Particles can cause health effects
It has been established for many years that exposure to particles can cause ill health in individuals or exposed populations. We have many examples of this which come from exposure arising from industrial activity and from environmental pollution. For example, in an occupational setting, exposure to coal dust is clearly linked to the onset of diseases such as pneumoconiosis and COPD. Exposure to certain types of asbestos is clearly linked with serious health consequences including asbestosis, mesothelioma and lung cancer. In an environmental context, recent evidence has suggested that exposure to particles in PM10 may be associated with increased hospitalisation rates and cardio-vascular disease.

 

Health effects relate to dose and toxicity
However, many millions of the population are exposed to particles in environmental pollution on a daily basis without any apparent ill effects. Whether or not exposure to particles causes health effects relates to the dose and the toxicity of the particles (and to the susceptibility of the individual) By dose we mean broadly "how much" gets to the organ where the disease occurs and "how long" it stays there. Toxicity, specifically for relatively insoluble particles, appears to relate to the total surface area of the particles.

 

Most of what is known about the health effects of particles relates to exposure by inhalation, and so in these cases it is the dose of particles reaching the lungs which is the key issue. The general mechanism understood to cause health effects in the lung is inflammation through a process known as oxidative stress. Inflammation due to oxidative stress can then lead to a number of different diseases including asthma, cancer and COPD.

 

Concerns about nanoparticles
Like other particles, it is understood that if nanoparticles accumulate in a particular area or location in the body they will cause inflammation which may lead ultimately to disease. Concerns about the potential risk from nanoparticles relate primarily to three main issues.

 

 Because of their small size, nanoparticles can reach parts of biological systems which are not normally accessible by other larger particles. This includes the increased possibility of crossing cell boundaries, or of passing directly from the lungs into the blood stream and so on to all of the organs in the body, or even through deposition in the nose, directly to the brain. This process is known as translocation and in general nanoparticles can translocate much more easily than other larger particles.

 

 Because of their small size, nanoparticles have a relatively much higher surface area than the same mass of larger particles. If surface area is a driver for toxicity this clearly implies potentially increased toxic effects. 

 

 A third issue is that the whole rationale for developing nanomaterials and nanoparticles is that they will have new and different properties than larger particles of the same material. If this is the case, it is certainly conceivable that these new properties could include increased toxicity.

 

A further but specific issue relates to comparisons between carbon nanotubes and asbestos. Asbestos has a fibrous nature with high aspect ratio (the ratio of length to diameter). Exposure to asbestos by inhalation causes disease because the particles can enter into the alveolar region of the lung. Macrophages (a type of cell) in the lung which act to clear particles by ingesting them and transporting them out of the lung, cannot take up the fibres because of their physical dimensions. Asbestos fibres are also highly durable and do not dissolve in the lung lining fluids. Hence they remain in the lung for a long period of time, causing inflammation and ultimately disease.  Some forms of carbon nanotubes are likely to have similar morphology (shape) to asbestos making it difficult for macrophages to absorb them and will be highly durable and so may also persist in the lungs giving rise to similar concerns.

 

These are the main issues about the potential risks of nanoparticles, and when coupled with increasing production volumes, lower costs and an increased general prevalence of these materials in industry and commerce, has led to the conclusion that more needs to be done to assess the potential risks associated with these materials.

 

Exposure, toxicity and risk

As indicated above, the likelihood (or risk) of disease occurring depends on the dose of the particles in the organ where the disease can occur, and the toxicity of these particles. Dose cannot be assessed directly but can be inferred from the exposure of particles, for example the concentration of particles in the air which a person breathes in. If there is no exposure (i.e. no particles in the air), no dose will accumulate and despite potential toxicity of the particles, there will be no risk to health.