Although the basic principles of fibrous filters have been well understood for capture of micron and submicron sized particles, questions arise when they are applied to nanoscale particles. In the first part of this review, the classical theory of fibrous filters is described with focus on the principles that are applicable to nanoparticle collection. The areas of recent developments reviewed include thermal rebound of nanoparticles and the effects of particle shape, aggregate morphology, flow regime, humidity, fiber size, and particle loading. One of the outstanding questions in nanoparticle collection is the particle size at which the effect of thermal rebound on collection efficiency can be observed. Theoretical calculations indicate that the effect probably can be observed only for particles smaller than 1 nm, but experimental confirmation is difficult at present because of lack of instruments for classifying and counting subnanoscale particles. Two promising devices based on filtration principles have been studied in recent years: multilayer filters and inertial fibrous filters. Multilayer filters, which are composed of nanofiber and microfiber mats, have potential to become an efficient and economical device for removing nanoparticles from gas streams. The inertial fibrous filter operates at high flow rates and relatively low pressure drop, thereby offering an attractive alternative to lowpressure impactors for nanoparticle sampling. Further development of these two types of filtration devices is needed to make them simple and reliable.

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We measured 24-hour ambulatory blood pressure and 16-hour noise exposure continuously for 20 automobile workers, and used linear mixed-effects regression models to estimate transient and sustained effects of noise exposure on blood pressure. The occupational noise levels of the high-exposure workers with 85 +/- 8 dBA were significantly higher than those of the low-exposure workers with 59 +/- 4 dBA (P < 0.05). We found a significant difference of 16 +/- 6 mm Hg in sleep-time systolic blood pressure (SBP) existed between 2 exposure groups, and a marginal increase of 1 mm Hg SBP per 1-dBA increase in occupational noise exposure at a 60-minute lag time during work (P = 0.07). Occupational noise exposure had both transient and sustained effects on workers' SBP.

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