Differences between a H-2 and H-3 occupancy

In terms of Fire and Building Codes, we have five (5) recognized areas associated with handling hazardous materials. We refer to these high-hazard areas/occupancies as “Group H”.  High-hazard Group H occupancy includes, among others, the use of a building or structure, or a portion thereof, that involves the manufacturing, processing, generation or storage of materials that constitute a PHYSICAL or HEALTH HAZARD in quantities over those allowed in control areas complying with Section 5003.8.3, based on the maximum allowable quantity limits for control areas outlined in Tables 5003.1.1(1) and 5003.1.1(2). Hazardous occupancies are classified in Groups H-1, H-2, H-3, H-4 and H-5 and shall be in accordance with this code and the requirements of Section 415 of the International Building Code. Hazardous materials stored or used on top of roofs or canopies shall be classified as outdoor storage or use and shall comply with this code.

H-1 buildings and structures contain materials that pose a detonation hazard, such as explosives.

H-5 is specific to semiconductor fabrication facilities and comparable research and development areas.

H-4 buildings and structures containing materials that are health hazards where materials such as Corrosives, Highly toxic materials, and Toxic materials are present.

So that leaves us H-2 and H-3 areas, which, by me posting this under my Flammable Liquids category, you might have figured out that these two (2) hazardous occupancies are about flammable liquids and gases.  But why have two (2) different ratings for the same hazard?

High-hazard Group H-2

Buildings and structures containing materials that pose a deflagration hazard or a hazard from accelerated burning shall be classified as Group H-2. Such materials shall include, but not be limited to, the following:

• Class I, II, or IIIA FLAMMABLE OR COMBUSTIBLE LIQUIDS that are used or stored in
  1. NORMALLY OPEN containers or systems, or
  2. in CLOSED CONTAINERS OR SYSTEMS pressurized at MORE THAN 15 psig

• COMBUSTIBLE DUSTS where manufactured, generated, or used in such a manner that the concentration and conditions create a fire or explosion hazard based on information prepared in accordance with Section 414.1.3 of the International Building Code

High-hazard Group H-3

Buildings and structures containing materials that readily support combustion or that pose a physical hazard shall be classified as Group H-3. Such materials shall include, but not be limited to, the following:

• Class I, II, or IIIA FLAMMABLE OR COMBUSTIBLE LIQUIDS that are used or stored in
  1. NORMALLY CLOSED containers or systems pressurized at 15 psig OR LESS

• Combustible fibers, other than densely packed baled cotton, where manufactured, generated or used in such a manner that the concentration and conditions create a fire or explosion hazard based on information prepared in accordance with Section 414.1.3 of the International Building Code

Pay close attention to the highlights above, especially the details about whether the process is OPEN or CLOSED and the pressures at which the systems/processes operate.