Yes, it does harm your teeth, less than the most aggressive drink, but still.
You could answer the question "Does unflavoured carbonated water cause damage to teeth?" in two ways. The first way by a "yes or no" answer, which correspond to my understanding of the question. The second way is "yes but no, so no" which is the point defended by Victorhooi.
Thanks to Oddthinking we found a study about unflavoured carbonated water.
"Sparkling mineral waters showed slightly greater dissolution (1) than
still waters, but levels remained low and were of the order of one
hundred times(2) less than the comparator soft drinks. De-gassing of a
sparkling mineral water reduced its dissolution (3), but the total levels
were still relatively low suggesting that carbonation of drinks may
not be an important factor per se in respect of erosive potential."
(1.1) This point shows that sparkling water is still more corrosive than still water. (We all agree that sparkling water is less corrosive some aggressive soda. But the question was not "is sparkling water better than flavoured soda or chloric acid?")
(1.2) It does dissolve, but "hundred times less than the comparator soft drinks". (The study could also have added "and probably several hundred time less than chloric acid"). Cf. point 1 above
(1.3) shows that carbonated gas causes erosion, which mean that you take relatively more risk by drinking carbonated water (but less than drinking a soda, or chloric acid).
According to the studies cited by Spork, the answer should be "yes it does harm your teeth" because a "negligible harm" is still a harm —that could be avoided. That's my point. On this topic Timothy Groote interestingly suggest that breathing causes carbonic acid. If it does, it suggest 3 things:
- (2.1) it would be interesting to measure the difference level of erosion
between our breathing and the carbonated water.
- (2.2) you can't probably avoid breathing through the mouth, but you can choose what you drink.
- (2.3) just to say: what is "natural", or "normal" is not necessary "good"
Concerning the difference between "none" and "negligible" and "high".
Those are very subjective: they may vary greatly depending on what your criteria of comparison (chloric acid, coca cola or milk?), but also of you consumption frequency and also of the type of mineral water you will ingest (in which the ph and the chemical compounds may vary greatly, because there is many kinds of carbonated water. Eg. The study above talk about calcium minerals that could slow the erosion). A corrosion index/scale would be much more precise.
The lack of precision of such terms such as "negligible" might more profit some companies than you teeth. Eg: "the total levels were still relatively low" "carbonation of drinks may not be an important factor" "their effects were moderated" (J Oral Rehabil. 2001 Aug;28(8):766-72). To get a complete idea we must also take into account that studies are often founded by lobbies.
My last point is about chemical reaction
In brief (any) acid source break teeth enamel. A wikipedia article talk about it.
(3.1) Although carbonated water contains nothing more than carbonated gas
and water (with a few minerals) it is still very acidic due to the
carbonation which can combine with the water to form carbonic acid.
(3.2) The pH value is clearly an important variable in the erosive potential of beverages (dental Acid erosion source )
We can safely assume that carbonated water has a (limited, but still) corrosion effect on teeth since the carbonic acid will dissolves the calcium compounds of enamel due to the acidity.
EDIT: I did update my answer according to Oddthinking study.