[blix]

Titanium is a common biocompatible implant material, widely considered safe in this context. One of the primary benefits of titanium implants is that that they readily form a TiO2 passivation layer. TiO2 is also common in many topical lotions and sunscreen, where it is considered safe.

TiO2 also has a signficant body of evidence demonstrating that it is a carcinogen when inhaled in particulate form. Just because some compound is safe in one context does NOT mean it is safe in all contexts.

> Do you understand that when you eat PTFE it comes out exactly the same way it went in?

I bet if I asked you to eat shards of glass, they would leave your body pretty close to how they went in :) Whether your body would remain unchanged is another question. While this is an extreme example, there are more subtle ways a compound could interact with the body. For instance, glyphosate has long been considered a safe compound because it "leaves the body the way it comes in." However, recent research has revealed substantial evidence that glyphosate may adversely interact with the human microbiome, which is linked to many various health conditions. This is an active thrust of a lot of new medical research that wasn't strongly considered 20 years ago. It is simply not enough to say the compound remains unchanged, the real question is whether the body is unchanged, and the body is pretty complex and our understanding is very incomplete.

It's not clear which version of the scientific method you subscribe to from your posts, but in general proving a negative is ~really, really~ hard. This list of all possible interactions in the human body with arbitrary chemicals is quite long. The list of things that have been 'proven safe' only to be considered unsafe when studied more carefully is quite long. To suggest that our knowlegde of the safety of ~any~ compound is complete is impossible.

[kragen]

You're confusing TiO2 with SiO2. There is some interesting evidence suggesting that TiO2 nanoparticles may accumulate in the body, in particular in the pancreas, but there is definitely not a significant body of evidence suggesting that inhaled TiO2 is a carcinogen.

It would be pretty interesting if it turned out that TiO2 added to food as a white colorant were responsible for the obesity pandemic --- the epidemiological correlations are about right --- but so far the evidence that it has any kind of harmful effect at all is fairly weak.

Shards of glass are commonly used as a mild abrasive in toothpaste, an inert excipient in pill formulations, and a gas reducing agent in simethicone; they go by the name "amorphous silica" or "silica gel". They're also added to grains as an insecticide in the form of diatomaceous earth. It's commonly accepted by people who study this stuff that they are absolutely harmless to humans at that size.

[blix]

What about larger sizes :) Physical effects can matter as much as chemical. Diatomacious earth has conflicting evidence anyway. Silica gel and glass aren't really the same thing.

I did mean TiO2. There is a reason the EU considers it a carcinogen and is banning it from several classes of products.

[kragen]

Silica gel is glass with nanometer-sized pores in it. Bottle glass has additional additives that lower its melting point and increase its solubility (oxides of sodium and calcium).

The EU has irrationally banned all sorts of things since the witch-burners have seized control of the government apparatus. Borate fire retardants and fertilizers is one of my favorite examples.

[blix]

Make some car windows out of silica gel and let me know how it goes :) I think you will find nanometer-sized pores can actually matter a lot!

I'm not sure dismissing all research that disagrees with me as witch-burning would be a good habit as a scientist.

[kragen]

Nanometer-sized pores matter enormously, which is why we make silica gel in the first place, but not in ways that make other forms of amorphous silica more toxic than silica gel.

I'm not dismissing research that disagrees with me as witch-burning. I'm dismissing people who disagree with literally all the research.