Now here is another ACGIH copyrighted page for your review:
Note for example p-dichlorobenzene, an insecticide that was used to make "mothballs." (I believe you can still buy it.) Contrasting the TLV, PEL, REL and MAK, you get 10, 75, (1.7), and 50 ppm. That's quite a spread, until you realize the 6 or more orders of magnitude that separate LD50s for different substances. The NIOSH REL is interesting. It says, since the chemical is carcinogenic, it should be kept to the lowest level practical, and that 1.7 ppm is as low as the common measuring instruments can detect. Here, if I were hired as an IH consultant, I would work to the 10 ppm, even though the employer could legally allow exposure up to 75 ppm.
You'll have to buy the TLV books if you want them. Here is a very handy NIOSH
booklet. NIOSH POCKET GUIDE TO CHEMICAL HAZARDS with PELs, RELs and lots of other information. Check it out, we'll be using it more in the future.
End of Submodule.