Safety Tidbit #12 - An Introduction to the OSHA Z-3 Table

We all know that the OSHA Permissible Exposure Limits are woefully out of date. They should be considered the legal limit but not the safe limit.  OSHA has three tables of limits one for gases and vapors (Table Z-1) and one for particulates (Table Z-3).  That makes sense, however, they also have a list of high hazard chemicals (Table Z-2).  Most of us are familiar with the 8-hour Time-weighted averaged Permissible Exposure Limit as we mentioned a second ago. However, are you familiar with the Ceiling Limit? This level is the workers' permitted instantaneous exposure level. By default if instantaneous measurements are not possible then a default 15-minute period is sampled.  Now here’s the twist, this is Table Z-1 world.  Table Z-2 for the high hazard chemicals is quite a different animal.

Table Z-2 chemicals have a third level OSHA calls a Maximum Peak Concentration.  OSHA says the worker exposure level shall never exceed. OSHA further complicates this by specifying a particular duration of time  (e.g., 5 minutes, 10 minutes, etc.) for this exposure (and it various by the chemical).  However, the worker can be exposed above the ceiling concentration (as found in Table Z-2) as long the overall daily exposure stays below the Permissible Exposure Limit.  I think an example is in order. We’ll take a simple one Toluene.

A worker cleans metal parts to remove grease or paint in preparation for the pickling process prior to coating the part.  The worker wipes the parts down using a rag soaked in the chemical stripper. The main ingredient in the stripper is toluene. The operation only lasts 10-15 minutes at a time and the worker cycles through the process 3-5 times per day.

Toluene – PEL = 200 ppm, Ceiling = 300 ppm, Maximum Peak Concentration = 500 for 10 minutes.

The industrial hygienist had one pump running to evaluate the 8-hour time-weighted average and one to evaluate the 10-15 minute exposures. The following results were collected:

TWA = 100 ppm

Cycle 1 = 400 ppm (15 minutes)

Cycle 2 = 300 ppm (15 minutes)

Cycle 3 = 600 ppm (10 minutes)

Questions:       Was the worker overexposed to Toluene according to OSHA’s regulations? 

How could the industrial hygienist have sampled the operation differently?

You thought I was going to give you the answer….that would be too easy.  Give me your thoughts.  Thanks for reading.