OSHA's says No; 1910.106(e)(2)(iii) does NOT require the exclusive use of fresh air for ventilation purposes. OSHA states that 1910.106(e)(2)(iii) is a performance requirement and does NOT explicitly require the introduction of fresh air to meet the performance requirements of the OSHA standard. OSHA, under §1910.106(e)(2)(iii), expects employers to provide adequate ventilation to maintain concentrations below 25% of the LEL. In other words, if an employer covered under §1910.106(e)(2)(iii) chooses to recirculate air, then the employer must take measures to provide adequate ventilation to maintain concentrations below 25% of the LEL. Note that the recirculation of air for ventilation purposes can result in the reintroduction of already exhausted flammable and combustible materials back to the ventilated area. This recirculation can result in a buildup of flammable and combustible materials in the area being ventilated to rise to concentrations which are considered dangerous, i.e., greater than the 25% of the LEL.
Paragraph 17.11 of NFPA 30-2008, which is applicable to all operations, including those at industrial plants, contains requirements for mechanical and natural exhaust ventilation. This paragraph REQUIRES exhaust ventilation to discharge to a safe location outside the building. In addition, this section prohibits the recirculation of the exhaust air with exception:
17.11.6 Recirculation of the exhaust air shall be permitted only when it is monitored continuously using a fail-safe system that is designed to automatically sound an alarm, stop recirculation, and provide full exhaust to the outside in the event that vapor-air mixtures in concentrations over one-fourth of the lower flammable limit are detected.
Therefore, although §1910.106(e)(2)(iii) does not explicitly require fresh air to prevent the short circuiting of the ventilation, when recirculated air is used, OSHA expects the controls listed in NFPA 30-2008 paragraph 17.11.6 to be used to control a potential fire/explosion hazard that could seriously or fatally injure employees. OSHA may find that the failure to do so constitutes a violation of the General Duty Clause of the OSHAct (29 USC §654(a)(1)).
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