In accordance with the Technical Rules for Hazardous Substances "Workplace limit values" (TRGS 900), the general dust limit value is the workplace limit value for sparingly soluble and insoluble dusts without specific toxicity. For dusts with specific toxicity, it should be considered as an upper limit. For dusts with produced nanomaterials, BekGS 527 applies accordingly .
In 2011, NIOSH, the US OSH institute, proposed recommended exposure limits of 2.4 mg/m³ and 0.3 mg/m³ for fine (> 0.1 µm) and ultrafine (including intentionally produced ultrafine) titanium dioxide respectively . British Standard BSi PD 6699-2:2007, "Nanotechnologies – Part 2: Guide to safe handling and disposal of manufactured nanomaterials" , adopts a pragmatic approach, proposing "benchmark exposure levels" in order for a defensible safety level to be attained. These values, however, do not offer the same safety as health-based workplace limit values. Based upon the NIOSH proposal, 0.066 times the workplace limit value is for example recommended as the mass concentration for insoluble nanomaterials. As an alternative, the lower limit for the ubiquitous concentrations in contaminated areas of 20 000 particles/cm³ is proposed as a benchmark. The authors probably had in mind a diameter range for which this maximum concentration should apply; such a range is not stated in the document, however.
A value of 10 000 fibres/m³ is recommended for fibrous nanomaterials, with reference to the British guide value for asbestos during remediation work.
A pragmatic proposal for assessment of the effectiveness of protective measures must take account of the following requirements, which if applied consistently are to some extent contradictory:
The OECD's Working Party on Manufactured Nanomaterials has agreed upon a prioritized list of nanomaterials which are to be addressed, and has revised this list again at its seventh conference . The table shows the particle number concentration for the majority of these materials (and also for typical respirable dust ) which is necessary in order for a mass concentration of 0.1 mg/m³ to be reached at a given dimension of the particles (20, 50, 100, 200 nm).
|Name||Density in kg/m³||N in cm-3|
|N in cm-3|
|N in cm-3 at|
|N in cm-3|
|CNT, com- |
|110||217 029 468||13 889 886||1 736 236||217 029|
|1 050||22 736 420||1 455 131||181 891||22 736|
|CNT1||1 350||17 683 883||1 131 768||141 471||17 684|
|nanoGBD2||1 500||15 915 494||1 018 592||127 324||15 915|
|1 650||14 468 631||925 992||115 749||14 469|
|2 500||9 549 297||611 155||76 394||9 549|
|Titanium dioxide||4 240||5 630 481||360 351||45 044||5 630|
|Zinc oxide||5 610||4 255 480||272 351||34 044||4 255|
|7 300||3 270 307||209 300||26 162||3 270|
|Iron||7 874||3 031 908||194 042||24 255||3 032|
|Silver||10 490||2 275 809||145 652||18 206||2 276|
|Gold||19 320||1 235 400||79 083||9 885||1 236|
N: particle number concentration required for attainment of a mass concentration of 0.1 mg/m³ with particles of the stated size in nm.
1 In order to illustrate the relationship between the particle number and the dimensions and density of the materials, two different densities of CNTs are used for calculation. The density of 1350 kg/m³ approximates to the density of CNT as a substance. The density of "CNT as a commercial product" approximates to the agglomerate density of the matted microscale agglomerates forming the basis of the commercial product, and is used as such in the work by Pauluhn (2011) . As the agglomeration size falls, this value approaches the substance density again. The density of inhaled CNTs and how a dose is then to be determined in relation to the mass was called into question by Oberdörster in a lecture in 2011 .
2GBD: granular bio-resistant dusts
The substances stated in the OECD list for which model calculations have not been performed here are:
As can be seen from the table, for 200 nm gold particles, a concentration of 1 236 of these particles per cm³ of air would result in a mass concentration of 0.1 mg/m³. Application of the value of 20 000 particles/cm³, as stated in the BSi PAS standard referred to above, to gold particles with a size of 200 nm results in a mass concentration of approximately 1.6 mg/m³. This concentration is above the existing general dust limit value for the respirable dust fraction, and is substantially higher than the threshold value currently under discussion, which is intended to prevent the inflammatory effects of the biopersistent granular dusts.
For all substances with a particle size of 200 nm and a density greater than 1, it may be assumed that a particle concentration of 20 000/cm³ corresponds to a mass concentration (or a multiple of it) of 0.1 mg/m³. Conversely, 20 000 gold particles with a size of 20 nm per cm³ of air corresponds to a mass concentration of only 0.0016 mg/m³. This would be substantially below the respirable dust limit value. At the same time, a concentration of 1 235 400 gold particles (with a size of 20 nm) per cm³, equivalent to 0.1 mg/m³, would be readily measurable and could be substantially reduced with application of the precautionary principle by engineered measures.
The table also shows that the range in both the size of the nanoparticles and their density over more than one order of magnitude results in a range in particle number concentration of over five orders of magnitude. This cannot be covered by current instruments. The size and density of the nanoparticles must therefore be employed as classification criteria for derivation of the recommended exposure limits.
In view of the prevailing uncertainty concerning the effect of nanoparticles and the need to find pragmatic solutions for company level, the IFA proposes the following recommended benchmark levels as increases over the background exposure during an entire shift (8 hrs) for monitoring the effectiveness of protective measures in the plants, based upon its experience in measurement and the detection limits of the measurement methods currently employed. Information on the measurement of this background exposure can be found in the proposal for a tiered type measurement strategy . These recommended benchmark levels are geared to minimizing the exposure in accordance with the state of the art, and are not substantiated toxicologically. Even where these recommended exposure levels are observed, a health risk may still exist for the employees. Depending upon the chemical composition, occupational exposure limit values for particular substances may have to be considered.
Reception and discussion of the proposed assessment metrics
Since the first proposal for assessment metrics was made in mid-2009, a number of limit values have been published at national and international level. In 2009 for example, the MAK Commission published a health-based value of 0.1 mg/m³ for zinc oxide (respirable fraction) in consideration of the effect of zinc oxide smoke . The value proposed by the IFA for zinc oxide nanoparticles of 40 000 particles/cm³ approximates to this mass concentration; for particles smaller than 100 nm, the value remains substantially below this mass concentration.
As already reported, NIOSH proposed a value of 0.3 mg/m³ in 2011 for ultrafine/nanoscale titanium dioxide, based upon toxicological findings for the avoidance of lung cancer. For titanium oxide particles 100 nm in size, this would correspond to a particle number concentration of approximately 135 000 particles/cm³. This value can be monitored easily using current measurement technology, and further reduction appears possible. Conversely, the particle number concentration of 40 000 particles/cm³ for particles 50 nm in size proposed by the IFA corresponds to a mass concentration of 0.011 mg/m³. This is therefore also substantially below the value proposed by NIOSH.
The Dutch parliament charged the Knowledge and Information Centre Risks of Nanotechnology (KIR-nano), in co-operation with the National Institute for Public Health and Environment (RIVM), with the task of examining whether provisional reference values could be derived for nanomaterials. The experts concluded that at the present time, they were not aware of any method better than that of the IFA for deriving provisional reference values for nanomaterials . In August 2010, nano reference values (NRVs) based upon the IFA concept were proposed by the Social and Economic Council (SER), an advisory comittee to the Dutch govenrnment and Parliament, and recommended to companies for application until such time that health-based limit values have been defined . At the same time, the Dutch government sponsored a project for evaluating the facility for implementation of the proposed values. The results of this project and the general suitability of the nano reference values were presented at a workshop in the Hague in September 2011 . It was reported that the IFA's concept for evaluation of the protective measures had proved effective in the Netherlands under the name "Nano Reference Values".
In 2008, BASF SE supplied information to the US Environmental Protection Agency in accordance with the provisions (Section 8e of the Toxic Substances Control Act) on a sub-chronic inhalation study on Wistar rats involving carbon nanotubes . BASF SE states that under the study conditions described, the NOEL must be below 0.1 mg/m³. Based upon this information, Nanocyl in Belgium reports a value of 0.0025 mg/m³ for the MW CNTs which it manufactures .
In the CIB of December 2010, NIOSH recommends limiting the concentration of carbon nanotubes in workplace atmospheres to below 0.007 mg/m³, measured in terms of elementary carbon in accordance with NIOSH method 5040. Information on comprehensive protective measures can be found in Chapter 6 of the CIB.
In short, it can be said that the sampling and measurement methods for CNTs are still at the scientific test stage. Suitable, simple methods for practical monitoring of exposure in commercial enterprises do not yet exist. The implementation of protective measures is therefore all the more important.
 Tonerstäube am Arbeitsplatz (in German)
 Pauluhn, J.: Poorly soluble particulates: searching for a unifying denominator of nanoparticles and fine particles for DNEL estimation. Toxicology 2011 Jan 11; 279(1-3):176-88
 Oberdörster, G.: Nanotoxicology: Hype and Reality, Concepts and Misconceptions,Real and Perceived Risks. 5th International Symposium on Nanotechnology OEH, 12 August 2011
 Tiered Approch to an Exposure Measurement and Assessment of Nanoscale Aerosols Released from Engineered Nanomaterials in Workplace Operations (PDF, 2,5 MB). Presented by: IUTA, BAuA, BG RCI, VCI, IFA, TUD
 Zink und seine anorganischen Verbindungen (MAK Value Documentation in German language, 2010)
 Tijdelijke nano-referentiewaarden. Bruikbaarheid van het concept en van de gepubliceerde methoden (PDF, 286 KB). RIVM Rapport 601044001/2010
 Provisional nano reference valuesfor engineered nanomaterials. Advisory Report. Sociaal-Economische Raad, März 2012
 van Broekhuizen, P.; van Veelen, W.; Streekstra, W.-H.; Schulte, P.; Reijnders, L.: Exposure Limits for Nanoparticles: Report of an International Workshop on Nano Reference Values. Ann. Occup. Hygiene 56 (2012), No. 5, pp. 515-524
 Schulte, P. A. et al.: Occupational exposure limits for nanomaterials: state of the art. J. of Nanoparticle Research 12 (2010) No. 6, pp. 1971-1987