Airlocks and Entry and Exit Procedures - Military Applications

Entry/exit is an important consideration for military collective protection shelters. It includes both hardware (ventilated airlocks) and procedures for transitioning from individual protection equipment to collective protection.

Airlocks and entry/exit procedures are employed with military collective protection systems because of the need to continue operations while agents may be present in the environment. For non-military applications, they are not required; however, airlocks can provide the benefit of helping maintain pressures within a protected area to ensure continuous protection during routine operations.

Battlefield experience has shown that chemical casualties are most likely to occur during protective transition; that is, when proceeding from a non-chemical to a chemical environment, from a protected to an unprotected posture after attack, or from individual protection to collective protection. Collectively, such transitions accounted for about three-fourths of the chemical casualties the U.S. Army suffered in World War I.

The transition from individual to collective protection is referred to as entry/exit. In World War I, one-fourth of all U.S. mustard casualties resulted from soldiers entering collective protection shelters. These crude shelters, which were not ventilated by filtered air, contributed to the casualty rate, although most of the injuries resulted from soldiers carrying mustard vapor on their clothing as they entered.

The mechanisms of contamination transfer that produced casualties in World War I also apply to modern collective protection shelters. For removal of contamination, entry/exit involves limited personnel decontamination procedures. These procedures are designed to minimize contamination transport to safe levels, and they are based upon the principle of contamination avoidance.

There are three mechanisms by which chemical contaminants are transported as people enter and exit into a shelter:

Indirect vapor transport
Indirect liquid transport
Direct vapor transport

Indirect Vapor Transport

When people are exposed to airborne contaminants, chemical vapors and gases are sorbed by their outer clothing and aerosols are deposited on their clothing from the air. If, after such exposure, they proceed into a clean environment, the agent desorbs or is reaerosolized in the enclosure. All surfaces can sorb vapor to some extent. Textiles do so in the greatest quantity because of their large surface areas. Desorption is a slower process than sorption. For example, what may be sorbed in a period of 5 minutes may require an hour or more to desorb in a clean environment, depending upon conditions.

Indirect Liquid Transport

Liquid agent can be transported into an enclosure if items taken in have been exposed to liquid chemical agent (through rain attack or contact with liquid deposited on surfaces). Liquid contamination on the surfaces of the boots, outer garments, gloves, hood, or mask can be transferred to the clothing, skin or hair when outer garments are removed. The agent evaporates or desorbs from the item once inside.

Direct Vapor Transport

Ambient vapor infiltrates an enclosure through doorways when they are opened for entry and exit. Any airborne contaminant is then purged by dilution as clean air from the filter unit flows through the enclosure. The purge rate is a function of the clean airflow rate, volume of the compartment, mixing efficiency, and the rate at which agent vapor is sorbed and desorbed by material within the enclosure. Solid and liquid aerosols also can enter directly and indirectly. Once deposited on supplies or clothing outdoors, they can become airborne inside the enclosure through movement of the item or the air around it.

Airlocks

An airlock is a transition enclosure, a protected entryway in which people pause for a specified period while entering to allow the purging of airborne contaminants introduced when opening the outer door. The main purpose of the airlock is to prevent direct vapor transport into the enclosure. In doing so, it also ensures that internal pressure is maintained continuously in the enclosure while entries and exits occur.

There are two types of airlocks: ventilated and unventilated. Ventilated airlocks are transition areas through which people enter and exit the protected air while an airborne hazard is present outside the shelter.

The airlock is purged by the flow of clean air. Normally the air flows from the main protected enclosure, through the airlock, and out a discharge vent on the outer airlock door. The flow required for purging can be estimated with the following equation:

In this equation, C is the concentration at time T; Co is the initial concentration; Q is the airflow rate in ft³/min; V is the volume of the enclosure in ft³; and T is the time in minutes. For a three-log (99.9%) reduction, the equation becomes: Q = 6.9 V/T

The purge period or dwell time is commonly no more than 5 minutes. The standard criterion is to achieve a three-log (99.9%) reduction in concentration of an airborne particulate or non-depositing gas within the dwell time. For a three-log reduction in 5 minutes, the equation becomes: Q = 6.9 V/5

This equation can be used to estimate the necessary flow rate, but it does not account for some factors:

It assumes perfect mixing. Depending upon the actual flow path, determined by the distribution of leakage points in the shelter, the actual purge rate may be lower.
It is based upon simple purging only and does not account for sorption and desorption of vapor on interior surfaces. Nor does it account for introducing a source of contamination, as occurs with indirect-liquid and indirect-vapor transport. When these become factors, the time required for purging increases.
It is based upon a ratio of beginning and ending concentrations. In practice, the beginning concentration is an unknown, and the ending concentration is the detection threshold of the chemical agent detector used to determine when to unmask.

An airlock is not an air shower; the velocity of the clean air through it is typically low and has a negligible direct effect upon the rate of desorption or evaporation from the contaminated clothing. It does provide an indirect effect upon indirect vapor transport. By imposing a delay that occurs in a relatively clean environment, it allows more time for desorption to occur, thus reducing agent transport that occurs through adsorption/desorption. The early collective protection filter units contained air-blaster units for the purpose of blowing off agent vapors clinging to clothing. This device, which was located in the airlock, consisted of a vertical pipe with up to 20 air nozzles. When the entering soldier stepped on a treadle, the air-blaster directed high pressure air onto his garments. Later tests showed the air-blaster to have little effect upon the rate of desorption, and its use was dropped.

One of the advantages of an airlock is that it can be used as a doffing area. If large enough, the airlock can be employed for the removal of the outer clothing, greatly reducing the potential for indirect vapor transport. The airlock can also be used for chemical agent monitoring. Its air flows can be adjusted to create an environment conducive to accurate monitoring. The airlock also can be used to introduce supplies and equipment.

Airflow Requirements for Airlocks

The flow of clean air from filter unit of a collective protection system serves four purposes:

Ventilation (respiratory requirements and comfort of crewmen, 15 to 20 cfm/person)
Cooling internal equipment
Producing overpressure in the enclosure
Purging airborne contaminants from the airlock

The total volume of clean airflow required for a collective protection system is equal to the largest among these four requirements. Adding an airlock creates an additional airflow requirement. If, for example, 500 ft³/min airflow is required to overcome leakage and establish overpressure, and the requirements for ventilation, cooling, and purging are each less than 500 ft³/min, the required total airflow is 500 ft³/min. If an airlock is a part of the system, the airflow needed to purge it (for example, 200 ft³/min) must be added to the airflow needed to achieve an overpressure, raising the total to 700 ft³/min. Makeup air that exits through distributed, unintentional leakage paths serves to purge the enclosure, but air to purge an airlock must be directed through the airlock at a rate sufficient for purging the airlock's volume at a required rate.

The rate at which an airlock can be purged of airborne contaminants is determined by the:

Flow rate of clean air
Flow path of clean air
Volume of the enclosure
Sorptive characteristics of the interior surfaces

Purging is most efficient when the path of clean airflow through the enclosure is of maximum length; that is, the clean air sweeps through all areas of the protected enclosure. In simplest terms, this means locating the vent through which air exits the enclosure (the controlled leakage point) at the furthest point from the vents through which clean air enters the enclosure.

Entry and Exit Procedures

Entry and exit procedures are designed to minimize, and ultimately to prevent the transport of agent by the three mechanisms of transfer.

Such procedures specify the steps of removing protective clothing to prevent agent transfer to the garments worn beneath the protective outer garments and are generally similar for all military applications. All involve removal of an outer layer of clothing and partial decontamination or exchange of the mask.

Entry and exit procedures vary according to type of ensemble worn by the crew or occupants, the type of entry system (airlock or no airlock), and the availability of support personnel and equipment.

To prevent the transfer of liquid agent to skin or to garments beneath a contaminated NBC ensemble, the general order of removal is from top to bottom. That is, the hood is removed or decontaminated first because it overlaps the jacket. The jacket is removed next because it overlaps the trousers. The trousers are then removed because they overlap the boots. The gloves are removed last to protect the hands while removing the overboots.

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