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With hot plastic being laid down layer after layer, I am worried about fumes. Should I only print in a well ventilated work space? Should I add additional ventilation?

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The short answer is: yes, it is always a good idea to print in a well-ventilated area. The longer answer can be articulated as follows:

Definition of "fumes"

"Fumes" is a fuzzy word that from a chemical/physical perspective includes at least three different things:

  • Vapour - the gas phase of a substance
  • Aerosol - a airborne suspension of tiny particles of liquid, solid, or both
  • Smoke - particles and gases emitted when a material undergoes combustion or pyrolysis (so really: a smoke is a combination of vapours and aerosols too... but the combustion/pyrolysis will have changed the very nature of the material, so it will be "vapours and aerosols of a different substance"

Interactions with the human body

Each of the above has a different way of interacting with the human body. The list of possible interactions is huge, and out-of-scope for this answer, but just to mention a few obvious ones:

  • Vapours tend to enter cells by osmotic pressure and can have carcinogenic effects by either attacking the genome of the cell or by disrupting its metabolic processes (think: benzene in car fuel)
  • Aerosols can trigger the immune system, and in return have the body develop allergies or autoimmune reactions.
  • Aerosols can deposit their particles on the cellular membrane, making it impossible for it to operate correctly and eventually fail (like neurons failing to transmit electrical impulses, for example)
  • ...

Composition of filaments

Modern filaments are a combination of different substances: the basic plastic (PLA, ABS, PETG...) that gives the name to the filament is almost always mixed with other plastics and additives that change its physical characteristics.

In some cases, the filament is host to particles of other materials (like wood, metals or phosphorescent compounds).

Each of the different materials have different transition and critical and flash points (the temperatures at which they will become vapour and ignite respectively), and different physical properties which in turn will affect differently the size of the particles in the aerosol coming out of the printer.

Conclusion

The bottom-line is that it is close to impossible to have a complete understanding of how a given "fume" affects human health.

Typically the safety of a substance is tested in a lab by directly observing its effect on cells, or by performing epidemiological studies in a population, if the exposure data to a given substance is known.

When people comment on PLA being "safe" for example, they typically refer to studies that tested inert, cold, chemically pure PLA. But the fumes of a PLA filament will probably not be inert, nor cold, nor be exclusively PLA.

Additionally, it has to be observed that it is much easier to rule a filament harmful than safe: for it to be considered harmful it is sufficient to know that one of its components is harmful (for ABS that is typically studies showing the adverse affect of ABS aerosols on health). For it to be deemed safe, one must know that all if its components are safe, but most filament do not go through the rigorous testing required to ascertain that.

In conclusion, it is always a good idea to get rid of the fumes from 3D printing regardless of the type filament being used. The ideal solution is a printing enclosure maintaining negative pressure, but an enclosure with air filtering or a well ventilated room are also good options (ventilation can have adverse effects on printing quality though, due to drafts and their cooling effect).

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There are some contradicting sources out there on whether plastics, especially ABS, have toxic fumes. It is well known that PLA is food safe, as it is an organic, biodegradable polymer being based on a particular cornstarch. This means that PLA is safe when printing, although it can produce foul smells from the dyes and other ingredients. As for the other plastics, it is most commonly said that the fumes are toxic, although, as stated earlier, there are some contradicting topics on this.

Here and here are some articles for further reading.

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    $\begingroup$ i wouldn't be so sure. PLA itself is safe, but filament is more than PLA. There's almost always dye and conditioners for example. Since a lot of filament is made overseas, the additive regulations might not be be as comprehensive as we'de like. I'm not saying it's bad, just that PLA alone is not the full story. $\endgroup$
    – dandavis
    Apr 6 '18 at 20:55
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    $\begingroup$ This is a well intentioned answer but - I am afraid - it is factually wrong and scientifically inaccurate. Most notably: A) the issue of ABS 3D printing being harmful for humans is not controversial. The linked article suggesting that there is unclarity about the ABS toxicity is about cold artefacts leeching chemicals, not about MDF extrusion. 3D printing ABS does emit dangerous fumes. B) The fact that PLA is food safe is totally unrelated to whether it is safe to 3D print with it, the same way an epoxy may be food safe but you may still have to apply it wearing a respirator and gloves. $\endgroup$
    – mac
    Apr 7 '18 at 1:18

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