OSHA requirements are set by statute, standards and regulations. Our interpretation letters explain these requirements and how they apply to particular circumstances, but they cannot create additional employer obligations. This letter constitutes OSHA's interpretation of the requirements discussed. Note that our enforcement guidance may be affected by changes to OSHA rules. Also, from time to time we update our guidance in response to new information. To keep apprised of such developments, you can consult OSHA's website at https://www.osha.gov.


August 28, 1991

Michael A. Sirbola
Air Monitoring Equipment, Inc.
2843 Samco Rd. Unit P
Rapid City, SD 57702

Dear Michael,

I have tried over the course of the past year to find time to make a simple determination that would answer the question you posed about the resistivity of the cowl. I appreciate your patience with me. I still have not absolutely determined a suitable response to your question. I have raised considerable questions of my own about what is going on in the cassette. The situation is far from simple. The intent of the requirement for the cassette cowl to be conductive was to eliminate electric fields in the interior and give charges a path out of the cassette.

Therefore, we require that the resistivity of the cowl be sufficient that the material be considered a conductor. The standards do not specify a particular resistivity, nor do they require the cowl to be black. Both 29 CFR 1910.1001 and [29 CFR 1926.1101] simply require that the cowl be "conductive". This is the aspect of the standard that is enforceable.

We will continue to study the issue in greater detail. Should you require further assistance, feel free to call.


Daniel T. Crane, Supervisory Physical Scientist
[Directorate of Science, Technology, and Medicine
Salt Lake Technical Center]

(Correction 6/2/2005)




8660 South Sandy Parkway
Sandy, UT 84070-6424]

July 9, 1990

Dear Dr. Crane,

I appreciated your time on the phone the other day. We at A.M.E. have spared no expense and exerted as best an effort as we could manage in order to comply with OSHA specifications and to provide our customers with as high a quality cassette as possible.

The single most critical factor in our determinations of product performance, general guidelines for manufacture, for quality control, and the physical design of our product, has been your published materials such as the ID 160 guidelines. Also of great importance has been the information provided us through phone consultation with you as OSHA representative.

We are now in the process of having all of our competitors cassettes tested by an independent testing laboratory for their carbon content and also for their conductivity readings. Nucleopore and Millipore cassettes will be quite adequate as per your guidelines I am sure, some others are questionable.

Pricing in the P.C.M. Cassette market has become highly competitive in this last year and a number of new companies have entered the market. All the better for a free market as long as the same rules apply to all of the players. Regretably for ourselves and for our innocent customers I am afraid that some of our competitors have made no efforts to comply with OSHA By their disregard of your guidelines I am afraid that the principle of using a representative sampling by use of standard cassettes and procedures such that comparability of results can be achieved has also been seriously undermined by them.

We at A.M.E. make use of the Highest conductivity carbon impregnated polypropylene available. It is not feasible to include a higher percentage of carbon without seriously degrading the structural and physical properties or the plastic. The DIFFERENCE in cost between this highly conductive polypropylene and a regular polypropylene with dark coloring easily amounts to as much as 15% of the total cost of the cassettes. A 15% additional margin is very significant now that some of these competitors are offering cassettes in the $40.00 price range.

It would be of great help to us to know more precisely what the requirements are for conductivity and charge dissipation for these P.C.M. Cassettes. We would like to continue our present high standards of manufacturing but cannot continue to do so as long as our competition feels no similar obligation to meet OSHA guidelines. They obviously feel no moral or business need to comply with your obvious intent, even if precise numbers have not yet been provided. It is for this reason that we find we must request this additional information from you either on the volume resistivity or on the carbon content.

Polypropylene with carbon powder filler has a volume resistivity (ASTM test method D-257) of 10 to 10 ohms squared. Two suppliers who can provide this material are:

Akzo 1-800-457-3764 LNP 1-215-644-5200

We have quite a few employees and also investors whom depend on us to perform in the marketplace and to provide employment. We therefore consider this to be a very serious matter. The summer period is a peak period for cassette usage and can account for up to 70% of the years total sales. We would therefore be quite appreciative of any efforts on your part that would provide this information to us as soon as is reasonably possible.


Michael A. Sirbola Pres., A.M.E.