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.


February 25, 2003

Douglas C. Hardy, CSP
Safety Coordinator Construction
Metropolitan Water Reclamation District of Greater Chicago
6001 West Pershing Road
Cicero, IL 60804-4112

Re: Tunnel/underground construction: whether flexible bag lines may be used for ventilation; whether the main ventilation fan is permitted to be located underground; reversible ventilation; §1926.800(k)(4).

Dear Mr. Hardy:

This is in response to your July 12, 2002, electronic correspondence [e-mail] to the Occupational Safety and Health Administration (OSHA) regarding our standards for underground construction (29 CFR 1926.800). We apologize for the delay in providing this response. Your question is paraphrased as follows:

Question: When using mechanical ventilation to meet the requirements in §1926.800(k), is it permissible to have the main fan located inside the tunnel bore, at the bore face, connected to a flexible "bag line" air ducting? It seems that locating the main fan at the face would make it susceptible to failure in an emergency, such as a fire. The need for the fan in an emergency appears to be reflected in the requirement in §1926.800(k)(4) that the ventilation system be "reversible." Does that requirement, in effect, prohibit this arrangement?


The ventilation requirements for underground construction are in §1926.800(k). Section 1926.800(k)(1) requires that there be a sufficient amount of fresh air to prevent hazards, and requires mechanical ventilation unless natural ventilation is shown to be sufficient:


(1)(i) Fresh air shall be supplied to all underground work areas in sufficient quantities to prevent dangerous or harmful accumulation of dusts, fumes, mists, vapors or gases.

(ii) Mechanical ventilation shall be provided in all underground work areas except when the employer can demonstrate that natural ventilation provides the necessary air quality through sufficient air volume and air flow.


Section 1926.800(k)(2)-(13) contains a number of specification requirements. Among these are §1926.800(k)(4), which state:


(4) The direction of mechanical air flow shall be reversible.


Irrespective of the location of the fan, a flexible bag line may be used only if it will not collapse when the air flow is reversed (only some bag lines are so designed). If reversing the air flow would collapse the bag line, the ventilation system would not meet the §1926.800(k)(4) requirement for reversibility.(1)

With respect to fan location, there is no requirement in the standard that explicitly requires the main fan to be at the surface. This contrasts with the Department of Labor's regulations for underground coal mines, in which 30 CFR 75.310 (a)(1) requires that each main mine fan be "installed on the surface in an incombustible housing."

Nor does the requirement for reversibility implicitly prohibit locating the main fan underground. In the rulemaking, the Agency considered a number of ventilation-related requirements in addition to reversibility for dealing with the possibility of explosions, fires, and other emergencies. OSHA ultimately promulgated additional measures for potentially gassy and gassy operations:


(11) Potentially gassy or gassy operations shall have ventilation systems installed which shall:
(i) Be constructed of fire-resistant materials; and
(ii) Have acceptable electrical systems, including fan motors.
(12) Gassy operations shall be provided with controls located above ground for reversing the air flow of ventilation systems. [Emphasis added.]



Specifically requiring the controls to be on the surface for gassy operations indicates that the Agency assumed that at least some fan equipment would be located underground. This is reflected in the rulemaking, in which a commenter objected to a proposed requirement for surface controls in all instances:



Kenny Construction argued that connecting the in-line booster fans to a central above-ground control will require the installation of a burdensome wiring and switch system which is unjustified, absent some compelling need for surface control . . .[54 FR 23837].


Although rejecting the concept of requiring above-ground controls in all circumstances, OSHA found that above-ground controls were necessary to deal with emergencies associated with gassy operations:


In light of . . . testimony about the value of above-ground controls for control from the surface after evacuation and during rescue work . . . OSHA believes that the record does support a decision to require above-ground controls in operations where such events are most likely to occur, that is, in 'gassy' operations. [54 FR 23837].



Nonetheless, a requirement to place the main fan above ground was not included in the standard.

Furthermore, in §1926.800(k)(13), the Agency required that if an employer uses a mine-type main fan on the surface for a potentially gassy or gassy operation, it must meet certain requirements:



(13) In potentially gassy or gassy operations, wherever mine-type ventilation systems using an offset main fan installed on the surface are used, they shall be equipped with explosion-doors or a weak-wall having an area at least equivalent to the cross-sectional area of the airway. [Emphasis added].



However, unlike the underground coal mine standard, this requirement is not coupled with a requirement that the main fan be located at the surface.

In sum, §1926.800(k)(4) does not require that the main fan be located at the surface. Note that a main electric fan located underground would have to meet the requirements of 29 CFR Part 1926 Subpart K (Electrical) and be of an approved type for the conditions.

If you need additional information, please do not hesitate to contact us by fax at: U.S. Department of Labor, OSHA, Directorate of Construction, Office of Construction Standards and Guidance, fax # 202-693-1689. You can also contact us by mail at the above office, Room N3468, 200 Constitution Avenue, N.W., Washington, D.C. 20210, although there will be a delay in our receiving correspondence by mail.


Russell B. Swanson, Director
Directorate of Construction



1. See our letter to Lawrence J. Keefe, dated May 28, 1998 (available on the OSHA website at www.osha.gov). [ back to text ]