In this post, we update our knowledge on inhalation of cellulose nanocrystal (CNC) and cellulose nano/micro fibril (CNF/CMF) dusts and discuss best practices for working safely with them.  

(i) Overview  

At the 2016 TAPPI Conference on Nanotechnology for Renewable Materials, health and safety of cellulose nanomaterials (CNs) were popular topics. We already know that the inhalation of poorly soluble dusts like cellulose/wood dust can be hazardous in large doses, or over long periods of time. Discussions with experts such as Chuck Geraci of the U.S. National Institute for Occupational Safety and Health (NIOSH) raised questions about whether CNs, the smaller form of cellulose dust, pose any new risks or should be handled differently than conventional methods used for poorly soluble dusts. To continue to be proactive on the safety of CNs, we discuss our current understanding of CN hazards by inhalation, and identify resources for the CN community to ensure we are all working safely with them. As with all new substances, avoiding and minimizing exposure and demonstrating safety through well-designed studies is important for safe commercialization.   

Cellulose is an abundant, natural, and renewable material that comes from plants and is used to make paper products and textiles, as a filler in medicine tablets, and is a key source of fiber in foods. When it is broken down into even smaller pieces, called cellulose nanomaterials (CNs), it becomes lighter, stiffer, and stronger, and therefore they can be used to improve existing products, or even replace harmful additives. Development of these materials is rapidly advancing and companies are moving toward large-scale commercialization. 

In 2015, we developed a roadmap of the health and environmental safety of CNs(1), where we reviewed and analyzed published studies to see if they provide new insights about the effects of working with powdered CNs and possible health effects that might occur when you breathe them in. In this review of studies published since then, the short answer is, the new studies don’t answer two key questions:  

1. Are cellulose nanomaterials more hazardous than conventional cellulose or poorly soluble low toxicity (PSLT) dusts? and,  
2. Are there special precautions to be taken when working with CNs (such as wearing respiratory protection, or applying a lower occupational exposure limit)?  

For a longer explanation, read on. 

(ii) What Do We Know about the Safety of CN in Dust?  

All dusts have the potential to irritate the lung when inhaled, including cellulose. When you breathe in dust at high enough levels, it can negatively affect your lungs and lung function, including triggering an immune system response. Dusts can also irritate the skin and eyes, and can accumulate and cause problems in the airways if exposures occur over time. We do not yet know whether CNs behave differently than conventional cellulose or may be similar to poorly soluble, low toxicity (PSLT) dusts in the lung. CNC and CNF must be in powder form, at a respirable size (< 10 micron particles), and suspended in the air for them to enter lungs in the first place if inhaled. If handled in solution or in a composite material, there is only a very low risk of exposure through inhalation, for example by sanding or grinding the composite. To be proactive, as with PSLT dusts, those working with CNs in powder form should be protected from inhalation exposures.  

Vireo recently reviewed and analyzed 10 studies(2-11) that have investigated the hazards of inhaled CNs. These studies need to be considered in a realistic context and it should be highlighted that they tell us little about the safety of CNs at occupational levels of exposure for several reasons:  

1. Doses used. Toxicology studies are often run at high dose levels to ensure an effect can be observed, where researchers can learn more about the mechanisms of toxic action.  
2. Exposures. The animal studies generally used a single high dose or were very short in duration. This does not allow us to assess whether effects can occur at low levels over a longer period of time, similar to workplace exposures. Two of the studies were short and ended after one day, before any short-term effects from the initial exposure could resolve, so it is not possible to determine if the lung inflammation observed was transitory or persistent over the long term.   
3. Experimental setup. While there were several studies in mice, most did not include a conventional material, such as cellulose dust, so we can’t conclude if the CNs behave any differently or with greater potency compared to PSLT dusts, including cellulose, that we already know cause inflammation when inhaled at high doses.  

Further, to date, only one study has been reported that exposed animals to an aerosol, as could be experienced in the workplace(3), and no adverse effects were seen in that study. The other studies, where some inflammation and other immune responses were observed, employed a technique called pharyngeal aspiration to deliver high bolus (i.e. all at once) doses of CNs to mouse lungs. This method does not realistically mimic the inhalation of CNs that could occur in an occupational setting, because the entire dose occurs in a moment, not over time. Our analysis of one of these papers(12), published in Particle and Fibre Toxicology, suggests that simulating these high doses by inhalation would require unrealistic workplace exposure concentrations in the gram per cubic meter (g/m^3) range (more than 1000 times the occupational exposure limit for respirable cellulose dust of 5 mg/m^3). In comparison, two of the studies(3,9), one in animals and another in non-animal model, where exposures conditions and doses were more realistic conditions, no negative effects were noted. 

While the analysis was too technical for discussion here, the design and weight of evidence from the studies did not allow evaluation of exposure dose-response relationships for the tested materials, meaning it is not known whether, or at what concentrations, these materials might cause lung inflammation at realistic exposure levels. What Vireo concluded from the review of studies is that when exposed at high doses, some of these materials may cause at least short term injury and impair lung function, but further investigations under more realistic conditions are required. This is similar to a variety of other PSLT dusts, as well as conventional cellulose, where inhalation of very high doses is associated with at least short term lung impairment and inflammation.   

Additional studies needed to allow assessment of the potential health risks of occupational exposures forward includes: 

1. Better techniques to detect CNs in different media (e.g. air, water, biological fluids). It is challenging to measure occupational exposure to CNs due to their composition: an organic carbohydrate material. This makes it difficult to identify and measure the material, often present at very low levels, and to distinguish it from background sources of particles in the atmosphere.    
2. More exposure assessments in actual facilities. Several (unpublished) investigations indicate exposure levels are low in CN production environments. Studies by NIOSH in pilot CN facilities have not measured high levels of particles, and other studies have shown that common equipment such as fume hoods can effectively remove a large proportion of airborne CNs(13). 
3. Studies that mimic realistic exposure scenarios. This includes using realistic exposure models (inhalation experiments instead of pharyngeal aspiration) that examine a range of realistic doses (show a dose-response curve) for different lengths of time, long enough to assess whether there is impairment when exposure occurs over time. Control materials need to be included that compare CNs to conventional cellulose to determine if there is any unique hazard associated with CNs or if they behave similar to PSLT dusts. The use of positive and negative controls will also allow comparisons across materials and studies.  These types of studies can be expensive, but are needed to assess differences between CNs and conventional cellulose and other PSLT dusts.  

(iii) Being Proactive: How to Avoid Exposure    

Until better information is available, the most prudent approach is to avoid inhalation exposure to dusts. Many PSLT dusts can affect the lungs, even below current occupational exposure limits (OELs). Currently, the U.S. Occupational Safety and Health Administration (OSHA) permissible exposure limit (PEL) for conventional celluloses is 15 mg/m^3 for total dust and 5 mg/m^3 for respirable dust. Many exposure limits were created a long time ago, and many organizations and occupational health professionals advocate for lower exposure limits based on growing data showing effects associated with chronic exposure to dusts. For example, authors report that Britain’s airborne exposure limits for dusts have not changed for over 30 years(14), and enough evidence is mounting that current exposure limits should be changed to 1 mg/m^3.  

To date, NIOSH has not observed respirable dust levels above 0.5 mg/m^3 in CN production facilities in their field investigations, even for a short time. Furthermore, studies looking at the release of CNs during pilot scale production activities that might generate dust (e.g. centrifugation, friction grinding, spray drying) have concluded that total particle concentrations measured in the air were not significantly elevated over background, and sometimes, were lower, perhaps due to the improved network of fibers with CNs in paper. However, these studies were performed on small scale operations and therefore more work needs to be done, especially in full-scale production plants. As mentioned, it is difficult to measure CNs, and specialized equipment is needed to accurately do so. 

That said, it is not clear from the studies whether more stringent protections are needed for CNs so the safest course of action is to prevent or minimize the potential for exposure. This can be achieved by (1) ensuring any dried material is not respirable, either by processing the material to be larger than 10 microns in size, or by working with material in solution; and (2) through industrial hygiene surveys and the use of engineering controls (e.g. ventilation systems or process specific enclosures) and personal protective equipment. The consensus is that if engineering safeguards and personal protective equipment are used properly, then worker exposure will be limited(15). 

Over the past decade or so, many governmental and private organizations have collaborated to develop methods, data sets, best practices and guidance documents to help workers, owners and managers minimize exposure when working with nanomaterials in the workplace. The TAPPI Nanotechnology Division Website includes a compilation of these resources.  

To move forward, the critical needs are to:   

1. Have a better understanding of safe airborne concentrations of CNs especially those resulting in low doses over the long term;  
2. Understand how CNs act in comparison to other conventional dusts;  
3. Ensure that current workplace controls and personal protection equipment is adequate for different sizes and surface modifications of CNs;  
4. Develop affordable and straightforward standardized methods to be able to detect CNs in the workplace, especially in the air. 

(iv) Efforts by P3Nano on the Safe and Responsible Commercialization of CNs  

Safe workplace practices are critical to successful commercialization of all technologies.  Vireo Advisors and the partners of the public private partnership to advance commercialization of cellulose nanomaterials (P3Nano) are collectively committed to the safe and responsible commercialization of CNs. P3Nano is working with partners to establish guidance for working safely with CNs, including communication tools and methods to detect and quantify workplace exposures. This is why Vireo actively engaged in updating readers on the latest research on nanomaterial safety, including our collaboration with the eminent Günter Oberdörster, Professor Emeritus of the University of Rochester. 

P3Nano, founded by the USDA and the US Endowment for Forestry and Communities, along with their partners have spearheaded several unique efforts and are committed to being proactive about safety in advance of the commercialization of cellulose nanomaterials. P3Nano has a Memorandum of Understanding with NIOSH to advance knowledge and methods. Overall, industry partners have shown tremendous support for the safe development of these materials, and the development of safety data to inform commercialization.  

Vireo’s efforts have so far included:  

1. Creating Safety Data Sheets for CNC/CNF and identification of knowledge gaps in terms of health, safety and the environment;(16)  
2. Publishing a life cycle risk assessment and environmental health and safety roadmap for CNCs;(1)
3. Developing methods to detect CNCs and CNFs in the workplace and using these to measure exposure levels during manufacture; 
4. Collaborating with NIOSH to develop data and guidance on workplace safety to limit/prevent exposure to CN dusts during the manufacture of CNs; 
5. Outreach to industry and governmental organizations to advance the development of safety data for CNs.  

References:  

1. Shatkin and Kim, 2015. Cellulose nanomaterials: life cycle risk assessment, and environmental health and safety roadmap. Environmental Science: Nano, 2(5), 477-499. 

2. Yanamala et al., 2014. In vivo evaluation of the pulmonary toxicity of cellulose nanocrystals: a renewable and sustainable nanomaterial of the future. ACS Sustainable Chemistry & Engineering, 2(7), 1691-1698. 

3. O’Connor et al., 2014. Commercialization of cellulose nanocrystal (NCC™) production: A business case focusing on the importance of proactive EHS management. Nanotechnology Environmental Health & Safety: Risks, Regulation, and Management, 225-246. 

4. Farcas et al., 2016. Pulmonary exposure to cellulose nanocrystals caused deleterious effects to reproductive system in male mice. Journal of Toxicology and Environmental Health, Part A, 79(21), 984-997. 

5. Shvedova et al., 2016. Gender differences in murine pulmonary responses elicited by cellulose nanocrystals. Particle and Fibre Toxicology, 13(1), 28. 

6. Catalán et al., 2017. Genotoxic and inflammatory effects of nanofibrillated cellulose in murine lungs. Mutagenesis, 32(1), 23-31. 

7. Yanamala et al., 2016. In vitro toxicity evaluation of lignin-(un) coated cellulose based nanomaterials on human A549 and THP-1 cells. Biomacromolecules, 17(11), 3464-3473. 

8. Clift et al., 2011. Investigating the interaction of cellulose nanofibers derived from cotton with a sophisticated 3D human lung cell coculture. Biomacromolecules, 12(10), 3666-3673. 

9. Endes et al., 2014. An in vitro testing strategy towards mimicking the inhalation of high aspect ratio nanoparticles. Particle and Fibre Toxicology, 11(1), 40. 

10. Menas et al., 2017. Fibrillar vs crystalline nanocellulose pulmonary epithelial cell responses: Cytotoxicity or inflammation? Chemosphere, 171, 671-680. 

11. Lopes et al., 2017. In vitro biological responses to nanofibrillated cellulose by human dermal, lung and immune cells: surface chemistry aspect. Particle and Fibre Toxicology, 14(1), 1 

12. Shatkin and Oberdörster, 2016. Comment on Shvedova et al. (2016), “Gender differences in murine pulmonary responses elicited by cellulose nanocrystals”. Particle and Fibre Toxicology, 13(59), 1 

13. Vartiainen et al., 2011. Health and environmental safety aspects of friction grinding and spray drying of microfibrillated cellulose. Cellulose, 18(3):775-786. 

14. Cherrie et al., 2013. Low-toxicity dusts: Current exposure guidelines are not sufficiently protective. Annals of Occupational Hygiene, 57(6): 685-691. 

15. Ding et al., 2017. Airborne engineered nanomaterials in the workplace—a review of release and worker exposure during nanomaterial production and handling processes. Journal of Hazardous Materials, 322, 17-28. 

16. Shatkin et al., 2016. Toward cellulose nanomaterial commercialization: Knowledge gap analysis for Safety Data Sheets according to the Globally Harmonized System. TAPPI Journal, 15(6), 425-437.