Occupational and Environmental Health in Carbon Nano-Tube Thread Manufacturing
Carbon nanotubes (CNT) have novel physicochemical properties accompanied by new risks and uncertainties (European Agency for Safety and Health at Work [EU-OSHA], 2009). CNT release into environment and worker exposure can occur during any stage of production, usage, transport, waste treatment or storage. Data on occupational exposure to CNT is limited, and measurement techniques are also not fully developed. Engineering controls and personal protective equipment used for aerosol pollution are recommended for CNT also. Though CNTs have a primary graphene structure, their properties are entirely different from carbon particles, for which there are established exposure limits. CNT emissions are <100nm in size, and can travel greater distance by diffusion in air. Further, CNTs commercial manufacture by chemical vapor deposition (CVD) technique employs a fluidized bed reactor and metal catalyst (Volder, Tawfick, Barghman and Hart, 2013). The manufacturing process itself can release CNTs with metallic impurities into the atmosphere.
Toxicological Effects
According to NIOSH’s report (2013), CNTs cause adverse pulmonary effects, inflammation, granulomas and pulmonary fibrosis. Dreher (2004) states that SWCNT causes dosage dependent lung lesions, and SWCNT is more toxic than quartz (considered to have the highest pulmonary toxicity). Further, SWCNT induces granulomas without cellular proliferation and cytotoxicity (Dreher, 2004). According to Donaldson et al (2006) this new mechanism of CNT toxicity may be because very small particles would not kindle normal phagocytic defenses, and since CNTs are insoluble in neutral or mild acidic pH, they become persistent. NIOSH (2013) has assessed that for an 8hour time weighted average (TWA) exposure to 1μg/m3 CNT, there is 0.5% to 16% risk of developing mild lung effects. Occupational Safety and Health Administration has set no permissible exposure limits (PEL) for CNTs. PELs are set for graphite at 5000μg/m3, and carbon black at 3500μg/m3. However, adopting these PELs for CNTs would not be relevant.
Monitoring Provisions
Instruments used to monitor workplace CNT exposures include cascade impactor, tapered element oscillating microbalance (TEOM), scanning mobility particle sizer (SMPS), condensation particle counter (CPC), and nanoparticle surface aerosol monitor (NSAM) (EU-OSHA, 2009). Cascade impactor allows aerosol sampling based on aerodynamic diameter of the particle, and TEOM allows measurement of concentration of collected aerosol. SMPS allows CNT counting through electric field scanning, and NSAM can be used to monitor the actual aerosol concentration that would deposit on the respiratory passage (EU-OSHA, 2009). Lee et al. (2010), assessed CNT exposure in 7 workplaces handling MWCNTs using SMPS and dust monitor. They found most nanoparticles were released following CVD cover opening and catalyst preparation. Further, they state that engineering controls and PPE can effectively reduce CNT exposure (Lee et al., 2010).
On the whole though there are monitoring methods, they are restricted by bulkiness of instrument and ease of handling. Specialized skill and technique is also essential to handle these instruments, which might affect their usage in several CNT handling facilities (EU-OSHA, 2009). Finally, background aerosol or carbon emission levels affect the efficiency of these instruments. Diesel vapors, and particulates from sources other than CNT can interfere with measurement (Lee et al., 2010). Thus there is still need for developing a suitable, practical measuring method for workplace CNT emissions.
Engineering Controls and PPE
Worker exposure to CNTs can be limited effectively using engineering controls such as isolated hoods for carrying out processes involving CNTs, High efficiency particulate air (HEPA) filters, electrostatic precipitators and good local exhaust ventilation systems (Schulte et al., 2008). But, Maynard and Kuempel (2005), highlight that these aerosol control methods are not meant for nanoparticles, and CNTs <20nm size can easily penetrate filters used in ventilation systems. NIOSH recommends PPE including appropriate clothing, gloves, and facemasks with respirators for personnel handling CNTs to reduce exposure through dermal or inhalation routes (2013).
Nanomaterial handling needs to be isolated from rest of the activity in the workplace within appropriate enclosures or hoods. This will restrict dispersion of CNT particulates. Further, CNT handling personnel should use respirators, gloves and appropriate overalls especially during opening of the CVD vessels, during cooling or mixing operations involving phase change, cleaning and maintenance. Usage of PPE must be strictly monitored, and a CNT specific safe operating procedure must be in place.
Role of Environmental Health & Safety (EHS) professional and EHS Programs
As there is very less data pertaining to occupational exposure of CNTs, the role of EHS professional in incorporating a nanomaterial risk management program into the overall H&S program can be very challenging (Schulte et al., 2008). An EHS professional’s role would include CNT risk management, training and communication, and establishing safe work practices (Schulte et al., 2008). CNT risk management includes hazard determination, exposure evaluation, risk characterization and prioritization, as well as establishing appropriate controls (Schulte et al., 2008). High risk processes in a workplace setup would include gas phase CNT handling, handling liquid phase mixing or agitation, spill clean up and maintenance activities, cleaning aerosol dust collection systems and pollution control equipment (Schulte et al., 2008). In CNT pollution management, health protection programs are an important administrative control measure. EHS professionals need to organize biomonitoring and medical screening programs, awareness programs, as well as training on safe operating procedures for workers handling CNTs (Schulte et al., 2008). Further, Top-level management’s commitment as well as adequate employee participation is critical for establishing safe environmental conditions in a CNT handling facility.
References
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