I'll give you a very long answer, sorry.

First- for rubio - 0.1% is low, for sure. Most hardeners contain 10-25% HDI. Their MSDS claims 0.5% of free HDI, not 0.1%, FWIW.

Claims that it is not toxic are wrong. They are a huge offender in the "sell people bullshit" category. Do not get me wrong - it is definitely less toxic - it is about two orders of magnitude less toxic to use than other things.

But it is not non-toxic. Don't take my word for it, look at the MSDS: https://ardec.ca/media/catalog/documents/MSDS_Rubio_Monocoat_Oil_Plus_2C_-_comp._B_EN.pdf

Look at what the PPE requirements are, etc.

If it was non-toxic, it would say it was not toxic and had nothing that required reporting at all. The MSDS for non-toxic things do not say "wear gloves and a respirator". They say "no hazardous chemicals or chemicals required to be reported".

Their MSDS does not say that, because it's not non-toxic.

If you look at older SDS from ~2008, it was classed also as specifically toxic to your respiratory tract, but no longer. Part of this is just that these are not sprayed, and warn against spraying. Isocyanate reacts really fast. So the most hazard is often when spraying it, from spray bounce back. It reacts fast enough that, say, rolling it does not generate a detectable level at breathing height. This means when they are meant for wiping, they are classed as less dangerous, and the precautions are mostly around skin contact.

So monocoat is better than most things. Despite hating them taking advantage of people, I would use it!

You can see it is low enough that it is generally classed as sensitizing rather than dangerous.

Compare to the bona traffic hd SDS: https://www.bona.com/globalassets/catalogassets/bona-traffic-hd-hardener-english-us-bona-us-united-states-us-sds-hcs-2012.pdf

But even for monocoat the PPE required is still gloves and respirator, and you should do that.

They probably mean that it's not toxic when cured, which they are required to test/prove. This is true of basically all wood finishes in the US due to various regulations.

In the end, look, lots of things in your body can be replaced. Not that you should live life trying to kill your organs, but like, liver and kidney transplants and such are not a complete death sentence. People often live full lives.

Lung issues, however, are often quite bad. IPF, etc are basically a death sentence. Lung transplants have one of the lowest 5 year survival rate of any organ. As a careful woodworker, i've seen too many woodworkers not use a mask with wood dust or spray finishing, and end up dying of lung issues at 55/60.

Don't fuck with your lungs, and don't let some company selling shit like this convince you to do it.

Second - VOC is not about direct human toxicity. At all I wish all these low-voc companies were literally required to tell people this. Companies know that people associate VOC with toxicity (IE low VOC = non toxic) and take serious advantage of this to try to sell things as better for you by being low VOC. Ignore all of it. It's all basically meaningless for the purpose of determining whether it's something safe to use in your house.

VOC = Volatile Organic Component. Theoretically about vapor pressure, actually. But in practice mostly about air pollution.

In the US (at least), VOC's basically equate to chemicals that cause various forms of air-pollution through photochemical reactivity (IE exposure to sunlight). The rest are exempt (acetone, etc). VOC's includes most things that smell good, like perfume, FWIW.

Isocyanate, while dangerous for humans, is not a VOC - it is not photochemically reactive. Instead, it reacts very quickly with moisture in air (which is why most danger is from bounce back from spraying it)

This is totally orthogonal to direct human toxicity. There are very low VOC things that are very dangerous to breathe. There are very high VOC components that are relatively okay.
As a first step, never confuse whether something is a VOC with whether it is safe to breathe directly.

What VOC compliance has often done a lot of is replace chemicals that are horrible for pollution but not that directly toxic to people, with chemicals that are better for pollution but more directly toxic to people. This is sort of a deliberate tradeoff, under the assumption that air pollution (which gets to everyone) is more dangerous overall than the toxicity (which gets to those spraying it).

For example, butyl acetate (a VOC), which is what makes a bunch of fruit smell/taste sweet, has been replaced with acetone (VOC exempt) in a lot of formulations.

This ends up worse for people spraying it - acetone is much harder to protect against with PPE. Butyl acetate is not carcinogenic, acetone is "unknown", etc

But better for the environment overall.

If you want to know whether and how something is toxic to you, ignore whatever marketing material they produce, and read the MSDS.