Bill Fox, Altairnano Inc./Aleks Stefaniak, Mark Hoover, NIOSH Photo
An image taken with a scanning electron microscope of agglomerated ultrafine titanium dioxide particles.
In a set of recommendations that could have far-reaching implications, the National Institute for Occupational Safety and Health has concluded that airborne super-small particles of titanium dioxide “should be considered a potential occupational carcinogen.”
The new document, called a “current intelligence bulletin,” outlines the agency’s suggestions for exposure levels that will help workers avoid long-term problems. Tucked inside the document are several tidbits that might foreshadow how federal regulators will approach the difficult task of setting rules for ultra-tiny “nanoparticles” of a wide range of substances.
For the first time, the NIOSH recommendations make a specific distinction between inhaling very small particles of titanium dioxide — the agency refers to them as “ultrafine” — and larger particles. After reviewing reams of studies about titanium dioxide, NIOSH scientists concluded that the ultrafine particles are more worrisome for workers.
The bulletin is emphatic that the “conclusions derived here should not be inferred to pertain to nonoccupational exposures,” and that the recommendations involve only airborne particles, which can be inhaled into the lungs. Titanium dioxide is widely used in commercial products, including paint, toothpaste and sunscreen, although generally in formulations that would make it very difficult to inhale the material.
But the observation that size may matter more than substance is one that nanotech-watchers have been waiting for. Whether it percolates into the deliberations of other agencies is an open question.
NIOSH’s approach — seeing size, and surface area, as the prime driver for whether a material is hazardous — is “I think more clearly stated and defended and justified here than I’ve seen before,” said Richard Denison, a senior scientist at the Environmental Defense Fund who closely follows regulatory policy involving nanomaterials.
Nanotechnology leverages super-small particles (a nanometer is a billionth of a meter) to create products with amazing properties. These materials can make bike frames lighter and stronger and sunscreen more transparent on the skin, as well as new medical instruments and medicines that can save lives.
There is broad agreement that nanomaterials hold great promise for a wide variety of applications. But shrinking these substances can change their properties; scientists are struggling to figure out whether, how and why that shift can make them dangerous in the process.
For example, a recently published study found that when pregnant mice were injected with nano-sized titanium dioxide and silica particles, the particles crossed the placenta barrier, and the mice had smaller fetuses. Such studies are increasingly common, although it’s unclear how much they tell us about the risks to people from consumer products.
Scientists and regulators know they’re already behind: More than 1,300 products containing some kind of nanomaterial are already on the market, according to the Project on Emerging Nanotechnologies at the Woodrow Wilson Center for International Scholars.
NIOSH, a division of the Centers for Disease Control, has been in the forefront of nano-related policy, at least where workplace safety is concerned. The new guidelines on titanium dioxide have been in the works since 2005. Late last year, the agency released a draft version of a similar document on occupational exposure and carbon nanotubes; a final version is expected soon.
For airborne titanium dioxide, NIOSH’s recommendation is a limit of 2.4 milligrams per cubic meter of air for larger particles, and .3 milligrams per cubic meter for air for “ultrafine” or nano-sized particles, defined as under 100 nanometers in size (that figure is the traditional threshold for a substance to be considered a nanoparticle). Those exposures are predicated on a 10-hour day during a 40-hour workweek, according to the recommendations.
Charles Geraci, coordinator of NIOSH’s Nanotechnology Research Center, said the agency’s recommendations are focused on the safety of those who are exposed to titanium dioxide during manufacturing, not while using consumer products.
Geraci said that while NIOSH is targeting manufacturers and workers with these guidelines, the agency can and does “collaborate and cooperate” with regulators that have jurisdiction over everyday products.
“We have been in discussions with other government agencies who do have broader consumer or public health responsibility,” he said.
The International Agency for Research on Cancer, an arm of the World Health Organization, classifies titanium dioxide as a possible human carcinogen. NIOSH reached its own conclusion separately, Geraci said.
He said that while the agency has made research on nanoparticles a priority, scientists weren’t necessarily expecting to find the distinctions among different sizes of titanium dioxide particles.
“You do see a difference and there is a different correlation,” Geraci said.
Philip Lippel, a member of the advisory board for the NanoBusiness Commercialization Association, an industry group, said the NIOSH recommendations should come as a relief to manufacturers. The report says that the hazards of exposure can be controlled through conditions that are “relatively readily obtained,” he said. Those methods are probably already in place in most, if not all, manufacturing plants, he said.
“This thing makes pretty clear that there’s no toxicity in any method except inhalation,” Lippel said.
Denison said the NIOSH guidelines are interesting for two other reasons. First, the report rejects an assertion, often made by manufacturers, that the hazard of titanium dioxide differs depending on its crystalline structure. Second, the guidelines suggest treating coatings used on the substance as basically irrelevant, and that risk evaluation should be done based on the underlying material.
Lippel said the coating comments caught his eye, too.
“When people coated it, sometimes it got worse, but nobody saw it get better,” he said.
Lippel said “there’s nothing particularly new” about the findings.
Denison called NIOSH’s approach a kind of “back to basics” take, though it’s unlikely to have much of an effect on the debate over whether creams or sunscreens are dangerous, since NIOSH’s warnings are about inhalation exposure over a long period of time.
But the ideas — chiefly that size is a major factor — could be applied to all kinds of other substances, Denison said.
The regulation of nanomaterials is almost nonexistent. According to critics, that’s mostly because the laws are aimed at evaluating chemicals, not different forms of a substance. If size determines what a substance does inside the body of people or animals, or in the air, soil, or water, then their logic is that a regular-sized particle may be fine, but a nano-sized one is not.
The NIOSH recommendations reflect that thinking, Denison said.
“This is reinforcing that actually risks that fall along a continuum but are greatly exacerbated as you get to smaller and smaller particle sizes are relevant and do require a different approach,” he said.
