Ventilation

Animal facilities must have dedicated supply and exhaust air handling units. The supply air must be 100% outside makeup air. The ventilation rate that has proven effective for most animal rooms expressed in terms of fresh air changes per hour (cph) is around 15 cph. However, this varies between 10 and 20 cph depending on the heat load as well as microbial, particulate, and gaseous contaminants generated in the room, which is dependent on the species and density of animals to be housed in the room. Control of the heat load in the room is the most critical concern, because high temperatures are stressful for all animals and may be lethal for laboratory species, especially rodents, not adapted to high temperatures, even at temperatures not normally dangerous for most species. The minimal lethal temperature for laboratory rodents is time and relative humidity dependent but may start at temperatures as low as 29.4°C (85°F). It is important to note that the temperature in the animal's microenvironment inside of microisolation caging can be several degrees higher than the macroenvironment. The prominent gaseous contaminant is ammonia, which is generated by urease positive bacteria from the feces splitting each urea molecule from urine to form two ammonia molecules. Ammonia production depends on many factors, including the species and density of animals, the sanitation level, and the relative humidity in the room and cage. As a general rule, a ventilation rate that adequately controls the heat load when air is delivered to the room at 12.8°C (55°F) is adequate to control the gaseous and particulate contaminants. Heat loads for various species of animals are listed in the ASHRAE Handbook.32

The ventilation rates noted above are not necessarily to be considered absolutes. Variable air volume (VAV) ventilation systems in which ventilation volume is based on actual heat load may achieve the objective while conserving energy. This would be consistent with the idea of using performance standards as noted in the Guide,4 as opposed to inflexible engineering standards. The same applies to other rooms in the facility, e.g., the cage sanitation area where loads range from very high when the sanitation equipment is being used to very low when it is not. If VAV is to be used, consideration must be given to how varying the volume may alter the ventilation efficiency in terms of distribution of fresh air in the room and removing particulates, including allergens and infectious agents.

Room ventilation patterns, with regard to the location and type of supply diffusers and location of return and exhaust grills, significantly affect the room ventilation efficiency; however, the most efficient pattern has yet to be definitively defined. The dogma for many years has been to supply high, typically from the ceiling down the center of the room, and exhaust low near the floor, preferably in all four corners. This dogma has been called into question by the result of some computational fluid dynamic studies (CFD) but is supported by other studies. CFD is the use of highly complex mathematical models to predict air circulation patterns in a space.35,36,37 It appears to be a power design tool for determining the optimal animal room ventilation pattern given the room configuration, the species and number of animals to be housed, and the type of caging. One published study suggests that high returns, preferably in each corner or above each cage rack, are the most effective,35 and another suggests that low returns, one in each corner, are the most effective.36 The problem is complex, and these CFD studies used different assumptions for key features, thus, additional study will be required to clarify this important issue. The Hughes'35 study suggests that an even more efficient configuration is to supply and exhaust room air from a soffit mounted in the center of the ceiling extending the full length of the long axis of the room. In this CFD model, supply air is directed from radial diffusers in the bottom of the soffit toward the floor. Exhaust inlets located along both sides of the soffit capture the air as it curls from the floor, up the wall parallel with the soffit, across the ceiling, and into the soffit, where it is removed from the room. A full-scale test model of an animal room fitted with this type of soffit is reported to have performed even better than predicted by the CFD model.35 Given the uncertainty, the high returns in each corner or the soffit configurations are tempting options in that they are less costly to construct than low returns and do not take up floor space; however, the best current answer is to do CFD studies specifically for the animal rooms in the facility being planned.

There is fixed equipment commonly used in research animal facilities that have special ventilation requirements. Fume hoods and certain types of biosafety cabinets require independent direct exhaust systems. Autoclaves require canopy exhaust hoods immediately above the autoclave doors with sufficient airflows to capture the heat, moisture, and odors that emanate from the autoclave when opened. This is especially important if the organic materials are to be autoclaved because they generate high odor levels.

The cage sanitation area has unique ventilation requirements, because high heat and moisture loads are generated in the room by cage washwater temperatures that are 82°C (180°F) or higher. Tunnel washers and often the cage and rack washers are connected directly to the exhaust system, and in addition, cage and rack washers must have exhaust canopies above the doors to capture the heat and moisture that emanates from the machines when the doors are opened (Figure 8.20). The high moisture levels in the cage sanitation area dictate having a dedicated independent exhaust system for this area, including the exhaust fan and the ducts. The canopies and all ducts must be nonferrous and acid resistant, and the ducts must be watertight and slopped and fitted to drain of the large amount of condensate released from the water-saturated hot air coming from the washers. The overall ventilation requirements for the cage

Figure 8.17 An individually ventilated microisolation cage rack with two filter and blower units on top of the rack. One supplies HEPA-filtered air to the cages, the other captures the air coming from the cages and hEpA filters it before blowing it into the room.

sanitation area must take into consideration the enormous heat load in the room that may include a significant mass of stainless steel coming out of the washers at temperatures of 82°C (180°F) or higher.

Ventilated rodent cage racks are an example of mobile equipment that may be connected directly to the ventilation system. Ventilated racks may be used as freestanding equipment with blower and filter units that supply HEPA-filtered room air to the cages. They may also be equipped with blower and filter units that capture air coming from the cages and HEPA-filter it before blowing it back into the room (Figure 8.17). The blower and filter units can be mounted on top of the cage racks but, ideally, are mounted on wall shelves and connected to the racks with flexible ducting (Figure 8.18). HEPA filtering the exhaust air from the cages removes particulate contaminants but does not remove gaseous contaminants and heat. This is best accomplished by coupling the rack exhaust directly to the room exhaust (Figures 8.16 and 8.19). There are many strategies for integrating supply and exhaust air of ventilated racks with the ventilation system.1,38,39 Regardless of which strategy is selected, it is important to decide early in the planning process because the design of the room ventilation system must be matched with the equipment to gain maximum benefit. Not only does the decision affect the physical couplings; it also impacts on the cubic feet of air per minute (cfm) of supply air that will be required in the room.

Figure 8.18 Upper portion of an individually ventilated microisolation cage rack showing the cage supply and exhaust filter and blower units sitting on a wall-mounted shelf. This arrangement facilitates rack changes by eliminating the need to transfer the filter and blower units from rack to rack.
Figure 8.19 An individually ventilated microisolation cage rack with two filter and blower units mounted or a mobile rack alongside the cage rack. The cage exhaust filter and fan unit is connected directly to the room exhaust at the ceiling, similar to that shown in Figure 8.16.

Air Balancing — Appropriate relative air pressures throughout the facility must be maintained to control airborne contaminants.9,30,31 This involves balancing supply and exhaust to maintain predetermined relative air pressures between adjoining spaces, typically between the room and corridor. Table 8.2 summarizes various balancing options, depending facility type, and corridor plan. Maintaining proper balance requires proper sealing of the room envelope and maintenance of the appropriate volumetric offset between supply and exhaust air to achieve adequate differential pressures, typically between 0.08 and 0.2 cm (0.03 and 0.075 in) of water. Proper air balance is important in controlling contaminants, but it has limitations.9 Most significant is to realize is that the relative air pressure in the spaces on either side of an opened door is essentially zero, allowing airborne contaminants to move freely between the spaces.

Table 8.2 Relative Air Pressure Between the Corridor and the Animal Rooms

Dual Corridor

Managed as a: Single Corridor Clean Soiled

+ Corridor positive to animal room - Corridor negative to animal room

+ or - Single-Corridor Conventional

In a conventional facility, the air pressure in the corridor is generally maintained positive to the animal rooms. The exceptions are facilities with mixed "conventional" and "barrier" rooms, where the air pressure in the "barrier rooms" is maintained positive to the corridor and in the "conventional rooms" is maintained negative to the corridor.

+ or - Single-Corridor Barrier

Both options are used. Following is a rationale for each:

Corridor negative to animal rooms — To keep airborne contaminants out of the animal room Corridor positive to animal rooms — To contain inadvertent contaminants

Infectious agents of concern are not ordinarily present in a barrier facility, so the rationale "to keep airborne contaminants out of the animal room," does not ordinarily apply as it does in a mixed facility. However, it must be assumed that a "break" will occur in a barrier room at some time. When this happens, the management objective is to contain the infectious agent, like in a biocontainment facility, until it can be detected and eliminated from the room and the facility.

Keeping air pressure in the corridor positive to the animal room has the added benefit of reducing animal allergens and odors in the corridors and throughout the facility.

+ or - Double-Corridor Containment

Both options are used, with negative being more common, but positive may be preferred in some situations.

Relative air pressures in animal rooms of a single-corridor facility are dependent on how the facility is to be managed: conventional, containment, or barrier. In a single-corridor conventional facility, animal rooms are typically balanced negative to the corridor, except for rooms that are designated as "barrier" or "clean" rooms, which are then balanced positive to the corridor. For this reason, the ability to automatically reverse room air pressure relative to the corridor without having to rebalance the entire system is a highly desirable feature in a single-corridor conventional animal facility. In a single-corridor containment facility, where the objective is to contain airborne contaminants, the relative air pressure in the animal rooms will be balanced negative to the corridor. The opposite does not necessarily hold for a single-corridor barrier facility, where the choice depends more on management philosophy. One philosophy calls for balancing animal rooms positive to the corridor in an effort to keep airborne contaminants out; the other calls for balancing animal rooms negative to the corridor with the objective being to contain a disease break until it can be detected and eliminated. Both management philosophies have merit, and neither is clearly right or wrong. However, one advantage to the latter is that it maintains corridors relatively free of animal allergens, which are well documented as a serious and common occupational hazard.8,40 In dual-corridor facilities, regardless of facility type, relative air pressures are typically balanced with the clean corridor positive to animal rooms and animal rooms positive to the soiled corridor; however, in some instances, both corridors may be balanced positive to the animal rooms.

Was this article helpful?

0 0

Post a comment