The explosive 3D printer market has made desktop versions affordable for schools, libraries, businesses and even consumers. But the growing influence of 3D printers has also raised concerns about possible negative health effects due to inhalation of toxic volatile organic compounds (VOCs) and particles given off by the devices.
By depositing molten plastic layer upon layer, three-dimensional printers can print almost anything, including toys, guns and artificial limbs. While several governments have set workplace standards for industrial environments where VOCs are released by 3D printers, the same is not true for homes, schools or libraries. Here, hardly any regulation applies, while then even vulnerable people like children come into contact with it. According to US environmental technologist Marina E. Vance, we need to take more account of harmful emissions from 3D printers. "Scientists know that particles and VOCs are bad for health, but they don't have enough information to create a legal standard for 3D printers."
Two years ago, Underwriters Laboratories, an independent global safety certification company, set up an advisory board and began funding research projects to answer fundamental questions about the amounts and types of compounds in 3D printer emissions: what levels are safe and how can we assess exposures. The company is working on a consistent testing and evaluation methodology so that researchers can compare data from different labs. "By this autumn, we will have an ANSI [American National Standards Institute] standard for measuring particles and VOCs for all to use," Marilyn S. Black, vice-president at UL.
Moreover, says Rodney J. Weber, aerosol chemist at the Georgia Institute of Technology, data from early studies of 3D printer emissions are difficult to use in the development of standards because of the variability in study conditions.
Harmful substances
A 3D printer makes an object by passing rolls of plastic filament through a nozzle that melts the plastic at temperatures up to 320°C and then extrudes it onto a moving base plate. A computer controls the movement of the base plate so that layers of material are built up until the predefined 3D shape emerges. "We know that when you melt plastic at high temperatures, the long chains of organic material in the plastic partially degrade and release potentially harmful volatile organic compounds and ultrafine dust into the air," says Brent Stephens, environmental engineer at Illinois Institute for Technology. He and other researchers have found that the most common filament materials can potentially cause unhealthy emissions when used in 3D printers. Petroleum-based acrylonitrile butadiene styrene (ABS), a plastic used in lego blocks, emits styrene and formaldehyde - the former suspected to be carcinogenic to humans, the latter is known to be. Nylon gives off caprolactam, irritating to the respiratory tract. Polylactide (PLA) - a corn-based plastic found in medical implants, drinking cups and disposable nappies - emits methyl methacrylate, a mild skin irritant. And all types of filaments squirt ultrafine particles, particles smaller than 100 nm in diameter that can penetrate deep into the lungs and enter the bloodstream. These particles are known to cause respiratory and cardiovascular diseases.
While laser printers heat and deposit ink, they emit numbers of ultrafine particles similar to those of 3D printers. However, that does not mean they have the same health risk, says Aleksandr B. Stefaniak, an industrial hygienist at the US National Institute for Occupational Safety and Health. "Although both types of printers use plastic 'ink', laser toner is heated only briefly to melt it on a sheet of paper," he says. A 3D printing job can take hours or days because the filament is continuously melted by the nozzle of the extruder. Because of the prolonged melting, 3D printer emissions include hundreds of VOCs and large numbers of particles of unknown composition.
Some early tests suggest that the use of 3D printers could lead to unhealthy aerosol levels. Weber measured VOCs given off by a 3D printer in a 1 m3 environmental chamber, while colleagues modelled levels that would be found in an office with the same machine. The model predicted that caprolactam room concentrations would reach 100 μg/m3 - more than 14 times greater than California's acceptable level of 7 μg/m3 (1.4 ppb) for an eight-hour exposure. Formaldehyde would reach concentrations higher than those recommended by the World Health Organisation for indoor air.
Testing on rats
Stefaniak and his team also looked at the health effects of 3D printer emissions on rats. Particles in outdoor air can lead to cardiovascular disease in humans, so Stefaniak looked for similar effects of dusts released by 3D printers. He and his team exposed rats to emissions from a printer containing black ABS for three hours and performed several tests on the rats' cardiovascular system before and after exposure. Twenty-four hours after exposure, the rats' blood pressure had increased by about 30% and their arteries had become stiff compared to before exposure. "Now we want to identify the causative factor and find out how it works," he says.
The filament
The composition of the filament and the temperature to which it is heated are crucial to the generation of particles and pollutants, according to Weber. "The higher the temperature, the more gases are produced and the more particles eventually form." Heat degrades the plastic and volatilises the compounds. As they cool, the gases form particles and also condense on small particles already present in the chamber.
Weber reasons that temperature is the reason why ABS filaments release more VOCs and particles compared to PLA: ABS softens at a higher temperature than PLA, so printers typically heat ABS to 240°C, while PLA is processed at 220°C. Filament additives, which are added for e.g. gloss, electrical conductivity, colour or other properties, can drastically alter emissions. PLA contains trace substances to make it impact-resistant, for example, generates more particles than standard ABS, says Weber.
Stefaniak has discovered particles containing chromium, nickel and aluminium during printing, possibly produced by metallic dyes in coloured ABS filaments. "These metals can generate reactive oxygen species that promote inflammation, a condition associated with certain lung diseases," he says. Related: Some 3-D printed parts can wash out toxic chemicals. Such findings suggest that additives are responsible for most of the release of fine particles. Yet manufacturers do not have to reveal the identity of the compounds on safety data sheets, Weber says. He has found that filaments from different brands produce very different particles, a finding he suspects is due to unknown additives.
Research in the Netherlands
In the Netherlands, the RIVM conducted research in 2016 into the safety of 3D printers and, in particular, their use by consumers. "Consumers may be exposed to chemicals when using the printed products. This can be via contact with the skin, as in the case of jewellery, and via the mouth, for example in the case of children's toys or drinking cups. These products must normally comply with safety requirements, so that there is no risk of poisoning, for example, when a child puts a toy in its mouth. With proprietary, 3D-printed products, there is no check on that. So consumers should be aware that the safety of 3D printed products is not guaranteed."
However, this body, like one of the US specialists, argues that the available data is still too unclear to draw conclusions about the risks. "During 3D printing, there is a risk of inhalation of the released substances as vapour or in the form of nanoparticles created during printing. The release of these substances depends on parameters such as the filament used, the printing conditions, and the size of the object to be printed. It is not possible to determine whether inhalation of the released substances leads to risks. Therefore, it is wise to advise consumers to properly ventilate the room where printing is taking place. When using 3D printed products, there may be dermal or oral exposure to chemicals migrating from the product. The nature of the product and its use determine the degree of exposure. Due to lack of (exposure) data, including data on use and migration, it is currently not possible to carry out quantitative risk assessments for this."
By: Kelly Bakker
Source: C&EN/RIVM
