State of discussion of the effects upon health

Atmospheric pollution caused by aerosols in the environment and the adverse effect of workplace dust upon human health have been known for centuries. Exposure to dust must be regulated and monitored. A brief outline of the history and development of dust measurement technology can be found in Möhlmann [1].

Dust fractions and their limit values

With the lowering of the workplace atmospheric limit value for the respirable dust fraction (PM₄) to 3 mg/m³ and the introduction in 2004 of an occupational limit value of 10 mg/m³ for the inhalable dust fraction (PM₁₀₀), a long discussion in OSH circles of the health effects of insoluble, biopersistent particulates without substance-specific toxicity was brought to a conclusion – provisionally, as is now found. An overview of the significance of the general dust limit value can be found in the lecture texts and slides of the BIA/BG symposium on the subject of general dust limit values [2] which was held in Hennef in 2002. At that time, ultrafine particles were explicitly excluded from the scope for the defined limit values. In July 2011, the Permanent Senate Commission for the Investigation of Health Hazards of Chemical Compounds in the Work Area of the German Research Foundation (MAK Commission) proposed a reduced value of 0.3 mg/m³ for a density of 1 kg/m³ for the respirable dust fraction of the general dust limit value [3].

Parallel to activity in the OSH sector, a possible adverse effect upon health of fine or ultrafine particles in the atmosphere has also been discussed in the environmental context with increasing intensity since the 1990s. In the context of its measurement, the term for particulate matter is defined differently in the German legislation governing the environmental and OSH sectors (PM₁₀ and PM₄ respectively) [4]. Correspondingly, "ultrafine" particles may be described as PM_0.1. In the public debate, the effects of different particle size classes are frequently attributed to an unclearly defined "particulate", even though the particles belong to different size fractions.

The results of epidemiological studies

In the early 1990s, Schwartz et al. [5] and Dockery et al. [6] reported a relationship between urban atmospheric pollution, particularly with regard to PM₁₀ and PM_2.5, and the disease patterns and mortality in the urban areas concerned.

In the years which followed, similar epidemiological studies were published, resulting in stricter environmental legislation governing air quality [7]. Pope and Dockery provide a critical overview of the progress over the past two decades [8].

The results of toxicological studies

It was necessary however for the epidemiological studies to be supplemented by toxicological studies which explain how and by means of what reaction chains and mechanisms within the body the various ultrafine particles or nanoparticles exert an effect upon health, and what damage may ensue. A brief summary of the mechanisms of action of ultrafine particles is provided by Krug [9]; a wider view of the discussion is provided by the report on the BIA's experts' workshop held in 2002 [10].

Oberdörster et al. demonstrated that ultrafine carbon is not retained in the respiratory tract, but is able to migrate from it to other organs, such as the liver and possibly even the brain [11; 12]. Conversely, Möller et al. [13] found that radioactively marked ultrafine carbon particles are retained for the most part in the human lung.

Schulte et al. [14] state a number of parameters influencing the potential toxicity of nanoparticles:

• Size
• Surface area
• Geometry
• Solubility
• Surface reactivity
• Charge
• Attached functionalized groups or coatings
• Crystal structure
• Status of agglomeration
• Contamination, for example by a residual quantity of catalyst which is employed during manufacture of the nanoparticles

With reference to the incidence of lung tumours in animal experiments, Roller [15] proposes the particulate volume combined with the particle diameter as a dosage criterion. Unfortunately, the particulates employed have been fully characterized in terms of the parameters stated above in only a small number of toxicological studies.

In addition, animal experiments are problematic, since for example effects caused by the suspension medium appear to be of greater significance when such small particles are involved. Standardization of the test arrangements is still in its infancy, and is being supported for example by co-ordinating activities on the part of the EU [16, 17].

Within the OECD, the "Working party on Manufactured Nanomaterials" is studying chemical test methods for their relevance and applicability to nanomaterials [18].

Carbon nanotubes (CNTs)

In the more recent past, discussion has arisen of whether carbon nanotubes (CNTs) are capable of inducing an effect comparable to that of asbestos, where the carbon nanotubes concerned are biopersistent and correspond in their dimensions to the definition of WHO fibres (length > 5 µm, diameter < 3 µm and length-to-diameter ratio > 3:1) [19].

Poland et al. [20] examined the effect of CNTs, some of which had been specifically manufactured for the purpose, in the abdominal cavity of mice. Experiments by Takagi et al. [21], who used a particularly sensitive strain of mice, had a similar objective. Both studies gave grounds for concern that ultrafine fibres of adequate length and biopersistence may cause cancer. Carbon nanotubes which resemble asbestos fibres in their geometry and length were deliberately used in the studies. In addition, results obtained by Shvedova et al. suggest that carbon nanotubes which do not comply with the WHO fibre definition but are instead present in agglomerated form could also have an adverse impact upon health [22].

An overview of more far-reaching toxicological findings is provided by NIOSH in the "Occupational Exposure to Carbon Nanotubes and Nanofibers" Current Intelligence Bulletin of December 2010 [23], and in more generally comprehensible, abridged form, by a dossier produced in May 2011 by the Austrian nano trust project [24].

Current assessments of nanoparticles

Of relevance to the consumer is however not the question of whether materials manufactured specifically for experimentation purposes may have an adverse effect upon health in the laboratory. Of decisive importance is adequate assurance that products actually available on the market are not a source of adverse effects upon health. The European Commission therefore explains how nanomaterials are to be dealt with in the context of REACH, and will review this process continually [25]. Within the "Carbon Nanotube Innovation Alliance" [26] launched and sponsored by the German government, research ("CarboSafe") is also being funded into the effects upon human beings and the environment over the entire life-cycle of the CNTs.

Despite the prevailing uncertainty concerning the impact of nanoparticles, Schulte et al. [14] conclude that sufficient provisional information exists to justify their being treated as hazardous substances. The NanoCommission of the German government reaches a similar conclusion. In its principles for the responsible handling of nanomaterials, Working Group 3 of the NanoCommission thus states the following under Principle 4, concerning the establishment of risk management:

"Until scientifically validated findings are available, a potential hazard of a certain nanomaterial cannot be ruled out. In such cases, the precautionary principle (in accordance with the EU Communication of February 2000) is to be applied" [27].