N.19 – Industrial air handlers microbiological monitoring


The microbiological efficiency of the filters used in buildings is an essential parameter that should be regularly monitored to be sure that the contamination is under control.

This Application Note reports a simple method used at 3M Development Laboratory in St. Paul, Minnesota, USA to measure the filter efficiency for fungi of industrial air handlers. The test can help to determine Indoor Air Quality in office buildings.


The material

Multiple heads microbial air sampler 

Malt Extract Agar Contact plates

Incubator at 25°C and 37°C.


The protocol

The Malt Extract Agar Contact plates are transferred to the aspirating heads of the air sampler.

Five hundreds litres of air are collected downstream of the filter.

Hundreds litres of air are collected upstream of the filter.

The test should be repeated for at least 20 times to obtain a data base and an average of the results.

For each sample set, one Contact plate is cultured at 25°C to simulate the temperature outside of the body and the other plate is cultured at 37°C to simulate the body temperature. The incubation time is 3-5 days.

The obtained number of Colony Forming Units (CFU’s per 1000 litres of air) after incubation are the basis for the efficiency calculation.


Efficiency calculation

Example: If the upstream sample has a count of 100 CFU and the downstream sample 50 CFU, the filter efficiency is 50%.



The test can be used to measure filter efficiency of air handlers used to cool, heat and move air in commercial space.



Reprinted from Bioscience World – The Newsletter of Bioscience International – Volume 7, N.1, Autumn 2005. Page 4. “3M T