Criteria for assessment of the effectiveness of protective measures

Limit values in Germany

The carcinogenic action of biopersistent granular particles of undefined toxicity is currently the subject of intense discussion in German scientific circles. This discussion could result in a drastic lowering of the general dust limit value from its present level of 3 mg/m³ for the respirable fraction.

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 (Announcement 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 [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).

N: particle number concentration required for attainment of a mass concentration of 0.1 mg/m³ with particles of the stated size in nm.

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 pelleted agglomerates 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 limits 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:

1. These recommended benchmark limits are geared to minimizing the exposure in accordance with the state of the art, and are not substantiated toxicologically. Even where these recommended exposure limits are observed, a health risk may still exist for the employees.
2. 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.
3. 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.
4. 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.
5. 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). 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 [8]. 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.
6. 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.
7. The recommended benchmark limit values 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 limit values proposed here be confused with health-based workplace limit values. These recommended benchmark limit values 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.