fig. 2. 
Schematic diagram for laboratory experiment.

Because large particles entering the nose are collected by nasal hair present in the anterior nares (Becquemin et al., 1991), in this study, two types of particle size (i.e., 2-5 μm and gt;5 μm) were the target of the estimation. To assess the particle blocking efficiency for the real indoor particles, the actual particles present in a closed experimental room were applied instead of the artificially generate particles. The number size distribution of background particles at our closed experimental room was varied from 99 to 218 L^-1 and 29 to 59 L^-1 at a diameter of 2-5 μm and gt;5 μm, respectively. According to three different types of breathing (i.e., rest, ordinary, and exercise), the respiration volume for once (Zuurbier et al., 2009), inhalation flow rate, and respiration time varied as shown in Table 1.

For the purpose of observing the morphology of the particles attached onto the nasal hairs, a SEM ( JEOL JSM-5400) equipped with an EDX (Philips, EDAX DX-4) was employed. Some nasal hairs were placed inside the SEM’s vacuum column (10^-6 Torr) through an airtight door. Particles were also distinguished and countered under 3,000 magnification and 15-20 kV working conditions.

Fig. 3 shows the SEM images of the particles attached onto an artificial nasal hair after laboratory experiment. As shown in Fig. 3, the nasal hair’s particle capturing was visually confirmed. In the present study, taking into consideration the particle deposition onto an artificial respirator and particle density changes in the experimental room, we set a time limit of 10 minutes for the total test breathing. During experiment (10 minutes), no meaningful particle deposition at the inner artificial respirator was found.

Fig. 3. 
SEM images of the particles attached onto an artificial nasal hair.

Table 1 summarizes the variation of particle number concentration for coarse particles of two different sizes (i.e., 2-5 μm and gt;5 μm) before and after breathing as a function of respiration pattern. After having completed in breathing, particle number concentration at both sizes showed significantly lower levels. Particle blocking efficiency (E_pb (%)) of nasal hairs can be evaluated simply by measuring the number of inspired (OPC-I) and expired (OPC-II) particles in each breath. The E_pb (%) of nasal hairs for particles of 2-5 μm and gt;5 μm varied from 43.14 to 44.88% and 51.60 to 56.83%, respectively. This result clearly indicates that nasal hair more effectively blocks the giant particles larger than 5 μm (such as soil particle and pollen), regardless of breathing pattern. High particle blocking feature for the giant particles was the expected result. However, it should be noted that while no significant change of E_pb (%) was shown at the particles of 2-5 μm, there was a continuously increasing at the particles larger than 5 μm in accordance with the increasing of respiration volume and time.

The fact that E_pb (%) of nasal hair for middle size particles was unchanged even though during exercise indicates that the considerable amount fine particles are easily inhaled into the deeper respiratory tract during exercise. For this reason, we must avoid a violent exercise during the episodically high PM_2.5 days.

It is suspicious point that after particles hit our nasal hairs, do they stay there? Or do they eventually get sucked in as we breathe more?

Table 2 shows the E_pb (%) of nasal hairs for two types of particle size according to breathing duration. In the case of rest and ordinary breathing patterns, the particles of 2-5 μm has no significant variation of E_pb (%) during the 9 minutes initial respiration. It varied from 42.26 to 43.88% and 41.43 to 45.07% at rest and ordinary breathing patterns, respectively. Meanwhile, at the same breathing pattern, the giant particles marked a relatively high E_pb (%) with from 47.10 to 57.16% without no continuous fluctuation.

On the other hand, the highest E_pb (%) (53.33 and 67.83% at particles of 2-5 μm and gt;5 μm, respectively) was shown by three minutes after breathing with exercise mode. The more interesting thing is that the E_pb (%) was rapidly decreased to a low level according to respiration duration. In particular, the highest E_pb (%) (67.83%) of the particles larger than 5 μm was marked at the first three minutes during exercise mode. This result might be caused by the inertial impaction. Inertial impaction affects mainly particles that are larger than 5 μm and refers to the particle’s inability to follow sudden change in air flow direction. Here, the sudden change of air flow direction is the turning of aerodynamic flow caused nasal hairs facing the entering air flow. The deviation of the particle from the air streamline increases its probability to deposit on nasal hair (Darquenne, 2012).

Particles with both sizes showed a continuous decrease in E_pb (%) with increasing breathing duration with exercise respiration pattern. One of the reasons of this result might be thought that after capturing, the particles gently attached on nasal hairs make their way to the inner wall of nasal passages by the constant breathing.

In general, during exercise, the mouth is more or less continuously open (or oral breathing totally replaces nasal breathing). This means that exercise aggravates the accumulation of particles (at least both middle and large sizes) to higher concentrations in our body even if we breathe through our nose.

It is thought that nasal hair thickness varies with respect to age, sex, and the race. Also, it is expected that nasal hair thickness is crucially important for particle blocking because it plays an important role in capturing particles. In order to make clear the particle cut-off efficiency with respect to nasal hair thickness, a theoretical calculation was performed. Fig. 4 shows how does the nasal hair block particles. Three particle cut-off efficiencies (E), i.e., E by Brownian diffusion (E_dif.), E by interception (E_int.), and E by inertial impaction processes (E_imp.) are fundamental mechanisms for the particle adhesion onto the nasal hair (Becquemin et al., 1991; Gerrity et al., 1979). A more advanced equation for particle cut-off efficiency (E_integ.) of nasal hairs that integrated three kinds efficiencies as follow:

Fig. 4. 
Fundamentals of particle adhesion onto the nasal hair.

In this study, the particle cut-off efficiency by inspiration for three kinds of nasal hair thickness (i.e., 50, 75, and 100 μm) was theoretically computed using this integrated E_integ. Among the important factors for the integrated E_integ., the Renolds number of nasal hairs based on its radius, the Schmidt number of captured particles, the Stokes number of captured particles, the Cunningham slip factor, the particle density, and the Critical stokes number are considered. More details for calculation like variable settings and computation processes were already described in the related articles (Ma et al., 2005; Ma et al., 2004).

Fig. 5 shows the E_integ. of nasal hairs according to particle size and nasal hair thickness. As one of outstanding features appeared in the E_integ. curves, the thinner nasal hair can block particles more efficiently through a whole particle size range. Another peculiarity for the E_integ. curves is that it is significantly greater for both nano size and giant size particles. For the particle larger than 2.0 μm, the E_integ. increases with increasing particle size because both inertial impaction and interception work more strongly. Meanwhile, for the particles smaller than 0.02 μm in diameter, the E_integ. increases with decreasing particle size because of increased Brownian diffusion. It was also found that no meaningful the E_integ. at particle size from 0.004 to 0.4 μm for all thickness ranges of nasal hair. This result might be caused by the fact that none of the main mechanisms (i.e., three-kind mechanisms mentioned above) were effective for particle cut-off in this particle size range.

Fig. 5. 
Particle cut-off efficiency of nasal hairs by inspiration according to particle size and nasal hair thickness.

Although this study estimated the E_integ. of nasal hairs by only inspiration, according to Wiesmiller et al. (2003), the particle deposition fraction in the anterior nasal segment was significantly higher during nose-only breathing (46.0%) compared with nose-in/mouth-out breathing (33.0%). Their study suggested that for inspiration as well as expiration the nasal particle blocking is very efficient and inspiratory efficiency is higher than expiratory.