Teeth that cannot be whitened are teeth with deep intrinsic stains, exposed roots or artificial restorations.
Artificial dental restorations that cannot be whitened include:
Crowns and caps that are made of porcelain, ceramic or metal as these materials don't respond to peroxide.
Veneers like porcelain or composite shells on the front of teeth stay their original shade and cannot be whitened.
Fillings and bonding as composite resin filling materials cannot be whitened as they don't change color, which can lead to uneven contrast if natural teeth lighten around them.
Teeth that have medication staining, trauma and root canals and severe fluorosis and teeth with exposed roots and thin enamel cannot be whitened.
Antibiotics like tetracycline cause deep gray, blue or brown internal discoloration which standard bleach rarely fixes.
Physical injury to a tooth or any internal damage after a root canal disrupts the tooth from the inside out and makes surface whitening of the tooth or teeth ineffective and heavy overexposure to fluoride during childhood also creates deep white or brown pitting and spots that don't bleach out evenly.
Receding gums can also expose the tooth root (cementum), which also lacks enamel and doesn't respond to teeth whitening gels.
And when tooth enamel wears away completely, the inner layer called dentin shows through as a natural yellowish tone that can't be restored or bleached whiter.
Teeth can be healthy but not white as it is normal to not have perfectly white teeth and having perfectly white teeth is not always a sign of healthy teeth.
It's sometimes unhealthy to have perfectly white teeth but not always unhealthy to have perfectly white teeth.
It's not always bad if your teeth are pure white, although having teeth with a pure, unnaturally bright white shade is rarely natural.
Healthy teeth are actually normally an off white, light gray or even slightly yellowish color because the hard outer tooth enamel is also somewhat translucent, which allows the naturally yellow inner layer also known as "dentin" to show through.
Everyone does have a unique natural tooth color that is determined by genetics and thickens of the tooth enamel.
Having off white or even slightly yellow teeth are completely healthy.
The illusion of white teeth, where you often seek sparkling white teeth in media are often a result of cosmetic bleaching, veneers or even digital editing and not standard human biology.
A person with bright white teeth may still have hidden gum disease, cavities or even receding gums.
And conversely, having teeth that are slightly stained or yellow can be structurally very strong and healthy teeth.
There are also risks involved when forcing pure whiteness on your teeth which include tooth enamel wear, tooth sensitivity, and some temporary pain or lasting sensitivity from over the counter whitening.
Achieving a blindingly white shade of teeth often requires use of aggressive chemical bleaching or abrasive products.
And overusing teeth whitening gels or teeth whitening strips can damage the microscopic structure of the tooth enamel, which leads to sharp tooth sensitivity and even irritated gums.
So if your teeth are not pure white they are not always unhealthy and having pure white teeth can be unhealthy in some cases.
Perfect teeth whiteness does not equal perfect tooth health as teeth naturally come in a range of natural shades that is very normal.