Criteria for assessment of the effectiveness of protective measures

Measurement of the particle number concentration

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Measurement of the particle number concentration during the use of nanomaterials
Source: IFA

Limit values in Germany

In accordance with the TRGS 900 technical rule for hazardous substances (workplace limit values), the general dust limit value does not apply to ultrafine (nor, it may also be said, to the nano-) particle fraction. In consideration of the possible reduction referred to above, it should nevertheless be regarded as an upper limit.

The German Federal Institute for Occupational Safety and Health (BAuA) has conducted a risk assessment for exposure to toner emissions from photocopiers. When the risk limits passed by the committee for hazardous substances of the German Ministry of Labour and Social Affairs (TRGS 910) [1] for tasks involving carcinogenic substances are applied, the following concentration values are determined for respirable biopersistant granular particles of toner: a tolerable risk of 0.6 mg/m³, acceptable risk currently of 0.06 mg/m³, and as of 2018, of 0.006 mg/m³ [2].

Proposals for limit values in the USA and UK

Binding limit values for nanoparticles are likewise lacking at international level. 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 [3]. British Standard BSi PD 6699-2:2007, "Nanotechnologies – Part 2: Guide to safe handling and disposal of manufactured nanomaterials" [4], 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.

Requirements for a provisional assessment criterion

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:

  • Where as a result of inadequate information on a product, a substance must be assumed to have a hazardous effect, the precautionary principle must be applied (in accordance with EU Communication of February 2000) [5].
  • Under no circumstances may the general dust limit value be exceeded as an upper limit.
  • Consideration must be given to the state of the art. In particular, no average shift values should be proposed which could easily be under-run by means of engineered measures.
  • The proposed recommended exposure limit value must permit simple technical monitoring. More far-reaching, complex, imaging study methods, such as scanning electron microscopy, cannot be employed in routine operations.

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 [6]. The table shows the particle number concentration for the majority of these materials (and also for typical respirable dust [7]) 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
20 nm
N in cm-3
50 nm
N in cm-3 at
100 nm
N in cm-3
200 nm
CNT, com-
mercial product
110 217 029 468 13 889 886 1 736 236 217 029
1 050 22 736 420 1 455 131 181 891 22 736
CNT2 1 350 17 683 883 1 131 768 141 471 17 684
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 At the seventh conference, polystyrene was deleted from the list.

2 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) [8]. 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 [9].

The substances stated in the OECD list for which model calculations have not been performed here are:

  • Carbon black, the true density of which based upon its microcrystallinity is approximately 1 850 kg/m³, whereas the density of pelletedagglomerates is in the region of 100 to 500 kg/m³.
  • The density of layered silicates (nanoclays) and silicon dioxide generally ranges from 2 200 kg/m³ (amorphous) to 2 650 kg/m³ (crystalline), and is thus in the magnitude of typical respirable dust.

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 in the region of 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 [10]. 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.

  1. For metals, metal oxides and other biopersistent granular nanomaterials with a density of > 6 000 kg/m³, a particle number concentration of 20 000 particles/cm³ in the range of measurement between 1 and 100 nm should not be exceeded.
  2. For biopersistent granular nanomaterials with a density below 6 000 kg/m³, a particle number concentration of 40 000 particles/cm³ in the measured range between 1 and 100 nm should not be exceeded.
  3. In our view, a substantial need for discussion remains with regard to evaluation of the effectiveness of protective measures against nanoscale particles or agglomerates/aggregates larger than 100 nm. For 500 nm titanium dioxide aggregates, 360 particles/cm³ corresponds to a mass concentration of 0.1 mg/m³. Measurement of this number concentration by means of the existing instruments would necessitate virtually clean-room conditions. In a typical industrial environment, this concentration would no longer be detectable, owing to the ubiquitous background level of 20 000 particles/cm³ or more. The corresponding mass concentration of 0.1 mg/m³ titanium dioxide can however be determined reliably by means of conventional analysis methods for documentation of the in-plant conditions. For 200 nm titanium dioxide aggregates, 20 000 particles/cm³ corresponds to a mass concentration of 0.35 mg/m³. This already exceeds the value of 0.3 mg/m³ proposed by NIOSH for nanoscale titanium dioxide (refer to the chapter entitled "Proposals for limit values in the USA and UK").
  4. Owing to the mounting evidence that biopersistent CNTs which satisfy the WHO fibre definition or have similar dimensions may harbour effects similar to those of asbestos, we urgently recommend that only CNTs be used that have been tested for this end point (according to the manufacturer's declaration) and which do not exhibit these properties. For carbon nanotubes (CNTs) for which no such manufacturer's declaration is available, a provisional fibre concentration of 10 000 fibres/m³ is proposed for assessment, based upon the exposure risk ratio for asbestos [11]. In addition to use of state-of-the-art protective measures, the wearing of respiratory protection and protective clothing is advisable even if the recommended exposure limits are observed. The demarcation of contaminated and non-contaminated zones should be reviewed.

    At present, however, monitoring of the above value in plants is hampered by a lack of collection methods of verified suitability, corresponding analysis methods, and criteria for counting of the fibres and determining of the fibre count concentration. An urgent need exists here for the development of analysis methods and conventions for interpretation.

    For a transitional period, a particle number concentration of 20 000 particles/cm³ should not be exceeded. In a worst-case scenario, however, this would correspond to a fibre concentration of 20 billion fibres/m³, and illustrates that the existing methods for determining the particle number concentration of CNTs at the workplace are unsatisfactory. Some companies have employed internal guide values as mass concentration values based upon the residual content of metallic catalysts in the CNTs.
  5. For ultrafine liquid particles (such as fats, hydrocarbons, siloxanes), the applicable maximum workplace limit (MAK) or workplace limit (AGW) values should be employed owing to the absence of effects of solid particles.
  6. The recommended benchmark levels stated above should not be applied to ultrafine particles (see definition). For some processes and technologies in which ultrafine particles are produced, proven protective measures and binding provisions exist for the handling of them. Welding fumes and diesel-engine emissions are examples. The body of rules and regulations which exists in this area and which has been drawn up with reference to the current state of knowledge should be applied until new findings become available.

Under no circumstances should the recommended benchmark levels proposed here be confused with health-based workplace limit values. These recommended benchmark levels and the measurement methods and strategies upon which they are based must be trialled in practice, and if necessary adjusted in consideration of new findings. The IFA will seek discussion of this issue in a suitable form with the users.

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 [12]. 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 [13]. 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 [14]. 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 [15]. 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 [16]. 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 [17].

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.

Further information

[1] TRGS 910: Risk-related concept of measures for activities involving carcinogenic hazardous substances

[2] Tonerstäube am Arbeitsplatz (in German)

[3] NIOSH Current Intelligence Bulletin: Evaluation of Health Hazard and Recommendations for Occupational Exposure to Titanium Dioxide (PDF, 3,07 MB)

[4] Guide to safe handling and disposal of manufactured nanomaterials

[5] Communication from the Commission on the precautionary principle

[6] List of Manufactured Nanomaterials and list of endpoints for phase one of the sponsorship programme for the testing of manufactured nanomaterials: revision (PDF, 244 KB). OECD

[7] IFA-Arbeitsmappe: Ultrafeine (Aerosol)- Teilchen und deren Agglomerate und Aggregate (in German)

[8] 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

[9] Oberdörster, G.: Nanotoxicology: Hype and Reality, Concepts and Misconceptions,Real and Perceived Risks. 5th International Symposium on Nanotechnology OEH, 12 August 2011

[10] 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

[11] Exposure-risk relationship for asbestos in BekGS 910 (PDF, 12 KB)

[12] Zink und seine anorganischen Verbindungen (MAK Value Documentation in German language, 2010)

[13] Tijdelijke nano-referentiewaarden. Bruikbaarheid van het concept en van de gepubliceerde methoden (PDF, 286 KB). RIVM Rapport 601044001/2010

[14] Provisional nano reference valuesfor engineered nanomaterials. Advisory Report. Sociaal-Economische Raad, März 2012

[15] 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

[16] Inhalation toxicity of multi-wall carbon nanotubes in rats exposed for 3 months (PDF, 2,4 MB). Study of BASF

[17] 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


Christian Schumacher

Division 3: Hazardous substances: handling, protective measures

Tel: +49 2241 231-2823
Fax: +49 2241 231-2234

Dr Dirk Pallapies

Institut für Prävention und Arbeitsmedizin der Deutschen Gesetzlichen Unfallversicherung (IPA)
Bürkle-de-la-Camp-Platz 1
44789 Bochum

Tel: +49 234 3024-501
Fax: +49 234 3024-505