Discussion

While acknowledging the potential benefits of nanotechnology, the main aspect of concern in the immediate future is the manufacture and release into the environment of free nanoparticles in the size range 1-100nm. The literature that exists on the health effects of nanoparticles indicates that a precautionary stance should be adopted, particularly because the relevant risk assessments have not yet been completed and the hazard characterizations have yet to

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Populations living in urban locations are routinely exposed to ambient levels of particulate aerosols that did not exist throughout evolution. There is strong evidence that our respiratory systems are not well adapted to cope with the smaller size fractions such as aerosols. In particular, the ultrafine fraction tends to be preferentially deposited in the alveolar portion of the lung, beyond the mucociliary escalator. In the alveolar region the alveolar macrophages, the final defence mechanism before particle internalization occurs, have difficulty recognizing the smallest particles and in addition they are easily overloaded by the numbers of particles arriving. Once internalized, insoluble particles appear to have the ability to translocate to other body compartments.

The influence of the size of particles on their toxicity is currently the subject of increasing research. The indication from current research is that there is a general tendency for acute toxicity, expressed through an ability to induce inflammation, to increase as the particle size decreases, particularly below 100nm diameter. The precise mechanism of this effect remains unknown but there are indications that it is associated with changes in the surface chemistry, possibly through an ability to produce free radicals. There are parallels with the action of heterogeneous catalysts.

Although many countries continue to use PM10 as the standard metric for assessing particle exposure, some countries are changing to the use of PM2 5. There is a debate within the scientific community concerning which fraction of PM10 is responsible for its toxicity. CAFE, a European scientific committee studying the effects of air pollution on health has recently recommended that the EU adopt a PM25 regulatory standard.

What appears to be beyond dispute is that health effects at the population level have been associated with both chronic and acute particulate aerosol exposure. The science is widely accepted and recognized to be of a high standard, despite some reservations from certain industries (HEI, 2000).

The positive aspect of this problem is that particulate aerosols are short lived and, unlike some other forms of pollution, do not persist in the environment. The associated health problems are therefore open to remediation. It is simply a matter of political willpower being sufficient for policy implementation to take place.

The negative aspect is that the ultrafine fraction of particulate aerosols, arguably the most hazardous part, is the most stubbornly resistant to abatement through regulation (Wichmann and Peters, 2000). Additionally, the major source of particulate emissions in cities is vehicular traffic. Exclusion of vehicles from areas of high population density is currently a difficult political problem. However, reduction in particulate aerosol concentrations would definitely lead to tangible health benefits in the relatively short term, and therefore should be actively pursued.

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