Last week, a series of articles on cellulose nanomaterials was published in the journal Industrial Biotechnology. The one most circulated by my network is a review paper by Maren Roman of Virginia Tech about the toxicity of cellulose nanocrystals (CNC). (I know, don’t we just want to tweet about Oscar winners on this chilly Monday morning!) What is compelling for me is that the story has not changed (despite new studies) since Vireo Advisors worked with the US Forest Service in 2013 to analyze the available studies for cellulose nanomaterials in a life cycle risk assessment and developed a Roadmap for establishing the Environmental Health and Safety aspects of this exciting bio-based material.
The available data indicate generally low toxicity, but there are too many gaps and uncertainties to reach a conclusion at this time. In the EHS Roadmap, we evaluated risks across the value chain into emerging applications with CNC (and cellulose nanofibrils, CNF), broadly considering occupational, consumer and environmental exposures to raw materials and products. Yet, we reached a similar conclusion, recommending a series of research priorities for filling the data gaps. In the EHS Roadmap, we found it particularly notable that the few available studies indicate there is little in the way of environmental toxicity, not too surprising when one considers that cellulose is the most abundant organic material on earth. Since it is plant based (although bacteria, algae and tunicates also generate CNC), cellulose and cellulose nanomaterials are biodegradable, a fact strengthened by the development of bio-based cellulosic fuels research.
Roman’s review focuses on toxicity to human health, and takes a systemic approach to reviewing the available studies for CNC by exposure pathway, explaining the physiology for uptake by that pathway, then going through the published whole animal studies for that pathway. One source of uncertainty is the impact associated with inhalation of CNC. Given that most products being developed would incorporate CNC into some composite material, the inhalation exposure pathway is generally of concern in the occupational environment, where if concentrated as a free powder, CNC could be released as dust and inhaled. The studies relevant for inhalation exposure are equivocal, meaning some indicate toxicity to the lung, and others don’t. This is partly due to differences in methodology, but on the practical side, avoiding any type of inhalation exposure to respirable low solubility dust particles is advisable, and further, limits established by OSHA and others for conventional cellulose dust based on prior studies suggest at a minimum, inhalation exposure can cause irritation, if not more serious effects. The bottom line here: inhalation exposures to dust in the workplace can be minimized, a practical solution to the current uncertainty in the toxicology literature.
One relevant finding: “Current studies of the oral and dermal toxicity of CNCs have shown a lack of adverse health effects. The available studies, however, are still very limited in number (two oral toxicity studies and three dermal toxicity studies) and in the variety of tested CNC materials,,,. Additional oral and dermal toxicity studies are needed to support the general conclusion that CNCs are nontoxic upon ingestion or contact with the skin….”.
In the EHS roadmap, we determined the highest priority is overcoming challenges to accurate measurement of cellulose nanomaterials in environmental media, to facilitate more definitive studies of exposure to these materials. While the few published toxicity studies suggest there may be adverse effects from inhalation, accurate exposure assessment is needed to assess the potential health risks from this pathway. As we know, every substance, even water, is hazardous to health at some exposure level. Hence, the roadmap focuses on better evaluation and control of exposure as a proactive approach to evaluating and managing risk. The other priorities include standardizing occupational exposure methods for cellulose nanomaterials, and greater focus on oral exposure studies. Cellulose is added to food, and of course paper-based products are widely used for food packaging. Thus, oral studies will inform the safety of this exposure pathway.
The key takeaway: while it is critical to fill the current data gaps, the current toxicology literature for cellulose nanomaterials provides strong indication that no novel risks exist for these emerging materials.
Roman, M. 2015. Toxicity of Cellulose Nanocrystals: A Review. Industrial Biotechnology 11(1): 25-33.