A few years ago, nanomaterials were curiosities from scientific laboratories. By now the ability to control the structure of materials at the nanoscale has been translated into numerous applications and nanomaterials have entered our daily lives. They are appearing in raw materials for new products that are lighter, more stable or have new properties. In some application, nano-objects (one or more dimensions in the nano-range) provide the benefit, for example as additives for ultra-stable concrete; or to make surfaces ultra-resistant, bactericidal or catalytically active. In other applications, the surface or material structure is modified, to create ultra-hydrophobic (extremely water-repellent) surfaces; glass screens that are non-reflective, reusable adhesive pads or super-efficient electronic chips.
When nanomaterials entered the public domain a little more than a decade ago, they aroused immense hopes but also great fears. While for some they represented the solution to all of humanity's problems, others predicted the demise of the planet through self-replicating nanobots. Today we can better distinguish between fiction and reality, and also say something about the chances and risks. Many of the (horror) visions were exaggerated. Nanomaterials will neither bring about the end of the world nor save humanity from itself. One area where a lot of research has been done is occupational safety. What do we know today about exposure to nanomaterials and the effectiveness of protective measures?
In principle, as soon as a material is handled, there is a possibility that some of the material will be released, allowing for exposure to occur. Toxicologically of interest are mostly nano-objects and their agglomerates and aggregates, while mere nanoscale structures of larger objects rarely pose nano-specific health issues. When studying exposures to nano-objects, it is helpful to look at the whole chain of events back to the release. Why was there a release? How could this release turn into an exposure? What is the role of the material properties and of the processing procedures? If we find the answer to these questions, we can often effectively prevent harmful exposures by design.
For example, if a nanocoating is applied in an immersion bath, the main thing is to prevent workers from dipping their hands into the bath. If, on the other hand, the coating is sprayed on, very high concentrations quickly occur in the air, which results in correspondingly complex protective measures. A few years ago, we analyzed publications on the release of nanoparticles from many different work processes. We saw that spraying processes often lead to many millions of nanoparticles per milliliter of air. This is similar to the amount produced during nanopowder manufacturing. But machine processing and powder transfer can also result in very high exposures. At the other end of the processing spectrum are ultrasonic baths, where even high energy rarely results in more than a few thousand particles in the air. Collaborating with material scientists also allowed us to understand how nano-objects embedded in a matrix do or do not get released as individual nano-objects.
Often, despite all the smart designs, protective measures are still needed to ensure that exposure does not occur after a release. The best way is to tackle the problem at the source. Ideally all processes would be conducted in closed systems. But that is not always possible. Flexible suction arms installed over equipment openings are well known. However, suction does not work when it is distant from the source. Since nano-objects move with the air, it is often better to create air flows that guide the particles to an extraction point. In complex interactions, for example when maintenance work is due on machines, the question of personal protective equipment (PPE) also arises. PPE are not very comfortable. Many people find it difficult to put them on correctly, as anyone can see for themselves in the current pandemic. Masks protect quite efficiently against nanoparticles provided they are worn correctly. However, this is often difficult, especially for women and young people, as there are few masks that fit their faces. Protective suits and gloves also work well to protect against nanomaterials. But it is important to choose the right protective material and follow the instructions of the manufacturers and technical experts.
Too often, safety rules are not observed by workers or not enforced by superiors. Because humans have no sensorial organ for nanomaterials in the air, they underestimate the exposure. A good example comes from "traditional industry": inert gas shielded welding has a reputation among many workers for being a clean process because, after all, there seems to be almost no smoke involved. Therefore, many welders do not wear respirators during this process. However, our measurements showed that the emitted particle mass in inert gas-shielded welding is almost the same as in conventional welding, but this mass is distributed among many millions of ultrafine and invisible nanoparticles. Thus, the eye is a poor measuring device for nanomaterials to distinguish clean from dirty processes.
For exposure determination, it is not enough to simply measure the number of particles in the air. A major challenge is the contextual information and metadata about the substances and processes involved. These are important for subsequent risk analysis, and also to better study the behaviour of nanomaterials in the workplace and the environment. Traditionally, measurement reports described mostly the measurement process. They rarely reported about the working environment, the production processes or the materials involved, and did not even provide the protective measures and PPE in place. The situation was even worse when it came to characterizing the particles. Fortunately, this has changed over the past few years, thanks to recommendations that I wrote with European colleagues, and the insistence of journal editors and research funding bodies that context and metadata are an essential part of exposure assessment. Research continues, and some of Europe's brightest workplace experts are currently working to translate basic research into practical protection concepts and guidance through a series of interlinked NanoSafetyCluster projects (https://www.nanosafetycluster.eu/safe-by-design-and-eu-funded-nanosafety-projects/).
So, is all this progress in exposure control beneficial to the health of workers? Unfortunately, we don't know. It seems wise to investigate the success of these measures with occupational health surveillance and epidemiology, the gold standard for documenting the existence or absence of health issues. Over a decade ago, we published a roadmap for doing this in a globally harmonized way. It is overdue and should be funded and supported at least on a European scale. Several nanomaterial risk governance frameworks were already proposed, and used to develop exposure scenarios that are believed to protect workers from ill-health. The "no effect levels", however, were derived from cell and animal studies. Confirming that the intended exposure levels can be indeed reached and that disease and early markers of ill-health are truly absent in humans at such levels will be crucial for a wide-spread adoption of these risk governance frameworks.
Dr. Michael Riediker is the founder and director of the Swiss Centre for Occupational and Environmental Health, SCOEH. He is certified occupational hygienist with decades of research and practical experience on the occupational and environmental health risks resulting from release of and exposure to nanoscaled and larger aerosols in air and water, allergens, gases and related agents and circumstances.
Website of SCOEH
LinkedIn-Profile Michael Riediker
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