Document ExQn4nvmbZLr5666jEKGDRda0
December 4, l?Sd
Hr. V. C. Carlisle, President, The Toledo Vitrified Biloi Company, 1220 Hadison Avenue, Toldeot Ohio.
Deer Hr. Carlisle!
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la response to jour letter of Deceaber 3, wield cay that the torsa "free silica* and "ooehlaed silica* refer to the state of the silica, that is ehethnr it is la coahiaatloa with another substance or act. Exas plea
of free silica are quarts, chert, or flint* Sxaaples of combined sllioa are sodiua silicate, magnesias silicate, aluminum silloate, etc.
Zt has been definitely established that aillooeia is canted
only by tho inhalation of duet containing sllioa in a free or unaoabinod state.
HiUi the exception of asbestos, there la no dust or combined sllioa which sill,
as far as we taw*, .produce n disabling fibrosis of the lungs.
v. "If*:-* r'
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~,y, 2THj> Therefor*, in endeavoring to estimate the eerioueneae of a
sllioa begard, it is necessary to know shat is the anouat of free sllioa in the
dust. .. Ordinary chemical analysis, as a rule, merely shoes the total silica pre
sent without attempting to differentiate mhether it ia present in a free or com
bined state* " Barre granite, for inatanoe, contains shout 35J of free sllioa. -
The ore In the copper mines in Butte is imbedded in e o outtry rode containing
about COJt free siliotj'whereas in the lead end elno mines in Missouri, Kansas, 2$
and Oklahoma District, the country rock is preotioally pore sllloe. X would esy
that any dust that ooataina otct T0 or 75JC of free eilica vaa a highly silloeoue '-
dust,
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There are three variables In connection with the oeude of
f?
siliooals, namely the amount or dost in the air, the amount cf silica in the dast, ^
and the constancy of exposure. Intermittent exposure to a dust containIng 363
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free silica is obviiusly less dengerous^than constant exposuze to dust containing %
90Jt free silica. 1 iagia that the free eilioa content or various sands varies'-^
greatly. The send timed in sand blasting, for lnstanoe, is usually highly silice- ^
ous end this,'in connection with the immense amount of dast produced ia sand hlasW^j
log makes this m'hasardoos occupation that meeds very caiefol supervision.
There are ho permissible standards of dustiness beoouge the multitude of various dusty conditions in the many industries concerned makAw.tbe,_S% elaboration of such standards isjraoti&able* The Onittd States Public Health Service In itiritudy of the enthreolti Industty detsruiAsd that dust conte inla| not more than t% free silloe eould not ecase disabling silicosis ia Us average life tine ia bomdcUtratioee iip to fifty nilltcn particles per cubic foot.'
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In ths Barra, Varaont, study, also aade by tha Public Ra<h 8ervioo, it u determined that granite duat, containing 55* free silica, would not oauaa disabling sllloosis in aonoeatratlcas of ton to twenty aillion partial** per oubio foot.
Thtra 1* plenty of evldanoe both froa this country and South Afrioa to ladloata that aora than fiwe aillion particles par oubio foot or* daeforou* wh*rw tha duat la highly ailloaoua, that la roughly froa 80* to 100* fraa ailioa. fhar* such highly siliceous duat la praaaot, >0X7 effort should b* aada to harp tha aount below tea Billion particles per oubio foot with the purpoae of getting the oount down to five Billion particle*, If preotioebla.
K you hate any questions or this explanation la not<olear, please let ua know and vt will U7 to be sore explicit?''
C T' .V
Slncsraly yours.
A. J. Loasa, E.D. Assistant Bediaal Dirrotor