Archive Notice - OSHA Archive

NOTICE: This is an OSHA Archive Document, and may no longer represent OSHA Policy. It is presented here as historical content, for research and review purposes only.

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

January 25, 1985

Dr. Bruce Karrh
Vice President
Safety, Health and Environmental Affairs
E.I. duPont de Nemours and Company
Wilmington, Delaware 19898

Dear Dr. Karrh:

This is in response to your recent request concerning the use of negative-pressure air-purifying respirators for protection against paint spray containing isocyanates.

Our technical staff has reviewed the respirator cartridge breakthrough studies on isocyanate containing paint spray as well as the respirator field performance studies on asbestos and lead which duPont has submitted. We agree with the conclusions made in the duPont cartridge study that the preferential breakthrough of solvent over the isocyanates can only be applied to a specific enamel formulation. As we stated in a letter to you previously, it is infeasible to test the preferential breakthrough of all different combinations of solvents, isocyanates and thinners used in spray painting. Our staff has found that there are no data to indicate that the painters would smell the odors of solvents or the isocyanates in the workplace while other odorous substances may be present.

Mine Safety and Health Administration/National Institute for Occupational Safety and Health (MSHA/NIOSH) have not approved negative-pressure air-purifying respirators for protection against isocyanates since these compounds do not have adequate odor warning properties. Additionally, the respirator wearers could be exposed, without knowing it, to isocyanates through leakage at the facepiece seal of a negative pressure respirator.

[This document was edited on 03/24/99 to strike information that no longer reflects current OSHA policy.]

Since the isocyanates are among the most toxic substances on OSHA's permissible exposure limit table and they cause irreversible damage to overexposed employees, the negative-pressure air-purifying respirator is not acceptable for protection against isocyanates containing paint spray because it does not provide a sufficient margin of safety.


John B. Miles, Jr.
Directorate of Field Operations

May 20, 1985

John B. Miles, Jr., Director
Directorate of Field Operations
U.S. Department of Labor
Occupational Safety and Health Administration
Washington. D.C. 20210

Dear Mr. Miles:

Thank you for your letter of January 25, 1985 in response to Dr. Karrh's letter concerning the use of negative-pressure air-purifying respirators for protection against Du Pont Imron(R) and Centari(R) paints.

We have taken the time to carefully consider each of the points raised and continue to have concern that the available data and information do not support the position expressed in your letter. We feel that it is vital that the lines of communication remain open between us until all questions can be clarified and resolved. To this end, this letter seeks clarification of several items of consideration which will assist us in understanding your position.

From our standpoint, the significant finding of the Vasta report is that air-purifying respirators protect the user from isocyanate exposure during application of Imron(R) and Centari(R) enamels by the preferential breakthrough of solvent before isocyanate and the odor warning properties of their solvent systems. We are pleased that your technical staff agrees with these conclusions. We also recognize, as you pointed out, that it is not feasible for Du Pont to similarly test and demonstrate cartridge characteristics for all possible combinations of solvents and isocyanates. Fortunately, there is no scientific need for this, since in the Imron(R) and Centari(R) paints the proportions of solvents and isocyanates remain essentially the same for all colors.

You indicated that your staff found no data which would show that solvent odors would be detected when there are other odorous substances present in the workplace. We are unaware of any substances that may mask the odor of ethyl acetate and render its odor warning properties ineffective. If the staff has identified any such masking substances, we would appreciate your providing us with that information. In this same regard, we have the following questions:

* What references and odor threshold data were used to evaluate odor masking substances?

* What references and data were considered that show the charcoal absorption characteristics of these substances?

You stated that MSHA/NIOSH have not approved negative-pressure air-purifying respirators for protection against isocyanates because isocyanates have inadequate odor warning properties. As we have discussed with you, it was precisely because of this characteristic that we conducted studies to show the selective breakthrough of solvents with unique, detectable odors. NIOSH suggested we seek an acceptance from OSHA for use of these respirators with Imron(R) and Centari(R) paints and that is the purpose behind our efforts. OSHA has accepted the use of such respirators for compounds such as vinyl chloride and acrylonitrile. We would appreciate clarification on whether you consider isocyanates to present a greater hazard to a worker than the other compounds for which OSHA has accepted breakthrough data and cartridge change schedules.

It was suggested that respirator wearers could be exposed, without knowing it, to isocyanates at the facepiece seal of a negative-pressure respirator. Solvents are always present at detectable levels whenever isocyanate is present. The same solvent odor that acts as a warning of cartridge breakthrough also acts as a warning of leakage. Therefore, we see no likelihood of unrecognized exposure. We would appreciate clarification of this point especially if your staff has information that would indicate that a leak would not be detectable.

Finally, you conclude your letter by stating that because isocyanates are highly toxic the negative-pressure air-purifying respirator is not acceptable for protection against isocyanates in paints. This statement is perhaps the most confusing to us. From our point of view, the question is whether the respirator is capable of reducing the ambient concentrations of isocyanate to or below the safe exposure limit. That is the basis of proper respirator practice and the primary consideration behind OSHA's regulations. We therefore pose the following questions, the answers to which would be most helpful in clarifying your position on this issue:

* What criteria was used by OSHA to rate the various substances according to toxicity?

* What margin of safety is OSHA applying to isocyanates and how does this margin of safety compare to OSHA health standard recommendations for respirator selection?

* Would OSHA accept the use of an air-purifying respirator with paints containing isocyanates if the concentration of isocyanate is 5 ppb or less, the concentration of solvents is less than 1000 ppm (or less than ten times the PEL for each individual solvent) and the concentration of paint mist solids is less than 100 mg/m(3); providing that a properly fitted and maintained half mask respirator with paint mist filters and organic cartridges is used?

We are hopeful that by each of us more fully understanding the rationale and considerations behind our respective points of view we can resolve our disagreements on the use of air-purifying vapor-particulate respirators with Imron(R) and Centari(R) paints. Your response to the questions posed in this letter should help us to better understand your position. I would propose that after we receive your response a meeting be scheduled to discuss these issues and resolve our differences.

The review and consideration of this letter by you and your staff are appreciated.


James T. Williamson

The data in the enclosed reports do indicate a long service life for both solvents and isocyanates under the stated test conditions. However, these studies do not encompass the wide temperature and humidity variations which may be present in paint spraying operations. For example, the DuPont studies were run at relative humidities below 61%. There was no indication of % RH in the Mobay study. Ernest Moyer and I reviewed the two studies. Our specific comments are as follows:


DuPont Study
Reference                               Comment
---------      --------------------------------------------------------
Page 4         This should read "NIOSH utilizes this flowrate in Par. 2
              their paint spray testing for certification."  The last
              line reference quoted does not state a
              recommendation that this is a heavy breathing rate.

Page 6         An initial 15 minute exposure setting may not be Par. 1
              indicative of the particulate level throughout the test.
              Mobay's result indicate that this can vary throughout
              the test.

Page 16        There is no collection time given for the impinger
last par.      samples.

Page 12        This data does reflect the true downstream par. 1
              concentration for this sample period.  This data supports
              Mobay's conclusion that cartridges should be discarded
              after each work shift.

Page 12        This statement may not be entirely true.  Depending on
Par. 2         the loading of the cartridge, a worker may be exposed to
              a long period of solvent vapor breakthrough.

Page 12        There was no data or reference given to support the
line 18        statement that this is a representative concentration.
              If the concentration of HDI used in the test are
              representative (ie. Table III levels) this point should
              be made more strongly since many of the exposure levels
              shown in your testing are below the ceiling level.

Table I,       Only one long term study with paint was done on the MSA
III, IV        cartridge.  This does not represent much data on which to
              base conclusions.

Table II, V    The upstream concentration of the vapors is relatively
              low.  Is this in agreement with the field survey of the
              body shop?

Table II       100 ppm of cellosolve acetate.  What does this mean?
              Was the upstream concentration a little above 100 ppm
              or could it have been over 1000 ppm?

Mobay Study

Reference Comment --------- ------------------------------------------------------ Page 1 Page 1 was missing. Table 1 The HDI and solvent levels are low (upstream). In fact the HDI is below the NIOSH recommended ceiling of 20 ppm. Table 1, 3, There is no indication of the relative humidity. and 4

Overall, both of these studies are interesting. Both studies indicated that for the conditions of the test (concentration, humidity, temperature, paint type) that the solvents breakthrough before HDI. However, further studies need to be performed to show that this remains true over a wide variety of conditions. The data used to determine what the representative work concentration is critical. If indeed the workplace concentrations of HDI is as low as the upstream concentrations in your tests, then OSHA may consider the use of APR for Imron, Centari, and Desmodur N based on the fact that the concentrations are normally below the ceiling.

It is difficult for us to comment on the MAK document. Only three test subjects were used in their determination of warning properties. The data on the effects of individuals with several years of exposure is restricted since workers were exposed to other toxic substances.

The question of determining adequate warning properties is a complicated one. The test methods used to determine physiological responses widely vary. Thus, the data cited in the literature must be carefully evaluated. The NIOSH report on isocyanates indicates that they do not have good warning properties. The lack of warning properties will eliminate the possibility of NIOSH approval for APR. This may be unfortunate since APR are so widely accepted.

We hope you will find our comments useful. We found the studies very interesting. It is good to know that others are also concerned over respiratory protection.

If I can be of any further assistance, please feel free to contact me.

Sincerely yours,

Nancy J. Bollinger Assistant Chief
Testing and Certification Branch
Division of Safety Research