A study published in Environmental Science & Technology raises new concerns about the toxicity of titanium dioxide (TiO2) nanoparticles in foods and cosmetics. Titanium Dioxide Nanoparticles in Food and Personal Care Products found that children potentially consume 2−4 times as much TiO2 per kilogram by weight as an adult. The study also found that some toothpastes and sunscreens can contain over 10% titanium. TiO2 is classified by the International Agency for Research on Cancer as “possibly carcinogen to humans,” and worldwide production is in the millions of tons per year.
With those concerns in mind, I asked my colleague, Professor Maria Vittoria Diamanti, for her thoughts on the current state of TiO2 toxicology. While not a toxicologist herself, Professor Diamanti is a faculty member at the Politecnico di Milano and author of numerous studies on TiO2, particularly in building applications.
GTF: Should we be concerned about the seemingly large quantities of TiO2 that children are ingesting, as this study reports?
MVD: Titanium dioxide, by itself, is considered to be a biologically inert material, as it is a naturally occurring compound. Yet, concern is growing about its nanosized forms. Animal and human studies show that inhaled nanoparticles (NPs) are less efficiently removed than larger particles by the macrophage clearance mechanisms in the lungs, causing lung damage, and that NPs can translocate through the circulatory, lymphatic, and nervous systems to many tissues and organs, including the brain. From this point of view, shape and structure have recently appeared to be even more determining than dimension: the generally round and equiaxial shape of TiO2 NPs, and the low-reactivity structure of powders used in such commercial items, surely helps in reducing the associated hazard.
GTF: The study cites other studies reporting risks from nanoparticular TiO2 due to inhalation, including inflammation and a possible link to asthma. Is inhalation a greater concern than ingestion?
MVD: In case of inhalation, this issue is more or less the same encountered in any situation of long-time dust inhalation, as in the case of people working in a dusty environment. After inhalation of NPs, macrophages and epithelial cells in the respiratory system that line the lungs may come into direct contact with them: further translocation to the lymphatic system could induce secretory immune responses.
Although inhalation is the most vulnerable entrance point, it probably exerts the lowest risk, given the low exposure time for common people; conversely, it can be a hazard for people working in the manufacturing of TiO2 NPs, or TiO2 containing materials. However, the human studies conducted so far do not suggest an association between occupational exposure to TiO2 and an increased risk for cancer.
GTF: How about absorption through the skin, as in the case of sunscreens?
MVD: Current experiments performed on TiO2-containing creams show that NPs do not penetrate through healthy skin, thus do not reach viable skin cells and distribute to other organs and tissues. For this reason, TiO2 nanoparticles contained in sunscreens or in other cosmetics that will be necessarily be exposed to sunlight usually have rutile structure, and several inorganic coatings are applied (magnesia, silica, etc) in order to shield the skin itself from the photoactivity of UV-irradiated TiO2.
In fact, in the particular case of TiO2 NPs, the interaction mechanism that generates the highest concern is the oxidative stress caused by the production of reactive oxygen species (ROS) and oxidative products, which are responsible for many inflammatory responses: their formation is mediated by TiO2 itself, specially if in the anatase structure, under UV irradiation.
GTF: The study estimated that for 4 years old children, the average daily exposure to TiO2 from food products could be up to 3 mg per kilogram of body weight. That sounds like a lot of titanium.
MVD: The National Cancer Institute tested TiO2 for possible carcinogenity by the oral route of exposure by feeding rats and mice approx. 5% of TiO2 in their daily diet for 2 years, and concluded that there was no evidence of carcinogenity. Yet, no information was given on particle size, therefore deeper studies are still needed to evaluate the outcomes in different exposure conditions. It is particularly important to achieve such information, since children are the main target for TiO2 exposure by ingestion, being the main sources of TiO2 candies, chewing gums and beverages.
GTF: Finally, the study points out that nearly 70% of all TiO2 produced is used as a pigment in paints, and that while several studies have indicated that TiO2 tends to be less hazardous to organisms than other nanomaterials such as multiwall carbon nanotubes, nanocerium oxide, and nanozinc oxide, others have shown it can bioaccumulate in plankton and inhibit algae growth.
MVD: The possible release of titanium dioxide NPs from the built environment (chalking effects of photocatalytic paints, plasters, raw concrete) may be a source of contamination of urban waters, leading to ecotoxicological issues on aquatic life. In this case, short term exposures revealed no toxic effects on aquatic life, but long term exposure should also be evaluated, due to the persistence and poor solubility of TiO2. Scarce information is available in the scientific literature; yet, the potential exposure dosages are extremely low.
Also the aerosol effect of NPs released from the building façade into the surrounding atmosphere can be considered a source of NP pollution, but the limited quantity of NPs released should not be considered a concern.
Professor Diamanti will be presenting her research on titanium dioxide nanoparticles in building materials at the 2012 Cleantech Conference in Santa Clara, California.
She also provided Green Technology Forum with an excellent bibliography for further study:
IARC Monographs on the Evaluation of Carcinogenic Risks to Humans, Volume 93: Carbon Black, Titanium Dioxide, and Talc (Lyon, France, 2010). Published by the World health organization international agency for research on cancer.
Alex Weir, Paul Westerhoff, Lars, Fabricius, Kiril Hristovski, Natalie von Goetz, Titanium dioxide nanoparticles in food and personal care product. Environmental science and technology, DOI: 10.1021/es204168d, online Jan 18 2012
Matej Skocaj, Metka Filipic, Jana Petkovic, Sasa Novak, Titanium dioxide in our everyday life; is it safe? Radiology Oncology 45 (2011) 227-247
Anne Kahru, Henri-Charles Dubourguier, From ecotoxicology to nanoecotoxicology. Toxicology 269 (2010) 105-119
Amanda M. Schrand, Mohammad F. Rahman, Saber M. Hussain, John J. Schlager, David A. Smith and Ali F. Syed, Metal-based nanoparticles and their toxicity assessment. Wiley Interdisciplinary Reviews: Nanomedicine and Nanobiotechnology 2 (2010) 544-568
R. Kaegi, A. Ulrich, B. Sinnet, R. Vonbank, A. Wichser, S. Zuleeg, H. Simmler, S. Brunner, H. Vonmont, M. Burkhardt, M. Boller, Synthetic TiO2 nanoparticle emission from exterior facades into the aquatic environment. Environmental pollution 156 (2008) 233-239.
Cristina Buzea, Ivan I. Pacheco, Kevin Robbie, Nanomaterials and nanoparticles: Sources and toxicity. Biointerphases 2 (2007) MR17-MR71
Paul M. Hext, John A. Tomenson and Peter Thompson, Titanium dioxide: Inhalation toxicology and epidemiology. The Annals of Occupational Hygiene 49 (2005) 461–472