Are Titanium Dioxide Nanoparticles in Food a New Health Risk?
A recent blog post in Mother Jones by Tom Philpott stokes concern that we should all worry about the apparent new trend of food products that contain nanoparticles. This argument boils down to a few specific points that I want to briefly summarize before continuing to examine them one by one:
- Nanoparticles have unique properties and we should be concerned about exposure.
- Food companies (and other consumer product companies) have recently started adding nanoparticles to our food (or other products).
- This is an entirely new kind of exposure that we don't know enough about.
There are concerns behind these points that bear careful watch, and the risks of nanomaterials are one of the hot topics in current research, but we shouldn't take up our torches and pitchforks just yet.
The Unique Properties of Nanoparticles
First, the Mother Jones article illustrates the unique properties of nanoparticles in consumer products by referencing socks and underwear that utilize silver nanoparticles for their antibacterial properties.
"It turns out that if you break common substances like silver and nickel into really, really tiny particles—measured in nanometers, which are billionths of a meter—they behave in radically different ways. For example, regular silver, the stuff of fancy tableware, doesn't have any obvious place in sock production. But nano-size silver particles apparently do. According to boosters, when embedded in the fabric of socks, microscopic silver particles are 'strongly antibacterial to a wide range of pathogens, absorb sweat, and by killing bacteria help eliminate unpleasant foot odor.' "
This anti-microbial property of silver is not unique to silver nanoparticles, but to anything made out of silver (including those silver teaspoons, which are just kind of awkward in socks). Silver metal when dissolved as ions into a liquid (like sweat) is highly toxic to many single celled organisms. Just read up on the "natural medicine" hype surrounding colloidal silver. And, contrary to the author's supposition, silver thread has been used for decades when antibacterial cloth was required. Typically this has been limited to hospital wards with patients experiencing extreme immunodeficiency, as well as astronauts who have no means to launder their clothing, and a trip to the cleaners is cost prohibitive. In this case, nano-silver particles allow these products to be as effective as before, but with a drastically reduced total silver content. Since, silver is expensive, a means of increasing the surface area per unit of mass is highly cost effective without reducing functionality of the product. Most regular consumers could not afford silver thread socks, but many could afford silver nanoparticle socks (if you really want to go that long without washing).
There is a potential environmental risk to be concerned about here since silver nanoparticles can easily wash out of the socks or underwear that contain them and make their way into natural waterways (link), a problem not associated with silver thread. Silver contamination outside of photographic film factories was once a major environmental issue. However, most of us would have to be using these socks (and similar products) before it amounted to a serious threat.
That said, there are nanoparticle-specific properties that we ought to be somewhat concerned about. Some nanomaterial structures like carbon nanotubes or graphene are so rare in nature that there is a legitimate possibility that they will react in unusual and unanticipated ways with biological systems. Other nanomaterials are specifically engineered to have a completely new set of material properties. Many of these are purposefully designed to be toxic, like anti-cancer drugs for example. As more and more new types of nanomaterials are invented, we need to put in place the risk assessment procedures to quickly determine which nanomaterials may be a problem and which are likely to be safe.
Have Companies Started Adding Nanoparticles to Our Food?
Almost all of the current concern with nanoparticles in the food supply is linked to one specific ingredient, food grade titanium dioxide (E171). This ingredient has been approved for use since 1966, and was only recently found to contain a certain percentage (~5-40%) of nanoparticles (link). Since aggregation is common, how much remains in nanoparticle form after being added to the final product is unclear (the nanoparticles have a tendency to clump together into larger particles). The FDA, in 1966, classified E171 as "generally recognized as safe", and other than stipulating limits on contaminants in the E171, and not allowing any food product to contain more than 1% by weight of E171, has allowed it in food and drug products since. Titanium dioxide products are generally produced from ilmenite or rutile ore, processed to remove contaminants like iron, and then ground up into a fine powder.
The 1960's passed well before any of us knew or imagined nanotechnology. So then, how does this material have nanoparticles in it? They are what I refer to as "inadvertent nanoparticles". They exist as a byproduct of a grinding process and are not intentionally created, except that many manufacturers would prefer a finer powder for mixing purposes rather than a coarser powder. It is probable that the proportion of nanoparticles in E171 has somewhat increased over time as our processing technology has improved our ability to create fine mineral powders. But, it is unlikely that earlier versions of E171 had insignificant amounts of nano-TiO2. We just did not know about the proportion of nanoparticles in E171 before because we lacked the instrumentation with which to measure them.
In this case then, manufacturers have not just suddenly started to add titanium dioxide nanoparticles to our food products. They have been using E171 (titanium dioxide powder) as a pigment (whitener) or opacity additive for a generation or two, and only recently have we discovered that all this time we have been creating (and ingesting) a certain quantity of TiO2 nanoparticles.
Is Exposure to Nanoparticles Completely New and Different?
Nanoparticles are the smallest things that we could classify as solids. If we start with individual atoms, the largest atoms (like uranium) have radii of approximately 0.2 nanometers (nm). A single water molecule has a diameter of approximately 0.3 nm. By connecting as few as a hundred to a thousand atoms or molecules, we can create a nanomaterial. Much of the buzz of nanotechnology surrounds materials typically built from the bottom up, and as more and more atoms are clumped together the properties of materials that we generally associate with large solids (bulk properties) start to emerge. But, they do not all emerge at the same size, and so we can have a clump of iron atoms with certain electrical properties that we expect and magnetic properties that we don't expect. This tunability means that engineers can create materials with individually selected combinations properties, rather than having to work around the limitations of specific traditional materials.
The size of nanomaterials is also something new, at least to our capability to observe and measure. Many biological macro-molecules could be referred to as nanoparticles such as proteins, long-chain lipids and carbohydrates, etc. The human protein immunoglobluin-G at 33 nm in size is larger than the smallest virus we know of, the parvovirus at 20 nm. What this means is that cells in the human body can ingest nanoparticles, and in that sense these solids act more like chemicals or drugs than larger solid particles. A human red blood cell by comparison has a diameter of roughly 6,000 nanometers. Like drugs and proteins, our cells can ingest these nanomaterials, although whether that is a good thing (better immune cell effectiveness) or a bad thing (new exposure pathway) has yet to be completely understood.
We are all exposed to natural ambient and inadvertently created nanoparticles. Dust in the air and water, smoke, and biological materials from plants and animals all are made up of a certain percentage of nanomaterials. Engineered nanomaterials are potentially something new and they necessitate surveillance. We are just starting to understand what the natural background of nanoparticles in our environment is, and how this new technology may change that.
It is true that TiO2 (not specifically nano-TiO2) is classified as a possible carcinogen. But, this a scientific categorization based more on our inability to disprove that TiO2 exposure causes elevated cancer risk in people than it is a specific concern. While we have seen TiO2 exposure result in cancer in lab animals, these animals have relatively high levels of cancer in general and it is difficult to extrapolate to what would happen in actual people. Even though epidemiological studies have not detected any elevated cancer levels in actual people who work with TiO2 on a regular basis, it is always possible with a long enough exposure or with a high enough dose or with some complicating factor (e.g., concurrent exposure to a specific chemical agent), that cancer may occur. In a recent review I conducted with the EPA of 25 animal studies involving inhalation exposure to nano- and micro-scale titanium dioxide particles, some rats and mice experienced fibrosis at relatively high dose levels, but no malignant tumors associated with the particles were observed. Overall, the lung inflammation caused by TiO2 nanoparticles was 5%-20% greater than that caused by larger TiO2 particles (this is neither inconsequential nor dramatic). Not as much research currently exists regarding the effects of ingestion exposure, but it is generally believed to be less risky than lung exposure (and remember, there are decades of human dietary exposure history with TiO2).
Nanotechnology including things like these nanoparticles have the potential to allow us to dramatically clean up our environment by using more benign nanomaterials to take the place of toxic chemicals we currently use. Furthermore, nanomaterials have the potential to make many of our manufacturing processes more sustainable by reducing the amount of raw materials required or increasing the longevity of manufactured goods. Researchers are actively trying to understand what can make these materials dangerous to humans or the environment, what types of regulations are needed, and how they can help us eliminate our dependency on other more dangerous substances. This field bares careful watch over the coming decade.
