NFPA 91 provides minimum requirements for the design, construction, installation, operation, testing, and maintenance of exhaust systems for air conveying of vapors, gases, mists, and noncombustible particulate solids except as modified or amplified by other applicable NFPA standards.

Changes to the 2004 edition include:

• Expanded conditions under which flammable concentrations can exceed 25 percent of the lower flammable limit
• Revised provisions for duct hanging and bracing design to account for both automatic sprinkler discharge as well as liquid cleaning solutions
• A new provision allowing ductwork to be wrapped or encased to achieve the intended fire resistance rating.