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 27, 1993

Mr. Tom Thoner
Mechanical Engineer
Department of the Army
Baltimore District
U.S. Army Corps of Engineers
Room 1219
P.O. Box 1715
Baltimore, Maryland 21203

Dear Mr. Thoner:

This is the Occupational Safety and Health Administration's (OSHA) third and final response to your concerns regarding minimum flow rate contained in 29 CFR 1910.252. The requirements contained in 29 CFR 1910.252 were adopted from American National Standards (ANSI Z54.1-1963) and contain "specification" language, e.g. specific flow rates for fixed enclosures. The current ANSI standards (ANSI/ASC Z49.1-88) are performance standards and do not contain specific flow rate requirements. OSHA is currently in the process of amending the welding standards contained in 29 CFR 1910.252 and will consider adopting these new ANSI standards. In the interim, OSHA has amended its enforcement policy regarding these "specification" requirements.

OSHA is not issuing Notices for failure to maintain specific flow rates for fixed enclosures, or freely movable hoods. Instead, OSHA issues Notices when "adequate" ventilation has not been provided. OSHA defines adequate ventilation as that ventilation required (natural or mechanical) such that personal exposures to hazardous concentrations of airborne contaminants are maintained below the allowable levels specified in 29 CFR 1910.1000. When permissible exposure limits are exceeded, Notices may be issued for those specific exposure limits which have been exceeded and may also be issued for failure to provide adequate ventilation under the general duty provisions of the Act.

Additionally, the welding standard does contain a section, 29 CFR 1910.252(c)(1)(iii) against which Notices may he issued for not providing adequate ventilation. Specifically this section states:

Local exhaust or general ventilation systems shall be provided and arranged to keep the amount of toxic fumes, gases, or dusts below the maximum allowable concentration as specified in section 1910.1000 of this part.

OSHA prefers not to enforce those portions of 29 CFR 1910.252 which require specific flow rates for fixed welding enclosures and freely moving hoods because OSHA believes that adequate ventilation may in some instances require flow rates in excess of those prescribed and in other instances a lesser rate may be adequate. It is difficult to classify fumes and gases from welding and cutting processes. The composition and quantity of fumes and gases are dependent upon the metal being worked, the process and consumable being used, coating on the work such as paint, galvanizing, or planting contaminants in the atmosphere such as halogenated hydrocarbon vapors from cleaning and decreasing activities. Additionally, adequate ventilation may depend upon (1) Volume and configuration of the space in which operations occur; (2) Number and type of operations generating contaminants; (3) Allowable levels of specific toxic or flammable contaminants being generated; (4) Natural air flow (rate and general atmospheric conditions where work is being done); and (5) Location of the welder's and other persons' breathing zones in relation to the contaminants or sources. All things considered, OSHA believes it is more protective to require that adequate ventilation be maintained than it would be to try to enforce specific flow rates which may not be effective.

Therefore, if adequate ventilation (as defined above) was not maintained, in the fixed welding enclosures or in the freely movable hoods, Notices may be issued for failure to provide adequate ventilation and also may be issued for specific permissible exposure limits which have been exceeded. Notices would not be issued for not maintaining specific flow rates prescribed by the standard.


John E. Plummer
Office of Federal Agency Programs