small in size, big problem
The Western world has come a long way in its drive to reduce air pollution from vehicles. Just over a decade back, the focus was on total suspended particulate matter (which includes natural dust too) and the black smoke vehicles emitted. Epidemiological evidence made governments shift the focus to respirable particles (particles smaller than 10 micron). With increasing evidence that vehicular emissions were a major health menace, these countries scaled up vehicle emission standards: Euro norms in the European Union and Tier I norms in the US. These norms reduced emissions from vehicles, but regulation kept up the demand for more stringency in the future. The automotive industry had no option but to find technological solutions -- which have moved from the engine to the tailpipe -- to make cleaner cars. The fuel industry, too, had to produce cleaner fuels, cleaning up other toxic components at the same time. (Thanks to the regulations, the task is now to go for almost sulphur-less fuel.) Yet, large regions in the US and in Europe still exceed the air quality standards for respirable particles. Responding to the scientific evidence that smaller particles do more harm, the US has also established air quality standard for smaller PM2.5. The problem, however, is that evolving science now shows that even these particles aren't the predominant kind of particles that actually swamp the air and are emitted in greater numbers by technologically sophisticated vehicles.
Paradoxically, while vehicular emission norms have taken care of reduction in mass of particles, many scientists point out that the number of particles emitted by vehicles has gone up. Measured in the scale of nanometre (a billionth of a metre), these particles are called nanoparticles and almost defy geometrical definition. The nanoparticles comprise only 1-20 per cent of the total particulate mass emitted from a diesel vehicle, but may constitute more than 90 per of the total number of the emitted particles. What is worrying the scientific and regulatory community is that it is still not easy to measure these extremely tiny particles, with diameter less than 50 nanometre (nm), let alone control their emissions.
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