Engineering Controls

Animal facilities should be designed to incorporate engineering controls to the extent feasible. The most likely limitations to the introduction of engineering controls are the constraints imposed by the existing facility and the need for significant capital investment. In existing facilities, the costs of retrofitting may be prohibitive, not least because to do so may mean business operations have to stop temporarily.

One of the problems associated with evaluating engineering controls is that there is relatively little evidence that specific building (ventilation or architect design) systems that may contribute to reduced total exposure to allergen actually help to prevent allergy. For instance, there is a general assumption that ventilation design contributes to reductions in particle counts and thereby leads to less allergy, but there is, until now, little published evidence regarding the biological significance of the various forms of technology, other than the indirect evidence provided that prevalence of symptoms is lower among low-exposed, than among medium- or high-exposed workers.18

Separation

Concerning allergen spread within facilities, the first consideration should be separation of the potential population at risk from the hazard. The hazard is not just the animals, but, most importantly, the allergen they produce. At the facility level, this can be interpreted as construction of the facility away from nonanimal workers. Within the facility, this can be achieved by clear segregation of work with animals from other work, such as administration and rest facilities. Boundaries can be established and, where necessary, access controls introduced to prevent exposure of people who are not directly involved in animal work or support of the animal areas. Even within the areas that animal work is directly carried out, it may be possible to widely separate the majority of workers from the areas where the potential exposure is highest (e.g., the animal-holding rooms). Separation can be facilitated by a two-corridor system where this is feasible, one corridor used for "clean," the other for "dirty"' activities.

Separation should also be considered in the specific context of the allergens. Where allergen exposure is foreseeable despite the absence of animals, this should be controlled. Key areas include places where cage waste is handled, in the laundry, and at the exhaust points from ventilation systems. Ventilation inlets, especially those to clean and nonanimal areas, should not be placed in proximity to or downwind from outlets from contaminated areas.

General Ventilation

Facility ventilation, including the control of temperature and humidity, contributes to the general control of allergen. General ventilation has an important influence on the microenvironment in the animal cage, and it is this factor more than allergen control that has usually been more influential in the development of ventilation systems. Task-specific local exhaust ventilation, rather than general ventilation, is the principal control method, being more effective, less costly, and probably easier to implement.

Studies have demonstrated that an increase in air-change frequency can reduce allergen levels.60 However, many different approaches to the general ventilation of animal facilities have been shown to be effective, not all of which are dependent on expensive high-frequency air changes.61 One-way airflow systems with sliding perforated screens, behind which are the cage racks and exhaust vents, have been shown to effectively draw allergens behind the screens, leaving minimal allergen levels in the room.62

Pressure gradients are an important adjunct to the control of allergen spread; these are a common feature of animal facilities. However, there are potential conflicts between the gradients required to protect the animals' health and those required to protect human health. In general, it is desirable to use a gradient that minimizes spread of allergen (and pathogens) from cage-cleaning areas and into "clean areas," such as offices and restrooms. Negative pressure "sinks" adjacent to animal-holding rooms can also be used.

Increasing the relative humidity has been shown to reduce the levels of airborne allergen.63,64 Presumably, in conditions of higher humidity, particles weigh more, are more adhesive, and will settle more readily. High humidity is, however, more uncomfortable for workers, increases growth of molds and mites, and may have an adverse effect on animal health.

Exhaust air from animal areas will be contaminated with allergens (and possibly pathogenic organisms). In some circumstances, it will be necessary or desirable to filter exhaust air. Exhaust air should not be recirculated without filtration. Exhaust air and filters are important sources of fugitive exposure. Controls should be in place to prevent exposure to exhaust air and to minimize the risk to people involved in the maintenance of ventilation systems and changing of filters.

Task Ventilation

Task ventilation, or local exhaust ventilation, is one of the most important control measures. These systems remove allergen at the source and can be designed (usually at relatively low cost) to accommodate the tasks with potential for the highest exposure. They contribute to reductions in both the spread of allergens and other contaminants.

Task ventilation includes biosafety cabinets, fume cupboards, and ventilated workstations that use downdraft or backdraft systems.65 Often, these systems have the advantages of being mobile and suitable for installation in established facilities. However, it may be difficult to demonstrate the effectiveness of these systems, especially novel designs, such as downdraft benches that are reliant upon undisturbed laminar flow. It is easier to demonstrate that novel ventilation systems function effectively when not in use than when used by operators under normal work conditions. For instance, covering too great a proportion of the ventilated surface of a downdraft table is likely to reduce the effectiveness of the exhaust system. If the effectiveness of the ventilation system cannot be confirmed under operational conditions, then it should not be relied upon as a primary control measure.

Automation

New technology is enabling the automation of many tasks. This benefit of automation is especially interesting where the tasks are labor-intensive and pose significant risk. These risks may be high allergen levels or other factors, such as exposure to potentially harmful pathogens or test substances or ergonom-ical risks. For instance, when cleaning cages and bottles, the risks will be due to both allergens and ergonomics. Automated cage cleaning and waste handling systems have now been introduced in some animal facilities.61 Automated cage cleaning systems have been shown to greatly reduce ambient levels of allergen and personal exposure of operators under normal operating conditions.59

Cage Systems

The introduction of filters to conventional open-top cages is associated with reductions in allergen concentrations of greater than 75% (see Figure 7.3).17,55,58,66 Individually ventilated cage systems are now widely available,67 and these have been shown to effectively reduce background aeroallergen levels in a number of studies.55,68,69 The most impressive reductions in aeroallergen levels in undisturbed animal rooms — almost 100% — arise when the system is operated with the cages under negative pressure.55 69 70

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