Heterotrophic plate count, air [HPC-A]

Published: July 8th, 2009

Revised: March 21st, 2023

This test measures total culturable bacteria contained in an air sample by culturing a measured amount of the sample on Tryptic Soy Agar and incubating at 35 °C for a minimum of 48 hr prior to analysis. At the end of the incubation period, bacterial colonies are counted and reported as Colony Forming Units per cubic metre of air (CFU/m³).

In general, air sampling for culturable heterotrophic bacteria in general and enteric (intestinal) bacterial in particular is not recommended as a companion practice in the routine evaluation of indoor environmental quality in commercial non-industrial and residential buildings.

Since the 1970s, it has been well known that indoor airborne levels of culturable bacteria and carbon dioxide are strongly positively correlated. Both, in turn, are strongly negatively correlated with air exchange rate. These observations led to the accepted conclusion that airborne culturable bacteria arise primarily from the activities of human occupants of buildings, and from the occupants themselves. Recent careful examinations of the taxonomic composition of the indoor environmental microbiota using high through-put DNA sequencing have confirmed humans as the source of a majority of indoor environmental bacteria in the general case. The predominant mechanism suggested is the aerosolization of mucosal bacteria during normal respiration, speech, coughing and sneezing. Secondarily, the aerosolization of enteric bacteria during toileting activities appears to be a normal contributor to the indoor microbiome. The recognition of humans as the primary normal emission source of indoor bacteria complicates the interpretation and negates the value of routine air sampling for these microbes.

Furthermore, routine bacterial air sampling has shown to be of little value in the elucidation of problem conditions in which components of the building system contribute to building related illness (BRI – specific diagnosed illness arising from exposure to a particular agent) or sick building syndrome (SBS – generalized symptoms unattributable to a specific agent experienced by multiple individuals in a common built environment). The characterization of these situations benefits from a thorough review of the building by an experienced assessor (informed inspection), coupled with targeted sampling from suspected source systems or materials (typically involving the collection of surface swabs or bulk materials such as humidification water or construction materials). Measurement of the bacterial surface load by swab or wipe testing for fecal bacteria is useful in the evaluation of sewage contamination resulting from flooding, and in the verification of effectiveness of remedial efforts.

This approach to the assessment of bacterial contaminants in the context of indoor environmental investigation has been advocated by relevant cognizant bodies (AIHA — American Industrial Hygiene Association; ACGIH – American Conference of Governmental Industrial Hygienists) and Health Canada.

Laboratory code: HPC-A

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