Hydrated Silica

Toothpaste does not clean teeth on its own. The abrasive does. Most modern toothpastes — and nearly all mid-range and premium products — use the same one: hydrated silica. Understanding how it works matters when you are choosing a paste for yourself or your family.

What RDA means

RDA stands for Relative Dentin Abrasivity. It is a standardised laboratory test: a dentin sample is treated with toothpaste under simulated brushing conditions, then the amount of dentin removed is measured and compared against a reference paste.

The test was developed in 1976 by Hefferren and remains the gold standard for abrasivity assessment to this day.

| RDA | Level | Typical use | |---|---|---| | 0–70 | Low | Children's pastes, sensitivity formulas | | 70–100 | Moderate | Most daily-use pastes | | 100–150 | High | Whitening pastes | | 150–250 | Maximum permitted | Some professional pastes | | > 250 | Unsafe threshold | ADA does not certify |

The American Dental Association sets the upper limit at 250 for daily-use toothpastes. For healthy teeth, values below 250 do not cause clinically significant enamel wear. Dentin wears faster — which is exactly why the test measures dentin abrasion: dentin becomes exposed with gum recession and is the tissue most vulnerable to aggressive brushing.

"RDA values below 250 are not associated with clinically observable changes to enamel when used with proper brushing technique." — ADA, Oral Health Topics: Toothpastes.

One important caveat: RDA is a property of the paste, not the raw abrasive. The final value depends on abrasive concentration, particle size, particle shape, and brush stiffness.

How an abrasive works

Tooth cleaning is a mechanical process. The brush creates movement; the paste is the working medium. Abrasive particles act as microscopic scrapers, physically removing plaque (bacterial biofilm), food debris, and surface stains.

Without an abrasive, fluoride and other active ingredients cannot reach the clean enamel surface — biofilm blocks them.

This is not the same mechanics as grinding. Biofilm is soft, and removing it requires relatively modest abrasive force — provided particles are the right size and shape.

Hydrated silica: why it is the best choice among abrasives

Hydrated silica is amorphous, hydrated silicon dioxide. Unlike crystalline quartz (Mohs hardness 7), hydrated silica has significantly lower hardness and rounded rather than sharp particles. This makes it gentler in practice than its mineral origin might suggest.

| Abrasive | Typical RDA | Particle shape | Notes | |---|---|---|---| | Hydrated silica | 70–120 | Rounded | Controllable, tunable | | Calcium carbonate (chalk) | 60–90 | Irregular | Cheaper, less consistent | | Baking soda (NaHCO₃) | 7–53 | Angular, coarse | Low RDA but large particles | | Kaolin | 45–65 | Platelet | Soft, weaker plaque removal | | Activated charcoal | 70–300+ | Sharp, irregular | No standard; enormous variation |

Worschech et al. (2001, PubMed 11413496) demonstrated that silica, at comparable particle sizes, shows higher abrasivity than calcium carbonate — but this is precisely what is needed for effective plaque removal. The operative phrase is controlled abrasivity.

Hydrated silica is highly amenable to process engineering: manufacturers can vary particle size and structure to achieve a target RDA. This is its primary advantage over calcium carbonate and baking soda, which have narrower adjustment ranges.

A separate word on charcoal. Activated charcoal has become a trend. The problem is the absence of standards: charcoal pastes from different manufacturers show RDA values ranging from 70 to over 300. Without testing the specific product, you do not know what you are dealing with.

Safety for daily use

The FDA classifies hydrated silica as GRAS (Generally Recognized As Safe). There are no toxicity data for the amounts that enter the body during normal brushing.

The risk to enamel at RDA below 100 is negligible for patients with normal enamel thickness. Risk zones to be aware of:

• Gum recession. Exposed dentin and cementum are less resistant to abrasion than enamel. With recession, choose pastes with RDA ≤ 70.
• Excessive brush pressure. RDA is a property of the paste, but actual wear depends on the force applied. High pressure causes abrasion even from low-RDA pastes.
• Brushing more than twice daily. Tissues need time to restore mineral balance between sessions.

Joiner et al. (2013, PubMed 23817063) note that RDA test methodology has inherent limitations — results from different laboratories can diverge by up to 30%. This means RDA is a reference point, not an absolute figure.

QDRO position

In v.daily pastes we use fine-particle hydrated silica with an RDA in the 70–90 range. This is moderate abrasivity: enough to reliably remove plaque, and well below the risk zone for healthy enamel.

The choice of silica over chalk or baking soda comes down to control. We know the abrasive load in each formula precisely. No surprises.