Page 427 - Book Hosokawa Nanoparticle Technology Handbook

7.3 SAFETY OF NANOPARTICLES FUNDAMENTALS
One typical example is the problem of dust explo- From the perspective of composition, the possibil-
sion, caused by the high surface reactivity of fine ity of explosion increase if materials that react easily
particles. In other words, since nanoparticles are with oxygen at low temperature are condensed into
extremely fine particles, dust explosion is more likely particles of small diameter. Therefore, with regard to
to occur. Explosion is more likely to occur because the effect of particle diameter on dust explosion,
fine particles are different in their composition, in that more careful attention needs to be paid in the case of
low boiling point metal can be easily condensed, as combined materials than in the case of uniform mate-
described in Section 7.2. rials, as assumed in Fig. 7.3.1. As described before,
However, of particular note here is that, since all however, since nanoparticles are considered to exist
particles do not necessarily exist independently in the often as agglomerates, it is necessary from the per-
form of a single particle, the possibility of dust explo- spective of the particle diameter to take into consid-
sion does not simply increase as particles become eration the diameter of not only primary particles but
finer. also of particles after agglomeration. To address the
Fine particles with sizes of 1 m or smaller such as safety of nanoparticles, it will be important in the
nanoparticles have an extremely high agglomeration future to elucidate their behavior in detail including
propensity and secondary particles can be easily gen- these factors.
erated. Therefore, in some cases they conversely
behave like large particles. These are the points to be
taken into consideration when studying the problems Reference
caused by nanoparticles.
As shown in Fig. 7.3.1 [1], the effect of the parti- [1] H. Enomoto: Funjin-Bakuhatsu–Kikensei hyouka to
cle diameter on dust explosion tends to be that the Boushi taisaku (Dust explosion–Estimation of danger
smaller the particle diameter of the dust, the lower and control policy), Ohmsha, Ltd., Tokyo, 17 (1991).
the minimum explosion concentration. In other
words, explosion can be induced under conditions of 7.3.2 Health effects on nanoparticles
lower concentration of particles in air as the particle
diameter becomes smaller. Due to the difficulty of The terms ‘nanoparticles’ and ‘nanomaterial’ have
conducting experiments to suspend particles with been used for particles of which one representative
the same size in a uniform concentration, this result dimension, for example, diameter of particles on
was obtained from particles far larger than nanopar- cross-sectional diameter of fibers has at least 100 nm
ticles; however, it has been clarified qualitatively or less. Some people hold that the majority of such
that the smaller the particle diameter, the higher the fine particles are exhaled without depositing in the
possibility of dust explosion. respirator tract, and that therefore the particles may
not cause pulmonary diseases. However, the proper-
ties of nanoparticles are known to be different from
the bulk material they are derived from. In cases
80 where the biological effects of bulk materials have
been reported, nanosized particles of these materials
may be expected to have stronger dose response for
70
the health effects. Every effort must be made to clar-
ify the uncertainty on the risks of these nanomaterials
Lower limit of particle concentration on explosion (g/m 3 ) 60 [1]. At the present time, there is no regulation or stan-
dard for assessing the biological effects of nanomate-
rials, and therefore there is a paucity of toxicological
data concerning nanomaterials. Much more system-
50
atic and strategic studies are needed to enable risk
assessments for human health [2–6].
40
As regards risk assessment and risk management
of nanomaterials, the characterization and identifica-
tion of anticipated risks should be first determined
30
for chemical substances or foods. Conventional
assessment methods are applicable for water-soluble
20 particles. For insoluble nanoparticles, the assessment
25 50 100 150 of potential health hazards should be made based on
Diameter (μm) their properties or toxicity and dose–response rela-
tionship. The risk is a product of hazard and expo-
Figure 7.3.1 sure; even if a nanoparticle has a hazard, the risk is
Influence of particle diameter on lower limit of particle lower when the possibility of exposure to the
concentration for explosion. nanoparticle is small [2].

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