HEPA filters come in several grades. Those rated for 0.3 micron are the ones that meet or exceed the requirements for air safety in Europe and the US.
Machining metals and plastics there is no real need to vacuum (unless one machines to make the materials burn!). Wood and friable (easily flaked materials like FR4, fiberglass and Renshape) materials generate some nasty particles when CNCed. Some exotic hardwoods - tropicals and such - release some real nasty compounds and even viruses when machined.
Ideally, it’s important to maintain a negative pressure inside the enclosure (lower pressure inside than outside) to ensure that things stay where they should. Sucking air out of an enclosure requires an equal amount of air to enter into it so the particles are taken out. When a job completes, let the vacuum run for a few minutes to ensure the air is cleared. Turn off the vacuum, open the enclosure, turn on the vacuum and immediately vacuum the inside of enclosure.
Don’t want to add a fitting to have negative pressure? That’s “ok” but still wait a few minutes for things to settle.
HEPA filters, at least the good/safe ones, are not cheap. One always tries to avoid unnecessary costs. Some HEPA filters are cleanable, at least for a few cycles.
In shops that generate a lot of dust (e.g. wood shops), one uses a dust collector with a cyclone dust separator ahead of it. The cyclone removes 99+% of the particles before the dust collector. The dust collector either dumps the air outside or through a filter. If the filter is in the room with workers, a 0.3 micron HEPA filter is called for.
ALWAYS, ALWAYS dump the collected particles outside. Ideally wearing a filter mask. Upwind works if you can pull it off.
For something as small as the Nomad, a high end dust collector and dust separator are overkill. There are a couple of approaches that one can pursue:
A) Festool makes top rated small dust collectors with HEPA filters. Fein is an alternative.
A good deal better than a shop vac… with a price to match.
B) A shop vac with a HEPA filter rated at 0.3 micron.
There are so many of these they are very easy to find. The HEPA filter rating is key.
For the safest air quality and the lowest HEPA filter replacement cost one can add a small cyclone dust separator. They can be added ahead of a shop vac or a Festool/Fein. There is even one for Festool that stacks on top of one, minimizing space.
Oneida and ClearVue sell dust separators that are priced fairly and their low end, small, plastic ones are inexpensive.
Budget tradeoffs with safety should err on the side of safety. The small particles are more dangerous than Asbestos and are significantly more damaging to children than adults.
Don’t do wood or friable materials (metals and plastics only)? You’re all set.
Tight budget? Shop vac with a 0.3 micron HEPA filter.
Generate a lot of dust or want really safe air quality? Add a cyclone (dramatically reduces your HEPA filter costs).
Very serious about air quality? Get a Festool with the special cyclone made for it.