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When selecting a measuring instrument for monitoring air quality, first you should clarify which variables must be determined (e.g., air temperature, humidity, particle size and concentration). If it is necessary to determine particle sizes in μm and to determine the quantity or concentration of certain sized particles in the air, for example, for the classification of cleanrooms, then an optical particle counter (OPC) is required.
Handheld particle counters are ideal for a discontinuous, random testing of the clean room environments 
in the field of research, development and production. Due to the low weight and small size, the mobile particle counters are flexibly used test devices for monitoring of the air quality. By means of adjustable test intervals of several hours, quasi-stationary test conditions can also be generated with mobile particle counters. However, the devices are not suitable for long-term measurement and inline monitoring. For the latter, stationary particle counters are the right choice. Permanently installed measuring systems can be used for clean room qualification as well as for monitoring of the air quality in operation rooms or production areas in the pharmaceutical industry. It is also possible to determine the degrees of filtration as well as to carry out leak tests.
An important standard is EN ISO 14644-1:2015, which is used to determine the clean room classes. The standard defines clean rooms according to the detectable particle sizes and number of particles. The classification of the particles takes place according to fixed size classes, which are between 0.1 ... 5 μm.
Measuring devices should represent the required size classes in order to be able to carry out an assessment according to the standard. In addition to EN ISO 14644, ISO 21501-4 describes the requirements and calibration regulations for scattered light particle counters. Important parameters are zero count rate, counting efficiency, size resolution and flow rate.
Airborne particles can be detected by various methods with the help of mobile or stationary measurement techniques. In addition to gravimetric methods, there are such known methods as: the extinction measurement, the laser diffraction or the scattered light method. The latter is used by the manufacturers in many particle-measuring devices. The common standards also frequently refer in the specifications, to the devices with corresponding measuring technology. In the scattered light method, the particles pass through the measuring volume individually and generate scattered light pulses in the detection zone. Depending on the particle size, these pulses are larger or smaller and are thus a measure for the particle diameter. The particle concentration can also be determined in a combination with the flow rate.
The zero-count rate indicates the number of particles measured in a channel when the particle-free air flows through the measuring volume. Particle-free air can, for example, be generated by means of a zero filter. Ideally, the number of particles in the smallest channel should be zero for particle-free air. If the values deviate strongly, this usually means that the measuring optics is contaminated. In this case, maintenance by the manufacturer, if necessary with adjustment and recalibration, should be carried out.
The counting efficiency also compares the measured number of particles within a size channel between a particle counter and identical reference devices. The counting efficiency is channel-dependent and, for the smallest channel, cannot be less than (50 +/- 20) % according to ISO 21501-4.
In many mobile hand-held units, the flow rate is 2.83 l / min or 0.1 cfm or 28.3 l / min or 1 cfm. 
This magnitude has its origin in the American US Federal Standard Norm. The flow rate has a direct influence on the test interval as the test times and the volumes tested depend on this. The probe openings are designed mostly as isokinetic sample receivers. The probe geometry ensures that the air flow is neither too much accelerated nor too much delayed. The air particles are thereby guided in a uniform laminar flow and "fall" into the measuring chamber.
In case of the temporal and spatial interaction of several light impulses, caused by a plenty of particles in the measuring volume of the meter, as a rule, two errors occur. The concentration is then displayed as too low and the particle size – as too large. The devices cannot distinguish, whether the light scattering is due to one large particle or to two particles of half the size. The higher the particle concentration in the air, the greater the possible influence of coincidence errors.
For the purpose of evaluation and documentation, particle counters with a sufficiently large storage capacity and evaluation software are recommended. With these auxiliary tools, measurement results can be automatically put on paper after each measurement.
Dust – it is an “enemy“ of the humanity, it can be found anywhere, and cannot be destroyed, 
no matter what measures are taken. The dangerous part about it is that dust is not easy to detect and see. The particles of it are of such a tiny size that we do not even notice how much we inhale - a particle counter can show it. In the case of dust, it is like that: the smaller the particles, the more dangerous they are. Besides just a hygienic side, there is one more to consider: dust provokes very hard diseases, and that is why the level of it must be monitored and measured by a particle counter all the time, especially in the work place. At the factories, plants, mining, textile, chemical industry, construction, in the agricultural sector, in various workshops, the amount of dust is sometimes exceeding any possible limits, but nothing can be done, since it is a part of the production process. Depending on its origin, dust may be organic, mineral, chemical, inorganic and others. Airborne dust particles are easily transported in the air and inevitable get inside of the human body. Partially, a protective role is performed by our body, since nose and throat are so-called filters that prevent harmful substances from penetrating deeper into the other systems. This function is helpful when it goes about bigger particles, which are inhalable, but not small enough to get though “natural filters”. A real problem occurs when it goes about respirable dust, since accumulation of these particles in the lungs and other systems, may over some time bring to serious devastating diseases, like cancer, allergies, asthma and similar. In this case a particle counter is always a important test instruments to protect your health.
Both, employers and employees should work in cooperation in order to reduce the exposure to dust during the working process. Protective masks and clothing should be worn regularly, there should be permanent monitoring with the help of respirable particle counter and search for safer ways of production, better materials, introduction of new technologies and equipment, sufficient cleaning, the main sources of dust should be detected.