Elsevier

Journal of Aerosol Science

Volume 79, January 2015, Pages 61-71
Journal of Aerosol Science

Airflow resistance and bio-filtering performance of carbon nanotube filters and current facepiece respirators

https://doi.org/10.1016/j.jaerosci.2014.10.003Get rights and content

Highlights

  • CNT loading significantly improved the aerosol filtering performances of the base filter.

  • CNT filter׳s QF at 0.2 mg/cm2 was higher than those of surgical and activated carbon masks.

  • CNT filters were observed to improve breathability over some available facepiece respirators.

Abstract

This study aimed to study airflow resistance of carbon nanotube (CNT)-based filtration material and compare its bioaerosol filtering performances with commercially available facepiece respirators. Firstly, we developed a manikin-based system to evaluate the filtering performances of six respiratory masks for biological aerosol particles using an Ultraviolet Aerodynamic Particle Sizer (UV-APS). Secondly, CNT filters with a base filter with a pore size of 10 μm were prepared using single-walled carbon nanotube (SWNT) loadings of 0.05, 0.1 and 0.2 mg/cm2. In addition, mask filter membranes were also made by manually cutting the materials of the tested respiratory masks, and their filtration efficiencies and pressure drops were measured and compared with those of CNT-based filters.

Results indicated that most of the studied respirator masks achieved a practical protection efficiency of >90%, while N95 types obtained more than 99% absolute protection efficiency under fully sealed conditions. Increasing CNT loading was shown to increase the quality factor (QF) for the CNT filter given all the sizes considered. For the loading of 0.2 mg/cm2, CNT filters achieved 87% filtration efficiency against indoor viable bioaerosols and 70% for outdoor aerosol particles; and its QF was significantly higher than those of base filter, activated carbon and surgical mask filters. Different from other mask filters, CNT filters were observed to have higher biological aerosol particle filtration efficiencies than the total aerosol particles. Future work in improving air permeability of CNT filter and CNT distribution on filter support would lead to next generation respiratory mask.

Introduction

Airborne particulate matter (PM), consisting of both biological and non-biological materials, when inhaled can cause respiratory, cardiovascular and lung diseases (Kampa and Castanas, 2008, Dockery, 2009, Raaschou-Nielsen et al., 2013). Recently, an association between a decrease of 10 µg/m3 in the concentration of PM2.5 and an increase in mean life expectancy of 0.35 years was detected (Correia et al., 2013). With the rapid progress of modernization, atmospheric pollution has become one of the serious environmental problems in China (Chan and Yao, 2008, Kan et al., 2012, Matus et al., 2012) and in the rest of the world. For example, in many parts of China, air quality has been characterized by heavy haze with increasing frequency and duration in recent years (Liu et al., 2013, Kang et al., 2013, Zhang et al., 2008). During haze days, PM concentrations can increase to levels much higher than usual. For example, 220 μg/m3 mass concentration of PM2.5 was reported in a study investigating regional haze in Beijing (Liu et al., 2013). Another study showed that increasing PM concentration also resulted in increased abundances of allergenic and pathogenic materials (Cao et al., 2014). Recently, dust collected by an automobile air conditioner filter in Beijing was found to harbor significant amounts of bacteria, fungi and endotoxin, which was shown to be re-aerosolized into the automobile cabin upon use of the air conditioner (Li et al., 2013). On another front, healthcare providers, e.g., in a respiratory clinic, and the public also face increased threats of pathogenic aerosol exposure arising from frequent outbreaks of infectious diseases such as SARS in 2003, H1N1 in 2009 and H7N9 in 2013 (Severe Acute Respiratory Syndrome (SARS),, World Health Organization, 2009, Gao et al., 2013). Correspondingly, there is a great interest for developing better personal inhalation exposure protection measures.

Among many approaches, filtering facepiece respirators (FFRs) are widely used to prevent inhalation of harmful substances from the air (Rengasamy et al., 2004). Wearing a respiratory mask is considered to be one of the most affordable and effective methods to reduce exposure to airborne pollutants. For various commercially available respirators, N95 FFRs, surgical mask and activated carbon mask are widely studied in the literature (Grinshpun et al., 2009, Loeb et al., 2009). N95 FFRs, certified by National Institute for Occupational Safety and Health (NIOSH), are described to achieve more than 95% filtration efficiency for 0.3 μm particles under certain test conditions (National Institute for Occupational Safety and Health, 1997). Qian et al. (1998) found that N95 respirators can achieve at least 95% protection against airborne particles in the absence of face leakage. However, relevant results varied greatly under different experimental conditions. When treated with airborne viruses and nanoparticles, the penetration of N95 FFRs can exceed 5% (Bałazy et al., 2006a, BaŁazy et al., 2006b). Lee et al. (2008) also showed that N95 masks may not offer expected protection against bacteria and viruses. On the other hand, surgical masks are widely used in the hospital and operating rooms for protecting wounds from infection (Lipp & Edwards, 2012), and their protective efficiency against airborne microbes attracts much attention. To further enhance the performance of biological protection, some novel masks were designed, e.g., an anti-influenza face mask containing copper oxide could filter above 99.85% of aerosolized influenza viruses (Borkow et al., 2010).

Despite the practicality of these respiratory masks, they often fall short of fully protecting healthcare professionals as well as the civilians from the pathogenic infection due to improper wearing and/or facial leakage. For example, a randomized trial for the nurses in Ontario tertiary care hospitals found that Influenza infection occurred in 23.6% participants wearing surgical masks and 22.9% in N95 respirator wearing group due to face leakage (Loeb et al., 2009). Nonetheless, surgical masks were found to have equal protection rates with N95 masks (Loeb et al., 2009). In addition to the failure of achieving desired protection efficiency, breathing resistance increases the wearing discomfort such as ‘heat’ and water moisture inside the mask. As a result of the breathing discomfort of those masks, people are often reluctant to wear them for a sustained time period, thus increasing the risks of respiratory infection especially during an infectious disease outbreak or a flu season. The pressure drop of a filtering material is closely related to the breathing resistance. A recent study indicated that the pressure differential (ΔP) was significantly greater across the N95 respirator compared with all face masks tested (Skaria & Smaldone, 2014). Arising from the difference of breathing resistance, lower heart rate was observed among the group wearing surgical masks than those wearing N95 masks (Li et al., 2005). In addition, it was also shown that N95 FFR dead-space has higher carbon dioxide and lower oxygen levels than the ambient workplace standards, respectively (Roberge et al., 2010). In developing more comfortable respiratory masks, an exhalation valve on the N95 respirator is being adopted and is shown not to affect the respiratory protection, while contributing to reducing the breathing resistance (Lee et al., 2008). In another work, a nanofiber mask was evaluated and it was found to have lower breathing resistance, even when sealed to the face (Skaria & Smaldone, 2014). Overall, higher protection efficiency often comes at the price of losing breathing comfort, and better respiratory filtration material is certainly needed.

As one of the most studied nanomaterials (Harris, 2009, Popov, 2004), carbon nanotube (CNT) is shown to have a gas permeability of several orders of magnitude higher than current available filtration materials (Cooper et al., 2003, Viswanathan et al., 2004). Over the past decade, a significant number of studies were conducted to prepare CNT-based filters (Park and Lee, 2006a, Park and Lee, 2006b, Karwa et al., 2012) and also water-borne inactivation and removal of biological agents such as bacteria and viruses by carbon nanotubes (Arias and Yang, 2009, Vecitis et al., 2011). It was found that direct cell contact with single-walled carbon nanotubes (SWNTs) can lead to severe membrane damage and subsequent cell inactivation (Kang et al., 2007). Vecitis et al. (2011) prepared multi-walled carbon nanotube (MWNT) filter by depositing MWNTs onto a 5 μm PTFE membrane, and then used the filter to remove and inactivate viruses (MS2) and bacteria (Escherichia coli) in the water. Results showed that without electrolysis, the MWNT filter was effective in completely removing bacteria by sieving and multi-log removal of viruses by depth-filtration (Vecitis et al., 2011). Brady‐Estévez et al. (2008) used SWNT-based filters with different concentrations ranging from 0.3 to 0.8 mg/cm2 in water purification, and found that the SWNT filter exhibited high antibacterial and antivirus activity. Compared with applications in water treatment, research of CNT applied in gas-phase filtration is scarce. Viswanathan et al. (2004) presented a filter media by depositing MWNTs onto cellulose fiber filters to filter airborne particulate matters; the results indicated that the prepared MWNT filters achieved over 99.9% filter efficiency for 0.3 μm particles even for low MWNT coverage (0.07 mg/cm2). Most recently, CNT-based filters were used to remove biological aerosols (Guan and Yao, 2010, Xu and Yao, 2011, Park et al., 2011). The results have shown that use of 0.64 mg/cm2 CNT can achieve 1–2 log inactivation of biological aerosols (Xu & Yao, 2011). CNT-based filtration material with outstanding gas permeability stands the great potential to revolutionize current facepiece respirators.

This study was designed to develop a novel CNT-based facepiece respirator filtration material and further compare its pressure drop and filtration efficiency of biological aerosols with other commercially available masks, including N95 FFRs, surgical, doctor and activated carbon masks. Here, a manikin-based system was used to test the practical protection efficiencies of various FFRs and CNT-based filtration material using an Ultraviolet Aerodynamic Particle Sizer (UV-APS). The results from this work provide important guidance in developing future respiratory mask filtration material.

Section snippets

Performance evaluation of commercially available FFRs

In this work, a manikin-based system was developed as shown in Fig. 1 for evaluating the protection efficiencies of various commercially available respiratory masks shown in Fig. S1 (Supporting information) against biological aerosols both in indoor and outdoor environments. Representative total particle size distributions in these two environments are presented in Fig. S2 (Supporting information). Typically, higher particle concentration levels were observed outdoors than indoors. In this

Statistical analysis

One-way ANOVA analysis was used to analyze the differences in protection efficiencies under different conditions, such as different air flow rates and respiratory mask models. A p-value of 0.05 indicates a statistically significant difference.

Protective performance of commercially available masks

In this part of study, we studied the protective efficiencies of various commercially available masks against airborne particles especially biological ones. Firstly, N95 masks (three different models), surgical, doctor and A.C. masks were fully sealed to the manikin and tested under three different air flow rates (12.5, 35, 85 L/min). Under the fully sealed conditions (no air leakage), the efficiency was treated as absolute protection efficiency (EAP). As observed in Fig. 2, all the tested N95

Conclusions

In this work, we utilized UV-APS to real-time monitor the performances of CNT filters and commercial FFRs filtering against biological aerosol particles, which has not been done in the literature. Under fully sealed condition, N95 FFRs were shown to achieve a very high filtration efficiency (>95%) for airborne particulates at the whole size range of 0.5–20 μm. Except for the activated carbon mask, all the other respirators tested were shown to have at least 80% practical protection efficiencies.

Acknowledgments

This study was supported by MOE grant (20130001110044), the NSF of China grants (41121004, 21077005, 21477003) and MOST grant (2015CB553401).

References (57)

  • S.J. Park et al.

    Development of CNT-metal-filters by direct growth of carbon nanotubes

    Current Applied Physics

    (2006)
  • S.J. Park et al.

    Performance improvement of micron-sized fibrous metal filters by direct growth of carbon nanotubes

    Carbon

    (2006)
  • O. Raaschou-Nielsen et al.

    Air pollution and lung cancer incidence in 17 European cohorts: prospective analyses from the European Study of Cohorts for Air Pollution Effects (ESCAPE)

    The Lancet Oncology

    (2013)
  • A. Rengasamy et al.

    Respiratory protection against bioaerosols: literature review and research needs

    American Journal of Infection Control

    (2004)
  • A. Weber et al.

    Aerosol penetration and leakage characteristics of masks used in the health care industry

    American Journal of Infection Control

    (1993)
  • Z. Xu et al.

    Effects of single-walled carbon nanotube filter on culturability and diversity of environmental bioaerosols

    Journal of Aerosol Science

    (2011)
  • Y.H. Zhang et al.

    Regional integrated experiments on air quality over Pearl River Delta 2004 (PRIDE-PRD2004): overview

    Atmospheric Environment

    (2008)
  • L.R. Arias et al.

    Inactivation of bacterial pathogens by carbon nanotubes in suspensions

    Langmuir

    (2009)
  • A. BaŁazy et al.

    Manikin-based performance evaluation of N95 filtering-facepiece respirators challenged with nanoparticles

    Annals of Occupational Hygiene

    (2006)
  • G. Borkow et al.

    A novel anti-influenza copper oxide containing respiratory face mask

    PloS One

    (2010)
  • A.S. Brady‐Estévez et al.

    A single‐walled‐carbon‐nanotube filter for removal of viral and bacterial pathogens

    Small

    (2008)
  • C. Cao et al.

    Inhalable microorganisms in Beijing”s PM2. 5 and PM10 pollutants during a severe smog event

    Environmental Science & Technology

    (2014)
  • C.C. Chen et al.

    Filter and leak penetration characteristics of a dust and mist filtering facepiece

    The American Industrial Hygiene Association Journal

    (1990)
  • C.C. Chen et al.

    Characteristics of face seal leakage in filtering facepieces

    The American Industrial Hygiene Association Journal

    (1992)
  • K.J. Cho et al.

    Large particle penetration through N95 respirator filters and facepiece leaks with cyclic flow

    Annals of Occupational Hygiene

    (2010)
  • S.M. Cooper et al.

    Gas permeability of a buckypaper membrane

    Nano Letters

    (2003)
  • A.W. Correia et al.

    Effect of air pollution control on life expectancy in the United States: an analysis of 545 US counties for the period from 2000 to 2007

    Epidemiology (Cambridge, MA)

    (2013)
  • B.J. Cowling et al.

    Face masks to prevent transmission of influenza virus: a systematic review

    Epidemiology and Infection

    (2010)
  • Cited by (31)

    • Pump-inject antimicrobial and biodegradable aerogel as mask intermediate filter layer for medical protection of air filtration

      2022, Materials Today Sustainability
      Citation Excerpt :

      In recent years, PM2.5 particles and infectious bio-aerosols have caused a serious threat to human health [1–3]. Wearing a mask has been believed to be one of the most cost-effective methods to prevent air pollutants from invading the human body [4,5]. Especially during the COVID-19 pandemic, masks becomes a daily necessity, thanks to the insufficient production capacity of the melt-blown fabric materials, resulting in the temporary shortage of masks in the market [6].

    • Air quality changes in cities during the COVID-19 lockdown: A critical review

      2021, Atmospheric Research
      Citation Excerpt :

      To effectively minimize the exposure dose, wearing face masks is necessary, especially when staying or entering a closed environment with a high possibility of SARS-CoV-2 accumulation and transmission (Kohanski et al., 2020; Lelieveld et al., 2020; Yao et al., 2020a). However, different face masks have different protection efficiencies and degrees of fitness (Yao et al., 2020a; Zou and Yao, 2015). Several previous studies have shown that N95 and surgical masks have effectively reduced the wearer’s exposure and community transmission of coronavirus and influenza viruses (Jefferson et al., 2008; Leung et al., 2020; MacIntyre et al., 2017).

    • Biodegradable and multifunctional surgical face masks: A brief review on demands during COVID-19 pandemic, recent developments, and future perspectives

      2021, Science of the Total Environment
      Citation Excerpt :

      Face masks that contained activated carbon are reported to protect the wearer from vaporized anticancer drug inhalation, thanks to their high adsorptive capacity (Sato et al., 2016). Carbon nanotubes (CNT)-based filters have shown higher biological aerosol PFE versus their total aerosol PFE (Zou and Yao, 2015). Graphical results of some studies on multifunctional structures for face masks are shown in Fig. 5.

    • Electrospun nanofibers for personal protection in mines

      2021, Chemical Engineering Journal
      Citation Excerpt :

      The filtration efficiency is almost inversely proportional to respiratory resistance. The better the filtration efficiency, the greater the respiratory resistance [85]. For example, N95 mask has the best filtration performance, but the longest continuous wearing time cannot exceed 4 h.

    View all citing articles on Scopus
    View full text