Richard Schumacher, in commenting on a posting by Harvey Miller dealing with recyclers being exposed to lead vapors, correctly points out that materials can vaporize below their boiling points. Here is an excerpt from an item I posted on April 23 (under the subject "Reality check", dealing with the OSHA permitted levels for lead and silver vapors in air).

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.) 

I later posted this:

Dan Kallin, in response to my earlier posting, brings up an important point that I failed to address: materials can become airborne by means other than simple vaporization. In the case of a wave solder machine, particularly during maintenance, solder can become airborne as an aerosol (suspension of fine powder particles) from such activities as liquid agitation or brushing. Given the predictable widespread usage of tin-silver-copper solder in electronics manufacturing, it looks as if maybe OSHA will need to consider setting a standard for how much of it can be present in the air.

As for recyclers being exposed to lead vapors, Harvey Miller is correct in saying that they aren't. But many environmental activists are irresponsible. They are much less interested in, for example, whether recyclers really are being exposed than in frightening people enough to keep the contributions flowing in. If a claim sounds plausible and it helps them reach their goals, they'll make it. Our analyses can keep us from believing them, but I have not been able to think of anything that would stop them from making irresponsible claims. For more on these people and their behavior, see my posting "Misleaders, misled, and victims, and how to deal with them", Aug. 14, 2001.

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


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