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Good Filters, Bad Filters: What Happens When Dirty Filters Are Left in Place

Everyone knows that keeping clean filters in an air handling system is important to maintaining good indoor air quality. What everyone may not know is that leaving dirty filters in place can have an equally negative impact on indoor air quality, becoming a significant source of indoor air pollution.

A recently published review of studies conducted by independent researchers throughout the last twenty years conclusively demonstrates that dirty filters are a key contributor to poor indoor air quality. The studies show that the problem was not with the filters themselves, their installation, or their location in the system, but with the dirt accumulated upon them. The dust and debris that accumulate on a filter trap odors, harbor microorganisms, and foster chemical reactions that produce air borne irritants.

“The dust and debris that accumulate on a filter trap odors, harbor microorganisms, and foster chemical reactions that produce air borne irritants.”

When filters are new and the dust load is very small, no pollution is created. As filters age and begin to ‘load up’, their pollution potential grows. Eventually, critical mass is acquired and a filter will cease cleaning the air and will instead start polluting it. The problem is exacerbated the longer a dirty filter is left in place.

According to these studies, the presence of dirty filters causes noticeable impacts. Building occupants expressed an overall dissatisfaction with indoor air quality, noting odors, dryness, and staleness. Adverse health effects are apparent, including increased nasal irritation, more frequent headaches, and decreased productivity. Continued exposure to the system aggravates these symptoms. In short, the presence of used filters creates characteristics indicative of Sick Building Syndrome. In these studies, removing the dirty filters and replacing them with clean filters immediately resulted in the perception of improved air quality and reduced the number of complaints.

Increasing outdoor air flow through the system in an attempt to dilute the symptoms and improve indoor air had no effect. In fact, in some studies, increased air flow over the dirty filters actually increased the odor strength of the filter. The only solution to controlling pollution caused by used filters is to replace them. Finally, the studies report that anywhere dirt is allowed to accumulate in a system also becomes a pollution source. Common culprits are the ducts themselves, coils, humidifiers, and rotary heat exchangers. As clean filters act to protect these areas also, regular filter change out becomes doubly important. Regular observation and seasonal cleaning of these components will control dirt and eliminate pollution.

Everyone has always known that regular filter replacement and good housekeeping contribute to good indoor air quality. As studies have now definitively shown, failure to keep up with these maintenance measures results in indoor air pollution.

– Julie K. Good, PE, LEED AP