Just two more fast comments and then, uncharacteristically, I'll shut up.
1.  Rob asked for documentation about OSHA's opinion of their PELs.  The statement is the Federal Register at 58 FR 35337-35351, June 30, 19 93 when OSHA had to formally reinstate the old PELs thus revoking both the upgraded levels and addition of new PELs on substances for which there were no standards in 1968:

"OSHA continues to believe that many of the old limits which it will n ow be enforcing are out of date (they predate 1968) and not sufficiently pr otective of employee health based on current scientific information and exp ert recommendations. In addition, many of the substances for which OSHA has no PELs present serious health hazard s to employees."

How I wish you would all add this short paragraph to your explanations of a ir quality standards somewhere for students to really understand the issue.

2.  Please also let students understand that there is no air quality l imit that is not "feasible" no matter how low it is set.  Th en the only consideration is money. 

For example, if we have to use a two component urethane system in an open s hop, I require the employer put our union workers in air-supplied respirato ry protection to keep below the 0.005 ppm TLV for the isocyanates.  On ce the employer has spent 4-5 large for the compressors and lines WHICH THEY SHOULD HAVE IN THE SHOP ANYWAY, it on ly cost about $100 more per employee than air-purifying systems. 

Done well, this system can keep the exposure to essentially zero.  And better yet, the employer can install local exhaust ventilation such as a w alk-in sized spray booth.

The only balancing act, then is between money and workers health.  As the old union song goes:  Whose Side Are You On?

Monona