Air changes per hour reflect the number of times air in a room is replaced per hour through normal circulation. Fresh air is critical for health and comfort in addition to structural integrity; stale, humid air can contribute to the growth of microorganisms that may damage structural components like beams and flooring. Many building codes set out recommendations for air changes per hour, and in some cases, a specific number is required by law for safety.
Improvement of building techniques makes air changes per hour especially important. In older structures, a certain amount of porosity is present, and air moves through cracks and chinks, creating some circulation even without fans, windows, and other ventilation options. Newer structures are more tightly sealed, especially in cases where they are designed to be clean and sterile. Laboratories and operating rooms, for example, rely heavily on mechanical ventilation for safety.
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In a space where activities are minimal, air changes per hour can be low. Somewhere like a warehouse only needs two to four changes to maintain appropriate circulation. Workers will be able to breathe comfortably, carbon dioxide is eliminated, and the supplies in storage will be safe from mold and mildew. In environments where hazards are present, more circulation is necessary. Gases and particulates need venting, both through specific hoods and ventilation systems, and some environments can feel closed and uncomfortable with poor ventilation.
Laboratories and other facilities where workers handle potentially hazardous substances may need 15 to 20 air changes per hour for safety. In a place like a commercial kitchen, as many as 60 may be necessary. This environment can feel hot, closed, and hard to work in as a result of fumes from food, consumption of oxygen by flames on the stoves, and heat vented from freezers and fridges used to keep food at safe temperatures.
Contractors and engineers will consider air changes per hour in the design of a space. They can install ventilation systems to meet the need and may build in flexibility so the system can increase ventilation easily when it becomes necessary. If the code requires a set number of air changes per hour, this must also be integrated into the design, with evidence to show building inspectors that the space will be safe for use. Personnel are also responsible for maintaining ventilation systems while a structure is in use, including changing filters and keeping vents clean so air can flow freely. Obstructions may limit air exchange and could lead to hazards like buildups of dangerous gases.