Page 424 - Book Hosokawa Nanoparticle Technology Handbook

FUNDAMENTALS CH. 7 ENVIRONMENTAL AND SAFETY ISSUES WITH NANOPARTICLES
Table 7.2.2 before smoking (N 0) has a bimodality, one with
db
Main sources of emission nanoparticles in cleanroom envi- the peak size of 0.2 m, and the other of 20 nm or
ronments. smaller. The former peak comes from atmospheric
aerosols as it decreases with the increase in number
Exhaled air of human of deep breaths in a clean booth (N ). The latter
db
originates from NVR particles of spittle droplets.
Corona-discharge ionizer (e.g. gas-to-particle conversion of
low-molecular-weight cyclosiloxane) Incidentally, since smoking induces the rapid
increase in number concentration of particles 0.1 m
Boron-containing particles from borosilicate glass fibers of or larger by 10 times or more and for nanoparticles
4
HEPA filter by about double, special attention should be paid to
Haze by chemical reaction on solid surfaces (precipitation the management of personnel’s clothes such as face
of ammonium salt and silica) mask when they enter a cleanroom after smoking.
Watermark on wafer surfaces at drying (2) Emission from ionizers
Leakage from thin film and nanoparticles processing Ionizers are commonly used in cleanrooms to elimi-
equipment nate electrostatic charge on substrates for precision
electronic devices. The most popular ionizer is a
corona-discharge type.
(1) Air exhaled by humans Corona-discharge type ionizers are categorized into
Emissions from human bodies are a minor contribution the following three groups; AC, DC and pulsed-DC
in ordinary indoor situations because airborne particle types. The issues of emission of contaminants such as
concentration in such places is quite high, whereas the ozone, NO and particles have been pointed out [3].
x
emission cannot be seen as negligible in cleanroom envi- These issues are also applicable to air cleaners using
ronments. The major human emissions are thought to be a corona discharger. Among these problems is that the
atmospheric dust deposited on clothes and skin frag- particle emission has a potential for particle contami-
ments, and most of these particles are submicrometer in nation onto product surfaces and eventually decline in
size. Meanwhile, particles in exhaled air are composed product yield.
of fine liquid droplets from spittle (99.5% of water), and The particle emission, which has been studied since
then evaporate to form nanoparticles of NVR. the 1990s, is caused by foreign particle deposition
In Fig. 7.2.12 an example of size distributions of onto electrodes, electrode erosion, and gas-to-particle
particles in exhaled air before and after smoking is conversion. The issue of electrode erosion can be
shown [2]. When measuring particles in exhaled air, solved by the improvement of electrode materials,
the air was introduced into a measuring device after whereas for the issue of gas-to-particle conversion,
drying them by passing them through a diffusion the airborne molecular contamination (AMC) control
dryer. The size distribution of particles in exhaled air to be ionized has to be made. It was reported that

Figure 7.2.12
Size distribution of particles in exhaled air for smoking subjects.

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