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Compartment for transfer between environments with different atmospheres

An airlock is a compartment which permits passage between environments of differing atmospheric pressure or composition while minimizing the mixing of environments or change in pressure in the adjoining spaces. "Airlock" is sometimes written as air lock or air lock, or abbreviated to just lock. An airlock consists of a chamber with two airtight doors or openings, usually arranged in series, which do not open simultaneously. Airlocks can be small scale mechanisms, such as those used in fermenting, or larger mechanisms, which often take the form of an antechamber. An airlock may also be used underwater to allow passage between the air environment in a pressure vessel, such as a submarine, and the water environment outside. In such cases the airlock can contain air or water. This is called a floodable airlock or underwater airlock, and is used to prevent water from entering a submersible vessel or underwater habitat.

The procedure of entering an airlock from the external or ambient pressure environment, sealing it, equalizing the pressure, and passing through the inner door is known as locking in. Conversely, locking out involves equalizing pressure, unsealing the outer door, then exiting the lock compartment to enter the ambient environment. Locking on and off refer to transfer under pressure where the two chambers are physically connected or disconnected prior to equalizing the pressure and locking in or out. Before opening either door, the air pressure of the airlock chamber is equalized with that of the environment beyond the next door. A gradual pressure transition minimizes air temperature fluctuations, which helps reduce fogging and condensation, decreases stresses on air seals, and allows safe verification of pressure and space suit operation. When a person who is not in a pressure suit moves between environments of greatly different pressures, an airlock changes the pressure slowly to help with internal air cavity equalization and to prevent decompression sickness. This is critical in underwater diving, and a diver or compressed air worker may have to wait in an airlock for a number of hours in accordance with a decompression schedule. A similar arrangement may be used for access to airtight clean spaces, contaminated spaces, or unbreathable atmospheres, which may not necessarily involve any differences in pressure; in these cases, a decontamination procedure and flushing are used instead of pressure change procedures.

On October 20, 1830, the first airlock patent was granted to Thomas Cochrane, who came up with the idea to help with underground tunnel construction. It was put into use in 1879 in an attempt to dig a tunnel under the Hudson river. There was no room that was specifically designated to be an airlock on the spacecraft used during the Apollo 11 mission in 1969; instead, the cabin served as an airlock, and thus it had to be evacuated and depressurized before the door was opened, and then once the door was closed it had to be re pressurized again before anyone could safely use it without a space suit. When the International Space Station (ISS) first began to house humans in November 2000, it did not include an airlock, and all extravehicular activity had to be facilitated by the airlock on the Space Shuttle until the Quest Joint Airlock module was installed in July 2001. The first ever commercial space airlock was the Nanoracks Bishop Airlock, installed on the ISS in December 2020. It is "bell shaped" and is designed to transfer payloads out from the ISS interior and into space. As of July 2023 it is the largest airlock of its kind on the station, fitting "payloads as large as a refrigerator."

Airlocks are used in air to air environments for a variety of reasons, most of which center around either preventing airborne contaminants from entering or exiting an area, or maintaining the air pressure of the interior chamber. One common use of airlock technology can be found in some cleanrooms, where harmful or otherwise undesired particulates can be excluded by maintaining the room at a higher pressure than the surroundings, alongside other measures. Conversely, particulates are prevented from escaping hazardous environments, such as nuclear reactors and some laboratories of biochemistry, by maintaining the room at a lower pressure than the surroundings, so that air (and any particulates that it carries) cannot escape easily. A lesser known application of an airlock is in architecture: pressurized domes, such as the USF Sun Dome, require the internal air pressure to be maintained within a specific range so that the structure doesn't collapse. Airlocks are utilized to maintain electron microscope interiors at near vacuum so that air does not affect the electron path. Fermentation locks, such as those used in alcohol brewing, are a type of airlock which allow gases to escape the fermentation vessel while keeping air out. Parachute airlocks are necessary because airfoil collapse due to depressurization can result in dangerous loss of altitude. Starting in the 1980s, airlock technology was used to explore newly detected chambers in the Egyptian pyramids, to prevent the contents from beginning to decompose due to air contamination.

Civil engineering projects that use air pressure to keep water and mud out of the workplace use an airlock to transfer personnel, equipment, and materials between the external normabaric environment and the pressurized workplace in a caisson or sealed tunnel. The airlock may need to be large enough to accommodate the whole working shift at the same time. Locking in is usually a quick procedure, taking only a few minutes, while the decompression required for locking out may take hours.

In saturation diving, airlocks are crucial safety elements; they serve as pressurized gateways to safely manage the transfer of divers and support personnel between the saturation system (living quarters) and the diving bell, which shuttles divers to their underwater worksite. Airlocks in saturation diving are equipped with safety features such as pressure gauges, manual overrides, and interlocks. Saturation systems typically feature a variety of airlocks, including a stores lock for the transfer of supplies and a medical lock for secure passage of medical necessities or emergency evacuations. Complex "split level" systems, which house divers at different pressure levels for varied work depths, may necessitate additional airlocks. Decompression post dive is a gradual process, often taking a full week. During this time, the airlocks allow divers to shift to a decompression chamber where pressure is progressively reduced back to surface levels. In emergencies, airlocks can facilitate transfer to a hyperbaric escape chamber or lifeboat without significant pressure changes.

In any hyperbaric treatment chamber capable of accommodating more than one person, and where it may be necessary to get a person or equipment into or out of the chamber while it is pressurized, an airlock is used. There will usually be a large airlock at the chamber entry capable of holding one or more persons, and a smaller medical lock for locking in medical supplies and food, and locking out waste.