A negative air pressure unit must have at least how many prefilters?

A negative air pressure unit is designed to create a controlled environment by maintaining lower air pressure inside a work area compared to the outside. This is crucial for containing contaminants, such as asbestos fibers, and preventing them from escaping into adjoining spaces.

Having at least one prefilter is essential for the effective operation of the unit. The prefilter captures larger particles before they can reach the more delicate and finer main filter. This primary function helps maintain the efficiency and longevity of the main filter, which is responsible for capturing smaller and potentially harmful airborne fibers. By ensuring that there is at least one prefilter, the system can help protect workers and maintain a safer job site environment.

The specified requirement of one prefilter ensures that the negative air pressure unit operates effectively, thereby reducing the risk of asbestos exposure to workers and preventing contamination of surrounding areas.