I certainly agree that the ventilation is primarily for flux. Ventilation is
only part of the industrial hygiene controls for soldering operations. There
is real potential for excessive exposure to lead in wave solder operations.
Primarily from de-drossing and dross management as the dross has a fine
powder which easily becomes airborne or attached to clothes.  I have not
seen leadfree dross yet, but I suspect it will have similar characteristics.


We monitor wave solder operators regularly.  It is not unccommon to find
lead exposures (but below the PEL) for operators. We are well below
vaporization temperatures of the lead but it still shows up in air sampling.


The low limit for OSHA seems to be due to US OSHA treating all silver as
soluble. ACGIH and DFG each have different limits for silver (0.1 mg/m3) and
soluble compounds of silver (0.01) . The Z1 table only has one entry.


Dan

-----Original Message-----
From: Davy, Gordon [mailto:[log in to unmask]]
Sent: Friday, April 23, 2004 12:49 PM
To: [log in to unmask]
Subject: Re: [LF] Reality check


Dan Kallin, in discussing the permissible levels for lead and silver in air,
noted that the OSHA limit for silver is one-fifth that for lead. He
commented that this might indicate a need to tighten controls where solder
containing silver is being used. As Dan knows, setting permissible levels is
an inexact science, and without more information it isn't obvious that the
factor of five difference is even real. Beyond that, however, knowing the
toxicity of a substance's vapor is necessary but not sufficient to indicate
what kind of controls are needed. The other piece of information needed is
the tendency of the substance to vaporize. (The need to make this
disctinction between toxicity and probability of actual entry into the body
is seen most clearly from the statements of environmental activists who
point out the toxicity of lead and other metals used in electronic products
but never say
how they think that the metals are going to poison people.) 

The measure of the tendency of a substance to vaporize is its vapor
pressure. The OSHA permissible levels in air can be converted to partial
pressures using the ideal gas law. The results I get are 4.5E-6 torr (i.e.,
4.5 microtorr or 6 nanoatmospheres) for Pb and 1.7E-6 torr for Ag. While
these may seem like very low pressures, it is interesting to see how hot
would these metals have to be for their vapor pressures to approach the
legal limits. I happen to have tables showing the temperatures at which the
vapor pressures of liquid and solid elements have various values (R.E.
Honig, RCA Review, vol. XXIII, No. 4, 1962). The temperature at which Pb has
a vapor pressure of 1E-6 torr (22 percent of the limit) is 429°C; the
temperature at which silver has a vapor pressure of 1E-6 torr (59 percent of
the limit) is 685°C. The temperatures at which the vapor pressures are 1E-11
(five orders of
magnitude lower, and lowest values in my table) are 243°C for Pb and 448°C
for Ag. (Since the metals are present in a molten alloy, the actual
temperatures at which their vapor pressures equals the tabulated pressures
will be higher than these values.) 

Clearly, the vapor pressures of lead and silver at soldering temperatures
are orders of magnitude below the OSHA limits. As Joe and Brian have already
stated, venting is to deal with fumes from the flux, not the solder.

Gordon Davy 
Baltimore, MD 
[log in to unmask]
410-993-7399 

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