Starting with the basics of titanium dioxide. 

Titanium is one of the most abundant metals found in the earth's crust. Yet, it doesn't occur in the metallic form naturally. Instead, it combines with oxygen to give its different oxides readily found in the indigenous dust, sands, and soils. Pure titanium dioxide comes in the form of fine, white powder.

As a compound, titanium dioxide is chemically inert. In simple words, unless subjected to any critical condition, it doesn't undergo a reaction and alter its chemical composition. Nor, it disrupts "the chemical milieu of the surrounding environment," points out a theory. 

Titanium dioxide (TiO2) is typically produced in two primary forms. 

1) Pigment grade 

• TiO2 undergoes processing to give a bright, white-colored pigment, which is opaque.
  
• The pigment grade TiO2 comprises over 98% of the total production.
  
• It is known for its excellent light-scattering properties and is always used in sunblocks.
  
• Its particles lie in the (200 - 300)nm dimensions (larger than the nano-material grade, thereby occupying a larger surface area). 
  

Note:

The maximum reflecting and scattering of light are observed by the particles of TiO2 when they are manufactured in the diameter range of (200-300) nm.

• A sunblock serves as a physical barrier between the UV radiations and your skin. It forms a white, thick cast on the skin and maybe aesthetically disappointing for some. TiO2 (in the pigment grade), as an active ingredient in a sunblock, reflects the rays falling on your skin.
  
• It is used in skincare products that aid in hiding the blemishes.
  
• TiO2 forms a solid white on a surface, where it is applied. Its white opacity and clear brightness, being its most prominent features. 
  
• It is also included in skin-brightening products and the thinner coatings of makeup material. 
  
• Specific classes of high purity TiO2 are used to coat drug tablets and capsules.
  
  It also makes a decorative supplement in food and acts as a base to various paints' main composition (mainly to increase their durability and longevity). 
  

2) Nano-material grade 

• The nano-sized or the micronized TiO2 particles foremost came out in the early 1990s. 
  

Interesting fact:

Nano or the ultra-fine particles are less than 100nm. Now, one nanometer(nm) is equal to one billionth of a meter(m).

• The nano grade TiO2 is used in the sunscreen, wherein it acts as the chemical barrier protecting your skin from the harmful UV rays. 
  
• In this case, the nanoparticles owing to their extremely small size and low concentration, become transparent to visible light. Hence, a sunscreen containing nano grade TiO2 does not form any white, opaque layer on application. 
  
• When used in sunscreen, it penetrates your skin. Because it is reduced to its nano versions, it can't reflect light anymore. Instead, it effectively absorbs the UV rays now. 
  
• The nano grade TiO2 also supports materials, which act as catalysts in different reactions and processes. 
  
• Access to the Scientific Committee on Consumer Safety, SCCS), the nano grade TiO2 is a non-sensitizer (doesn't cause any sensitivity to the skin) and a mild or a non-irritant.
  

The nano-sized TiO2 can be troubling. How exactly? 

• It is photoreactive, i.e., to say that the nano-sized TiO2, after absorbing the UV radiations, undergoes a chemical reaction. And produces more free radicals in the body. This situation is further aggravated when your skin gets exposed to the UVA and UVB rays.
  

Note:

Free radicals are the highly reactive oxygen species that go around hunting for electrons. In the process, they attack cells and damage their DNA.

• The extremely small size of the nanoparticles, in all likeliness, enables them to penetrate across the dermal (skin), respiratory, and gastrointestinal barriers. They may spread throughout a particular organ and can become a potential risk factor.
  
  However, the former issue has been resolved to a certain extent by coating the nanoparticles with alumina, silica, dimethicone, etc. 
  

The coating of the nanoparticles brings in a couple of benefits. Some being, 

• It reduces the number of free radicals formed. It makes the TiO2 particles more stable.
  
•  It improves the dispersion of TiO2 nanoparticles on the skin, and as you apply your sunscreen, it immediately rubs in without leaving a white residue. 
  
• It enhances their ability to provide better sun protection. 
  
• The coating further prevents the nanoparticles from going beyond the outer layers of skin (limiting it to or before the stratum corneum).
  
• It increases the compatibility of the TiO2 nanoparticles, not permitting their interaction with the other ingredients (as it may do in the presence of sunlight). 
  

What does research say about nano grade TiO2? 

According to a 2010 IARC (International Agency for Research on Cancer) study conducted to evaluate humans' possible carcinogenic risks, titanium dioxide was considered a carcinogen. It was all based upon the increased incidence of lung cancers in animals after exposure to a medium, wherein they could inhale the concerning particles.

Studies indicate that the pigment grade TiO2 does not penetrate through healthy skin, and in no way can it damage or disturb the systematic functions of a human being.

However, experts raised concerns over the ultra-fine or nano-sized TiO2 particles that may readily seep into the human skin's deeper layers. In a 2012 experiment, sunscreen having nano-sized TiO2 was applied to the sunburnt ears of the pigs. TiO2 didn't reach the tissue's deeper layers (the UV rays damaged that).

Also, the IARC has found that the commercial pigments involve no TiO2 particles smaller than 100nm.

However, the nano grade TiO2 poses a greater threat when it is available in products, which can be inhaled or taken in easily with air (i.e., loose or pressed powders, sprays). Now, the TiO2 particles must be small enough to travel across to the lungs' alveoli (where the exchange of oxygen takes place).

In the experiments carried out over the years, it has been observed that some mammals could inhale the nano-sized TiO2 particles. This further leads to more serious doubts about human inhalation.

As a response to these doubts, further research has been done. Their findings state that the TiO2 particles present in cosmetics will be predominantly inhaled in clusters, even if by accident. And the reason why they don't stand a chance to reach the alveoli.

A 2005 experiment saw that when the rats and mice were exposed to nano grade TiO2, they suffered significant lung inflammation and cell abnormalities. In other experiments (2007), the inflammatory responses resolved on their own. Whereas, in some, no development of tumors took place at all.

Titanium Dioxide Rutile Tio2 POWDER Titanio Dioxide Pigment

More quality tests are needed to assess whether or not the nano-sized TiO2 particles can be retained in the respiratory passage. Or, do these particles have the potential to induce damage to the DNA and other genetic material. 

What do dermatologists say? 

They often point out that the people who work at the TiO2 manufacturing units are continuously exposed to the ingredient. But the research carried out to date confirms no direct association between the onset of inflammatory responses or tumor development and the workplace exposure to TiO2.

"The EU was the first body to state guidelines for the safe use of TiO2 (as a UV filter) in cosmetic products. The SCCNFP approved a maximum concentration of TiO2 at 25% in the products in October 2000," informs Dr. Andrea Suarez, MD, Colorado, USA.

"Moreover, after reviewing the reports given by IARC on TiO2 being a possible carcinogen (2006), it added another part to its previous statement. That being, the use of nano-material TiO2 in the form of spray products is not safe because it may expose the lungs of the consumer to the concerned ingredient, using inhalation." 

All about the topical use of titanium dioxide 

Titanium dioxide comes as a vital component of eye shadow, blush, loose and pressed powders, other cosmetic products, etc.

When it pertains to skincare, TiO2 plays an active ingredient in the composition of sunscreens, and now, mostly in the nano grade format. Lotions, creams, gels, oils, and even sprays, are the different ways in-store to make topical use of sunscreens. 

Note:

When a sunscreen product marks TiO2 as an active ingredient, it should be mentioning the exact concentration of TiO2 used in it.

According to the latest report (2019), in the case of psoriatic skin (one with a compromised skin barrier), the nanoparticles reach a level deeper but never come in contact with the living cells when treated with a sunscreen having TiO2. Their reach is limited to the outer layers of skin.

The nano-grade TiO2 sunscreen gives you broad-spectrum protection, i.e., it defends you against UVA, UVB, and UVC (though the majority of UVC is blocked by the earth's atmosphere itself.)

It is much suited for people having sensitive and redness-prone skin. It can be easily used around the eyes.
The other common ingredients you will find in a sunscreen alongside TiO2 are - stearic acid, isostearic acid, dimethicone polymer, polyhydroxy stearic acid, etc.

Iron oxide, a UV light blocking agent, is also present in most of the sunscreens. It reduces the white opaque layer on the skin after you put on a TiO2 sunscreen. It also upgrades the active ingredient's efficacy (TiO2), helping it deliver maximum sun protection. 

Interesting fact:

A sunscreen absorbs the UV light (falling on your skin) in the high energy state and converts it into the low energy state. Thereby releasing it to the surroundings, without any harm caused to the skin cells.

Precautionary measures, you should follow. 

Experts always advise avoiding loose powders and blushes that contain TiO2. Pressed powders, too, should be handled with care.

It is better not to use the spray-on sunscreens because you may inhale some of the particles.

Lip balms also contain the nano grade TiO2. You may ingest some of it, and the particles reach your internal system. Yet, most of the experiments point out that such oral exposures don't cause much trouble, as the penetration of the particles through the oral path is very limited.