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.

July 1, 1999

NAVOSH Air Branch
Naval Facilities Engineering Service Center
Directorate of Compliance Programs
SUBJECT: NFESC e-mail dated May 13, 1999

This memorandum is in response to your e-mail of May 13, 1999 and confirms subsequent e-mail correspondence between you and Ron Cain of my staff.

OSHA considers the Department of Defense corrosion control hangars described in the NFESC memo dated May 13, 1999 as "spray areas." As such, the spray areas must comply with the requirements of NFPA 33, 1995 edition for "Spray Application Using Flammable or Combustible Materials," and with subpart Z of 29 CFR 1910 for hazardous substances. Non-compliance with table G-10 in 29 CFR 1910.94 will be considered de minimis by OSHA as long as the above requirements are met.

Should you require further assistance in this or any other matter, feel free to contact John Plummer or Ron Cain of my staff at 202-693-2122.


DATE: May 13, 1999


Office of Federal Agency Programs,
Occupational Safety and Health Administration, Washington, DC 20210
Office of Federal Agency Programs
Occupational Safety and Health Administration, Washington, DC 20210
Naval Facilities Engineering Service Center
Naval Occpational Safety and Health - Air(ESC 425),
1100 23rd Avenue
Port Hueneme, CA 93043-4370
Commercial:(805)982-4984, DSN:551-4984, Fax:(805)982-1409
Internet: Paulsonkm@nfesc.navy.mil
Web Page: http://www.nfesc.navy.mil/enviro/esc425/NoshArBr.htm
SUBJECT: Industrial Ventilation Flow Rates in Aircraft Hangars


We appreciate your offer to revisit the OSHA standard interpretation you provided to the Department of the Navy, Office of the Assistant Secretary, (Installations and Environment) regarding spray painting in aircraft hangars. See Enclosures (1) and (2). When we tried to apply the interpretation that you provided to us dated April 8, 1997, Letter we discovered discrepancies in our characterization of the processes performed in Navy Final Finish and Corrosion Control Hangars. Enclosure (3) defines the operations performed in each of the various level hangars.

Our questions are:

  1. What is your definition of a production spray finishing operation?
  2. How do you characterize the five operational levels of hangars disucssed in Enclosure 3?
  3. What airflow rate criteria is required for each of the five levels?
  4. If 100 cubic feet per minute per square foot of cross-sectional area is required for any of the five operational levels, please define the term cross-sectional area. Is it:
    1. Area of the exhaust filter bank?
    2. Area of the exhaust filter bank?
    3. Air envelope around the plane, which excludes the "empty" area where there will be no aircraft parts?
    4. Full opening of the hangar, for instance the approximate side of the hangar door opening plus about 5 feet on the top and sides of the hangar reserved for maneuverability?
    5. Full opening of the hangar including open space for roof trusses?

Naval Facilities Engineering Command (NAVFAC) assigned the NAVOSH Air Branch of NFESC to revise Military Handbook 1003/17, Industrial Venitlation Systems. The handbook defines engineering design criteria for use by all components of the Department of Defense. We are adding a new chapter to the MIL-HDBK discussing the criteria for spray painting in aircraft hangars. We are having difficulties applying the interpretation to our criteria. To add to the urgency, NAVFAC is also in the process of designing several new aircraft hangars. Reducing the flow rate from 100 cubic feet per minute per square foot cross-sectional area will provide a significant reduction in equipment first costs and annual operating costs.

Our position is -Aircraft hangars should not be designed for 100 cubic feet per minute per square foot of cross-sectional area due to the size of the space and the dilution effect. Regardless of the flow rate, not all the paint overspray will reach the filters and we acknowledge some will drop to the floor. This is particularly true for the portion of the aircraft farthest from the exhaust filter bank. Paint spray criteria in the ACHIH Industrial Ventilation Manual permits airflow in large spaces as low as 50 cubic feet per minute per square foot of cross-sectional area. Both the NFPA 33 and the ANSI Z9.3 consensus standards require a sufficient ventilation rate to prevent vapor build-up by requiring airflow to keep the vapor less than 25% of the LEL. Airflow calculations based on LEL are typically 10-25% of rates required for health protection. Enclosure (4) reiterates our understanding of the pertinent regulations.

Our experience shows that even in spray painting operations using flow rates of 100 cubic feet per minute per square foot of cross-sectional area, some employee's occupational exposure exceeds the PEL for certain paints and paint components. Therefore, our employees use respiratory protection when painting in hangars.

Thank you for continuing to consider our concern. Based on our phone conversation today, I understand that you are also working on this issue with the US Air Force. Could you direct us to thier point of contact? Our contacts are Kappy Paulson and Trinh Do (805)982-4984.