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Air quality is defined as the degree to which the ambient air in a particular environment is pollution-free. The environment can be the clean room of a pharmaceutical company, food producing industry, hospital operating rooms/wards and beverage companies. Air quality monitoring is a measurement of the amount of contaminants in the air; and it gives a description of the healthiness and safety of the atmosphere of a given area at a given time. The quality of air in any environment could be regarded as being good and free from pollution or contamination (inclusive of biological, physical and chemical contaminants) when it has been assessed by measuring a number of indicators of pollution such as the presence or absence of contaminating microbes (e.g., moulds and bacteria). The air is inundated with innumerable microorganisms including spoilage organisms, pathogens and spores of microbes that find their way into factories, clean areas and in the manufacturing areas of drugs and other products where they cause economic damage.

The monitoring of the quality of air in a pharmaceutical production company or industry is critical to the production of products with standard quality. Thus, clean air standards should be a top priority for all manufacturing industries since it helps to ensure the safety and comfort of workers in commercial and industrial environments as well as ensure the provision of sterile environments to produce quality products. Bad air could be a major source of microbial contamination in manufacturing industries. All the clean rooms or sterile areas in such organizations should be monitored from time to time to ensure that the quality of air in these environments is devoid of microbes capable of causing any form of contamination or spoilage of finished products and other raw materials. Airborne bacterial and fungal cells and even their spores may be present in droplets as bioaerosols, and as very small particles that stay suspended for long periods in the air. These microbial spores can also be suspended in the air as larger clumps and aggregates that settle rapidly onto manufacturing surfaces from where they can cause contamination of the finished products. Airborne bacterial and fungal spores can also be an important source of contamination and infection in medical facilities. These organisms can also contaminate sensitive manufacturing operations, equipment and other instrument involved in the manufacturing processes.

It is noteworthy that these microbes thrive in these environments especially in facilities where biopharmaceuticals and other medical devices are manufactured because the organization failed to imbibe the practice of regular monitoring of the air entering their facility so that airborne microbes could be detected and kept at bay. The microbiological monitoring of the air in facilities where biopharmaceuticals, food and medical devices are produced is essential for the total success of any manufacturing process. This is important because if these microbes find their way into these products, the finished products could serve as routes (fomites) via which infectious disease agents can spread from person to person and from place to place. Microorganisms (including spoilage microbes and pathogens) are ubiquitous; and thus, environmental monitoring techniques in manufacturing areas should be instituted and imbibed to keep these organisms at bay. Such environmental monitoring of the quality of air should include the use of active air monitoring techniques such as air settling or contact culture media plates and other equipment that help to filter the air entering the clean areas.

There are commercially available air monitoring instrument/equipment that detect and monitor a variety of gases, chemicals, and particulates in ambient air especially in the environment and even in industries. Air monitoring equipment/instrument incorporates photoionization detectors (PIDs), flame ionization detectors (FIDs), single-gas meters, and multi-gas detectors that help researchers to monitor in real-time the quality of air in a particular environment (Figure 1). The amount of pollution in air can be measured in several ways either passively or actively. In passive monitoring of air quality, tubes known as diffusion tubes are usually used to absorb a specific amount of pollutant from ambient air in the outdoor environment. These diffusion tubes (which are usually monitored for about 2 to 4 weeks) are later sent to the laboratory for physicochemical and microbiological analysis to determine the level of contamination in the sampled air. Active air sampling techniques makes use of instrument/equipment that pulls ambient air across series of filters over a certain period.

These filters are later sent to the laboratory for analysis as aforementioned to determine the level of contamination in the air. Some countries have well established regulatory requirements and internationally recognized standards for biocontamination control in clean rooms and other controlled environments in hospitals, food factories and pharmaceutical companies. And it is required that factories and industries involved in the production of food and other materials or products that are ingestible by man and animals imbibe and follow these standards to ensure the presence of quality air in their manufacturing environment. Air pollution comes from many different sources including factories, power plants, cars, buses, planes, trucks, and train, windblown dust and from volcanic eruptions. The pollution emitted from these sources can affect the quality of the air in each area, and thus affect the quality of the finished product being produced in such environment.

Figure 1. An air monitoring equipment. Photo courtesy:

Air can also play a central role as a reservoir for microorganisms (e.g., bacteria, moulds, and microbial spores) in controlled environments such as clean rooms of manufacturing facilities and operating theatres in hospitals. Therefore, the regular monitoring of the microbial load in the air of such environments is useful to measure air quality and identify critical situations that require urgent attention. The quality of ambient air can be monitored at individual level, organizational level or even at national levels. In some developed economies, advanced equipment stationed at strategic areas are used to monitor the quality of air in those environments (Figure 2). Air quality monitoring stations are usually built and stationed at strategic areas where they measure and determine the quality of air in a particular environment. Data from such stations help the authorities to know the level of contamination in the air of a particular area. This equipment generate data from time to time; and the extrapolation or analysis of such data generated at real-time helps the authorities to know the level of air contamination or pollution in a particular area.       

Figure 2. An air quality monitoring station. Photo courtesy:


Passive air monitoring is usually done using special type of Petri dish plates known as settle plates. These culture plates are standard Petri dishes (measuring about 90 mm in diameter) that contain appropriate culture media that are opened and exposed for a given time and then incubated to allow visible colonies to develop and be counted (Figure 3). The settle plates or sedimentation plates are usually exposed for a period of 30 to 60 minutes prior to incubation at ambient temperature; and plates for passive air monitoring is not exposed for more than four hours. The results from a settle plate are expressed in CFU/plate/time or in CFU/m2/hour. CFU is the acronym for colony forming unit. These culture plates help to monitor viable microbes that sediment out of the air and settle onto a surface of the culture plate over the time of exposure. Nevertheless, settle plates are very limited in their application since they are only capable of monitoring viable biological particles that sediment out of the air and settle onto a surface over the time of exposure. The results obtained in using the settle plates are not quantitative because they do not detect smaller particles or droplets suspended in the air and they cannot also sample specific volumes of air. They are also vulnerable to interference and contamination from non-airborne sources during usage.

Figure 3. Ready to use Settle Plates for Microbial Air Monitoring. Photo courtesy:

Settle plates are most suitable for the qualitative analysis of airborne microbes such as moulds and bacteria. They are also suitable for personnel monitoring such as monitoring the presence of microbes on personnel wears and hand gloves. Culture media plates for passive air monitoring should include growth media such as soya bean casein digest agar and Sabouraud dextrose agar (SDA) that support the growth of a wide variety of microbes. In addition, the agar (growth) medium in the plates may deteriorate if they are exposed for a long time. Settle plates may easily become overgrown in heavily contaminated conditions and this makes result interpretation to be difficult. Despite the several disadvantages of using the settle plates for the passive monitoring of air quality, this method of air monitoring is usually cheap and easy to use.

The settle plates are inexpensive and easy to use, and they require no special equipment. They are useful for qualitative analysis of airborne microorganisms inclusive of bacteria and fungi. The data they produce may detect underlying trends in airborne contamination, and such data may also provide early warning of possible contamination in any given environment. Settle plates are also useful for direct monitoring of possible airborne contamination of specific surfaces. In an environment such as a low-risk food factory or a hospital or research facility, settle plates may provide an adequate means of monitoring biological air quality. The best use of settle plate for air monitoring is in those test methods for which the use of other, more efficient sampling methods may not be possible or may have limited applicability.


Active air monitoring also involves the use of settle plates or sedimentation culture plates (as is applicable in passive air sampling) and contact plates for the monitoring of air quality. It involves extracting a set volume of air within a given environment into a calibrated sampler which is then passed onto the surface on an agar plate. The agar culture plates are then incubated at ambient temperature to allow the growth of colonies of airborne microbes including moulds and bacteria. There is an increasing need for effective air monitoring in all clean areas with filtered air where airborne microorganisms may contaminate or affect industrial products and processes such as in biopharmaceutical companies, hospitals and food industries. Active air monitoring with contact plates requires the use of a microbiological air sampler to physically draw a certain, pre-determined volume of air and pass it over the agar culture plate on the air sampling device also known as particle collection devices (Figure 4). As aforementioned, the exposed culture media plate is removed from the microbiological air sampler and directly incubated at ambient temperature for microbial growth. The resulting colonies which give an estimate of the number of colony forming units in the sampled air are counted and reported quantitatively per taxon identified in colony forming units (CFU) per cubic meter of air (CFU/m³). Contact plates are usually used in the active air monitoring techniques.

Figure 4. Illustration of particle collection devices used for active air sampling.After the sampling period has been concluded, carefully remove the sampling medium from the device, taking care not to touch the surface of the medium. Photo courtesy:

The contact plates are used to measure microbial contamination of work surfaces in manufacturing or clean rooms of food industries and pharmaceutical companies. The surface of the medium is pressed against a flat sampling surface to pick up any microorganisms that may have settled onto the location by operator contact or from the environment. Active air monitoring requires the use of a microbiological air sampler to physically draw a known volume of air over, or through a particle collection device. There are various types of these particle collection devices but only the impingers particle collection device and the impactor particle collection device or samplers are highlighted in this section.

  • Impingers or impinger samplers use a liquid medium for particle collection during active air sampling. In the use of this device, the sampled air is drawn by a suction pump through a narrow inlet tube into a small flask containing the collection culture medium. This accelerates the air towards the surface of the collection medium and the flow rate is determined by the diameter of the inlet tube. When the air hits the surface of the liquid, it changes direction abruptly and any suspended particles are impinged into the collection culture media. The collection culture medium is then cultured to enumerate viable microorganisms once the sampling is complete. The result of this technique is quantitative since the sample volume can be calculated using the flow rate and sampling time. The use of impingers for active air monitoring has some disadvantages associated with its use. Most impingers are usually made of glass, and this is undesirable in food and pharmaceutical production sites. The impingement of the sampled air into culture media may also damage some microbial cells and thus affect the viability of the result. Also, the longtime of sampling may allow some cells to multiply in the liquid collection medium.
  • Impactors or impactor samplers use a solid or adhesive culture medium (e.g. agar) for particle collection unlike the impinger devices that use liquid culture medium in active air sampling technique. They are commonly used for active air sampling because of their convenience. In a typical impactor sampler, air is drawn into a sampling head by a pump or fan and accelerated, usually through a perforated plate (sieve samplers), or through a narrow slit (slit samplers). This produces laminar air flow onto the collection surface that is usually a standard agar culture plate or contact plate filled with a suitable agar medium that favour microbial growth. The velocity of the air is determined by the diameter of the holes in sieve samplers and the width of the slit in slit samplers. When the air hits the collection surface of the culture medium, it makes a tangential change of direction and any suspended particles are thrown out by the force of inertia, thus allowing the particles to impact onto the collection surface. When the correct volume of air has been passed through the sampling head, the agar culture plate is removed and incubated at ambient temperature for microbial growth. After incubation, the visible microbial growth (colonies) on the agar plate is counted and reported as CFU/m³. This gives a direct quantitative estimate of the number of colony forming units in the sampled air. The impactor/impaction samplers have several benefits over the impinger samplers. It is convenient to use, and it makes use of pre-poured solid culture media plates. They are also able to handle higher flow rates and the large sample volumes necessary to monitor air quality in clean rooms where the number of microbes present is likely to be very low. Proper care must be taken not to allow agar plates to remain in the sampler heads for too long, or the culture medium may dry out and deteriorate. Microbial cells may also be damaged by mechanical stress during the sampling process and lose viability. Impaction samplers offer benefits in terms of convenience and pre-poured, gamma-irradiated contact plates and standard Petri dishes from specialist suppliers can be used with them to minimize the risk of contamination and variation. 


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