Dr. Ivan Ong, Vice President of Research and Development, and Tara Conley, Director of Regulatory Affairs at Microban International discuss the potential safety issues posed by TiO2 when used in indoor ceramic surfaces.
What Is TiO2?
TiO2 is Titanium Dioxide, a chemical that has many industrial uses such as in paints, adhesives, floors, roofing materials, and even cosmetics.
A feature of TiO2 is its ability to interact with strong UV light and moisture to produce Reactive Oxygen Species (ROS). ROS are highly reactive transient chemical species (known as free radicals) that can react with, and oxidize, organic matter. Hence, TiO2 coatings have been used outdoors for decades to provide surfaces (such as glass and construction materials) with a “self-cleaning” attribute. In addition, these ROS species are also able to rapidly oxidize and destroy a range of microbes.
The use of TiO2 solutions in outdoor applications has a lot of precedent and those use cases are reasonable for consideration. However, the application of TiO2 solutions in surfaces meant for the indoor built environment is disquieting based on major concerns over the health and safety of both workers in the manufacturing and construction arenas, and ultimately building/home occupants.
How Is TiO2 Different from Built-In Antimicrobial Technology?
Built-in antimicrobial technologies are permanently and durably incorporated into a variety of products and can impart antibacterial, antiviral, antialgae, and antifungal attributes to the product. Unlike TiO2 technologies, built-in antimicrobials do not require the simultaneous presence of strong UV light and water to effectively kill microbes.
TiO2 Performance Concerns on Products Used in Indoor Spaces
TiO2 requires strong sunlight (in particular, the UV components of sunlight) to effectively generate ROS species to kill microbes. As such, the technology’s utility has primarily been on outdoor surfaces. This is because indoor light is of insufficient intensity to provide efficient generation of ROS species. (1)
However, in recent months, manufacturers of ceramic and glass products have increasingly offered a new variant of TiO2. By doping TiO2 with additional chemicals, they claim that surfaces treated with this new chemistry can generate ROS species under room lighting (and even, in some cases, in the dark) to kill microbes, including bacteria and viruses. Here, we list out four key concerns related to TiO2 variants and the related marketing claims.
1. TiO2 as a Carcinogen and its Potential Impact on Construction Professionals and Homeowners
IARC (the International Agency for Research on Cancer) has classified titanium dioxide as a category 2B carcinogen, which carries the designation of “possibly carcinogenic to humans” by the inhalation route of exposure. Similarly, ECHA published an opinion in 2020 that titanium dioxide should be classified as a category 2 carcinogen (suspected of causing cancer), such that the harmonized classification for TiO2 in the EU bears the Carc 2 designation.
In a ceramic tile application, TiO2 could be made readily available for exposure by inhalation through the manufacture and cutting of tiles. This could impact not only workers during product manufacture but also installation contractors and homeowners cutting and laying tiles without proper ventilation systems or PPE to minimize inhalable particulate matter. The huge levels of inhalable dust triggered by cutting and grinding of tiles would be a significant risk of exposure to a suspected carcinogenic substance by the very route of exposure that has been identified as the source of the risk.
The safety of the new class of visible light-activated doped TiO2 has not been properly studied. Given the lack of peer-reviewed safety studies, there is a risk that the use of heavy metal dopants to make TiO2 active under visible light conditions could increase the toxicity (e.g., carcinogenicity) of these new chemistries, especially when they are aerosolized during processing. As examples:
- The toxicological profiles of SnO2 and TiO2 have been assessed under REACH, but the blending of these two products together could enhance or exacerbate toxicity in either moiety. While this is not known, it stands that no toxicological assessment has been performed on the blended SnO2+ TiO2 technology to determine its safety for workers and/or consumers.
- Similarly, while SnO2 is indicated as being present in some ceramic products, it is just one of the available dopants used to modify the bandgap to increase the photoactivity of TiO2 in visible light – other dopants include, amongst other substances, silvers including nanosilver. As with SnO2, the toxicological assessment of the TiO2 paired with these dopants has not been conclusively presented to provide certainty that the blended technologies are safe for workers or consumers.
2. Nanocomposites and Toxicity
Many of the newly developed light-activated TiO2 chemistries (e.g., SnO2+ TiO2) are actually nanocomposites. Most regulatory agencies are increasingly concerned about nanomaterials because risk assessments performed on standard chemistries are not adequately representative of the toxicological profile expected of the same chemistry in its nanoform. This is due to much higher potentials for exposure by inhalation and dermal penetration. As a result, the toxicological profiles of chemistries are often underestimates of the toxicological profiles of their corresponding nanoforms.
3. Potential Danger of Constant Indoor Exposure and Presence of Reactive Oxygen Species/Free Radicals
The technology, as described by multiple manufacturers of ceramic tiles, results in the generation of free radicals which function to inhibit viruses and bacteria on the surface of the ceramic tile. However, there is not a full and comprehensive understanding of the potential toxicity expected with direct and continuous exposure to free radicals, which can react with body systems if released from the tile and permitted to penetrate and get into the bloodstream (e.g., via a barefoot).
Too many free radicals released into the body can result in oxidative stress. Similarly, free radical reactions in the body caused by exposure can trigger diseases such as atherosclerosis, asthma, and diabetes. Free radicals are also known to the beauty industry as triggers for the breakdown of skin collagen and subsequent wrinkling and sagging. A ceramic flooring system installed in a home would typically remain in situ for several years, which means many years of free radicals potentially being released into the air or onto the skin of the people walking or crawling on the surface.
4. Potential of Creating Dangerous Compounds Indoors
TiO2’s oxidizing potential has been largely confined to outdoor applications where free radicals generated are quickly reacted with atmospheric pollutants and should pose no danger to humans. The new classes of visible light-activated TiO2 technologies are now able to continuously generate free radicals indoors on an unchecked basis.
TiO2 has been touted as being able to oxidize and remove airborne pollutants such as organic molecules released from cigarettes, cooking, hair spray products, and carpet VOCs. However, there has been no rigorous research conducted to identify and assess the safety of the downstream products created as a result of the action of free radicals on these indoors organic compounds. Similarly to the point above, the reaction of free radicals with organic molecules (e.g., air pollutants) can result in free radical components of those organic pollutant molecules being generated.
There has, to the best of our knowledge, been no scientific analysis of those free radical moieties investigated or presented to show that there is no hazard to the synthesis or fate of those subsequent radicals, as they can further react with other molecules in the surrounding air, on surfaces, or even in the body. As such, there is no conclusive evidence showing that the breakdown of pollutants in the air through the free radical mechanism does not, in its function, generate downstream toxic substances of concern.
Microban® views the adoption of TiO2 technologies for ceramic products and surfaces used indoors as premature. Consumers interested in using ceramic products containing TiO2 technologies should request that manufacturers provide peer-reviewed tests and documentation to vouch for the safety of these products.
Summary of Safety Concerns:
- TiO2 has been re-classified as a suspected carcinogen.
- The potential carcinogenic impact on homeowners and construction professionals installing ceramic surfaces treated with TiO2 has not been fully studied.
- There is a lack of safety/toxicity data on the new class of visible light-activated TiO2 chemistries.
- The nanocomposite nature of the material poses a potential risky route of dermal or inhalation exposure.
- The potential long-term impact on surfaces that constantly generate free radicals in an indoor environment under room lighting has not been studied.
- There is the potential for extensive exposure profiles for finished ceramic products that will be in situ for long periods of time.