N.12 – Number of location and measurement for a reliable microbiological air testing



The people involved in a daily microbiology air sampling activity know quite well the frequent difficulties to obtain reliable results even in the same closed environment where the conditions are constant.

The reasons are several: uneven distribution of micro-organisms in the air, inconstant biological activity of micro-organisms, influence of operator, air sampler, media, etc.

The goal is to obtain results that are comparable with the previous data.

We can receive some help looking at what ISO14644 and Federal Standard 209E say for airborne particulate cleanliness classes for Clean Room. It is in fact possible to extrapolate the physical facts of the electronics world into the microbiology world.

The responsible of the hygiene in the production department of an agro-food-dairy industry may found in the cited document the correct compromise to obtain more reliable results to improve the food shelf-life and to reduce the risk of customer’s complaints with consequent recall.

The production activity is so different from Company to Company that it is not possible to give absolute figures. Everybody has to adjust the general indications to his specific case and needs.


ISO 14644 and Federal Standard 209E

The documents ISO 14644 and Federal Standard 209E give precise normatives and formulas to be applied to the Clean Room in the electronic industry for airborne particulate.

Both standard specify:

(a) the Minimum Number of Locations within the Clean Room at which measurements of particle concentration must be made;

(b) the Minimum Volume of Air that must be sampled during each measurement;

(c) the Minimum Number of Measurements that must be taken at each location;

(d) the Minimum Number of Measurements that must be taken throughout the Clean Room;

(e) the statistical methods that must be followed in interpreting the measurements.


Minimum Number of Measurements of viable airborne micro-organisms that should be taken at each location

Depending on several factors like:

(a) previous experience

(b) movement of personnel

(c) frequence of door opening

(d) classification of the environment

(e) type of sampling (e.g.: interval sampling)

the responsible of sampling will decide if a single sample for location is suitable or if it is better to take 3 samples and then to obtain an average figure.


Minimum Number of Measurements of viable airborne micro-organisms that should be taken throughout the closed environment

Both ISO 14644 and Federal Standard 209E allow just one measurement to be made at any one location but specify a larger minimum number of measurements per Clean Room or Clean Zone.

Fed-Std-209E requires a minimum of five individual measurements of particle concentration per Clean Room. These five measurements can be divided in any manner between the minimum required two locations – four at the first location and one at the second or three and two.

ISO 14644 requires only three individual measurements per Clean Room. When only one location is required, these three measurements will, of course, be at that one location.

This principle for particle concentration can be applied for viable airborne micro-organisms.


Volume of air that must be sampled during each measurement

In this case the ISO 14644 and Fed-Std-209E cannot be of help because the microbiologist is bounded by the number of colonies that can be counted on a single agar plate. As a rule of thumb, considering that the number of colony forming unit (cfu) per plate, at the end of incubation, should not be more than approx. 100 per contact plate (approx. 300 per standard Petri dish), the suggested volumes to be aspirated by air sampler are reported :



Clean Room  Min 500 Max 1000-1800

Grey Area Min 500 Max 1000

Biotechnology Min 500 Max 1000

Operating theatre Min 250-500 Max 1000-1800

High Risk Area Min 500 Max 1000

Low Risk Area Min 100-Max 200 300

Intensive Treatment Unit Min 500 Max1000

Isolation suite Min 500 Max 1000-1800

Aseptic filling Min 500 Max 1000

Home building Min 100-200 Max 250-300

Office Min 100- Max 200 300

Public building Min 50-100-Max 200 250

Bus, Train, Airplane, Car Min 100 Max 200

Wastewater treat.plant aeros Min 50-Max 100 200

Heating Ventil.Air Conditioning Min 100- Max 200 250



Federal Standard 209E

ISO 14644