The National Institute for Occupational Safety and Health set two recommended exposure limits for titanium dioxide April 18, one for fine and one for ultrafine materials.
The exposure limit for ultrafine materials would be the first time such limits were applied to nanoparticles.
The recommended limits, 0.3 milligrams per cubic meter for ultrafine and nanoscale particles and 2.4 milligrams per cubic meter for fine particles, were set forth in a NIOSH “current intelligence bulletin,” Occupational Exposure to Titanium Dioxide, which also reviews carcinogenicity data, exposure monitoring techniques, and control strategies. It comes on the heels of a draft NIOSH limit for carbon nanotubes issued in December (34 CRR 1169, 12/6/10).
Titanium dioxide is an insoluble white powder used in commercial products, such as paint, plastics, and food; it is not known how many workers are exposed to the compound, the institute said.
NIOSH Director John Howard said in an April 18 statement, “As NIOSH continues to conduct research into the occupational health implications of nanotechnology, guidance of this nature will play an increasingly important role in fashioning protective occupational safety and health practices.”
The Occupational Safety and Health Administration's permissible exposure limit for titanium dioxide is 15 milligrams per cubic meter.
The decision to promulgate two recommended exposure limits was made for several reasons, including evidence suggesting the genotoxicity of titanium dioxide is related to particle surface area rather than the compound itself.
The institute found insufficient data to suggest fine titanium dioxide causes cancer, pointing to a lack of workplace studies, and only some animal studies that showed increased tumors at 250 milligrams per cubic meter, the document said.
“The potency of ultrafine [titanium dioxide], which has a much higher surface area per unit mass than fine [titanium dioxide], was many times greater than fine [titanium dioxide], with malignant tumors, with malignant tumors observed at the lowest dose level of ultrafine [titanium dioxide] tested (10 milligrams per cubic meter),” it said.
Furthermore, “because agglomerated ultrafine particles are frequently measured as fine-sized but behave biologically as ultrafine particles due to surface area of the constituent particles, exposures to agglomerated ultrafine particles should be controlled to the ultrafine [recommended exposure limit],” the document added.
NIOSH added its findings suggest other poorly soluble, low-toxicity particles, such as coal dust and barium sulfate, among others, could pose similar hazards as titanium dioxide.
NIOSH recommended a multi-tiered exposure assessment to analyze airborne titanium dioxide levels, as personal sampling devices cannot detect ultrafine particles of the aerosol compound.
Initial exposure assessment should include two simultaneous measurements, the institute said.
One sample should use a hydrophobic filter while the other sample should use a mixed cellulose ester filter (MCEF).
Monitors should then evaluate the duplicate respirable sample collected on a MCEF using transmission electron microscopy to size particles and determine the percentage of fine and ultrafine titanium dioxide, it added.
Meanwhile, workplaces should use the traditional hierarchy of controls to reduce exposures to the material, NIOSH said, with a primary emphasis on engineering controls.
Employers should use ventilation systems, administrative controls reducing the number of employees exposed to particles, and respirators or other personal protective equipment as necessary, in that order, it said in the document.
By Greg Hellman
A copy of NIOSH's current intelligence bulletin, Occupational Exposure to Titanium Dioxide, is available at http://op.bna.com/env.nsf/r?Open=sbra-8g9ucx.
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