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Microbiological contamination of HVAC filters.

Duling M; Berardinelli S; Calvert C; Lawrence R; Coffey C
American Industrial Hygiene Conference and Exposition, May 10-15, 2003, Dallas, Texas. Fairfax, VA: American Industrial Hygiene Association, 2003 May; :62
Heating, ventilating, and air-conditioning (HVAC) filters that were under investigation for filtration performance became contaminated with microbiological growth after the humidification system within the air-handling unit (AHU) malfunctioned. The HVAC system is a constant volume system operating 24 hours a day, 7 days a week that serves a biomedical research facility. The HVAC filters have a two-year recommended service life and were rated at 90% efficiency as per American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE) 52.1 standard. These filters were also approved by the manufacturer for use in high humidity, turbulent airflow, and elevated operating temperature environments. The AHU humidification system malfunctioned causing some of the filters to become water-logged, restricting airflow. As a result, the filters became contaminated with a visible black microbiological growth within 24 hours of becoming saturated. Samples collected from the ventilation filters with sterile swabs were cultured. The predominate fungal species found on the contaminated filters were Aureobasidium pullulans, Epicoccum nigrum, Penicillium, Cladosporium, Alternaria alternata, and other yeasts. The bacterial species identified were Staphylococcus, Rhodococcus, Micrococcus luteus, and Bacillus. Filters with no visible contamination from the same filter bank were also tested and no detectible microbial growth was found. Within 48 hours of the humidification system malfunction, the system was repaired and normal airflow resumed through the filters. The black, gelatinous microbial contamination found on the filters was completely desiccated, leaving a slight black stain on the filtration material. It is recommended that contaminated filters be replaced, but due to the ongoing filtration study this was delayed. This incident documents the role humidification can play in microbial contamination. This incident also documents the unpredictable, spontaneous nature of contamination that can occur within AHUs with potential adverse impact on indoor air quality. This contamination can go unnoticed without routine inspection of the ventilation system.
Microbiology; Heating-systems; Air-conditioning; Ventilation; Filtration; Filters; Humidity; Air-flow; Indoor-air-pollution; Microorganisms; Indoor-environmental-quality
Publication Date
Document Type
Fiscal Year
NIOSH Division
Priority Area
Work Environment and Workforce: Indoor Environment
Source Name
American Industrial Hygiene Conference and Exposition, May 10-15, 2003, Dallas, Texas
Page last reviewed: May 11, 2023
Content source: National Institute for Occupational Safety and Health Education and Information Division