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Air Particle Counter FAQ

Scientific studies have found an association between exposure to particulate matter emissions and significant health problems. The Air Particle Counters are affordable instruments, which can monitor indoor air quality in homes and workplaces.

Aerosol particle counters are used to determine the air quality by counting and sizing the number of particles in the air. This information is useful in determining the amount of particles inside a building or in the ambient air. It also is useful in understanding the cleanliness level in a controlled environment. A common controlled environment aerosol particle counters are used is a cleanroom. Cleanrooms are used extensively in semiconductor device fabrication, biotechnology, pharmaceutics, disk drives, aerospace and other fields that are very sensitive to environmental contamination.

Different locations have varying levels of acceptable particulate concentrations, driven primarily by health and comfort concerns (i.e. homes, offices, paint booths) or contamination (i.e. hospitals, food and beverage plants, clean- rooms). Excessive levels can result in medical conditions such as Sick Building Syndrome, lower productivity, contaminated product, or all of the above. 

Maintaining acceptable air quality levels may not only lower the costs associated with downtime, but also reduce or remove costs associated with expensive fixes in the future. The first step in establishing an IAQ maintenance program is to determine if a problem currently exists.

A correct interpretation of the data requires an understanding of the test area. Is the area residential or commercial? Is the location exposed to tobacco smoke or animals? Is there construction at or near the location? A proper assessment of the environment can help narrow down the list of problem particulates.

Concentration limits vary widely according to the size and type of facility, among other variables. However, a high-level assessment can provide direction on whether or not a problem exists. The following outside air readings provide a high-level point of reference for the technician:


Figure A.

Scenario 1: The particulate levels displayed in Figure B are from a new residence (< 5 years), and do not indicate any concentrations outside of the norm. In a residential setting, particle levels are sometimes higher than outside readings due to more potential particle sources (i.e. pet dander), smaller diffusion area, and often less sophisticated filtration.


Figure B.

Scenario 2: The particulate levels displayed in Figure C are representative of an average office workspace, and do not indicate any concentrations outside of the norm. In a commercial setting, particle levels should be significantly less than outside readings due to better filtration and better dilution with outside air.


Figure C.

Scenario 3: The particulate levels in Figure D are from an older residential location with visible mold. The readings are significantly higher, and steps should be taken to remediate the mold and address the root cause of the problem.


Figure D.

Scenario 4: If the particle source in Scenario 3 is not visible, use particle size tables such as Figure E to identify possible sources. Obtain a sample of the particles and submit to a lab for further analysis.


A Cleanroom Exercise

Cleanrooms are an excellent application for a particle counter. For illustration purposes, let’s put the CEM DT-9881 to the test in evaluating an ISO Class 5 (per ISO 14644-1:1999) cleanroom. To qualify as a Class 5 cleanroom, levels cannot exceed the limits for the class in each particle size stated in the following table:

Our test is concerned with the concentration of 0.3 μm particulates in the room. Several 2-liter samples are taken from six different locations inside the cleanroom, with the following results:

The individual readings are well within the limitations for the cleanroom; however, we can take the following steps to determine the statistical validity of the readings:

Step 1: Calculate the mean average particulate concentration

M=(AC1 +AC2 +AC3+AC4+AC5+AC6)/L

995 = (674 + 1154 + 1097 + 841 + 828 + 1376) / 6

Step 2: Calculate the standard deviation of the averages

SD = ( (AC1-M)2 + ... + (AC6-M)2) / (L-1)

116 = ( (674-995)2 + (1154-995)2 + (1097-995)2 + (841-995)2 + (828-995)2 + (1376-995)2) / (6-1)

Step 3: Calculate the standard error of the mean of the averages


47.36=116/( 6)

Step 4: Establish the upper confidence limit (UCL)


The resulting mean count for all locations is within the requirements of a Class 5 cleanroom.

The DT-9881 provides particulate data over six channels on a single display, allowing the technician to view all readings at a glance. Though the cleanroom exercise focused on 0.3 μm particulates, the single display would immediately alert the technician to anomalies in other particle size concentrations.

Ambient air quality varies greatly even within a period of 24 hours, windows and doors are opened or closed, the air conditioning comes on, building or other work activity commences or any number of other factors. The air particle counter will show variation in the levels of particulate pollution throughout the day.

Airborne particle counters detect and measure particle contamination in air. Typically, they monitor particle contamination in clean environments, such as cleanrooms or minienvironments. In addition to monitoring air within a room, airborne particle counters can monitor particles in the air inside a large processing tool. 

Filter efficiency monitoring is another common application. The particle counter samples air as it enters and exits the filter. Filter efficiency is the ratio of particles trapped by the filter to the total number of particles found in the air upstream of the filter. If unattended testing is desired, the particle counter can include alarms for acceptable particle limits and send a notification when the filter fails efficiency tests. 

Cleanroom monitoring, verification, and testing are the most common applications for airborne particle counters. These particle counters sit near a process under test and constantly gather data. When contamination rises above the particle counter’s programmable limits, an audible and/or visual alarm alerts the manufacturing personnel. 

In any application, a particle counter should sample enough media so that it provides statistically valid data. Specifically,if sampling a large cleanroom, the particle counter should sample several different locations within the room. ISO documents offer suggestions for the number of sample locations: 

Number of sample locations = Area(m2 ) Therefore, if a cleanroom measures 10,000 square feet, first convert to square meters, next find the square root, then round up. 

Effective monitoring of this cleanroom should include thirty different sampling locations.

Alternatively, one can perform cleanroom testing using one of the following methods:

  • Using an Aerosol Manifold (described below)
  • Moving the particle counter from location to location
  • Demonstrating that a statistically valid sample can be taken at a single location Selecting a particular airborne particle counter requires some decisions. Channel sizes range from 0.06 μm at the smallest to several hundred μm at the largest, and depending upon the model, the number of channels and size range is either preset or programmable. Other features include different flow rates, statistics processing, automated certification modes, and almost any feature to meet most airborne particle applications.

You may be exposed to harmful levels in boiler rooms, breweries, warehouses, petroleum refineries, pulp and paper production, and steel production; around docks, blast furnaces, or coke ovens

Carbon monoxide (CO) is a deadly, colorless, odorless, poisonous gas. It is produced by the incomplete burning of various fuels, including coal, wood, charcoal, oil, kerosene, propane, and natural gas.

Carbon monoxide is harmful when breathed because it displaces oxygen in the blood and deprives the heart, brain, and other vital organs of oxygen. Large amounts of CO can overcome you in minutes without warning—causing you to lose consciousness and suffocate.

Besides tightness across the chest, initial symptoms of CO poisoning may include headache, fatigue, dizziness, drowsiness, or nausea. Sudden chest pain may occur in people with angina. During prolonged or high exposures, symptoms may worsen and include vomiting, confusion, and collapse in addition to loss of consciousness and muscle weakness. Symptoms vary widely from person to person. CO poisoning may occur sooner in those most susceptible: young children, elderly people, people with lung or heart disease, people at high altitudes, or those who already have elevated CO blood levels, such as smokers. Also, CO poisoning poses a special risk to fetuses.

CO poisoning can be reversed if caught in time. But even if you recover, acute poisoning may result in permanent damage to the parts of your body that require a lot of oxygen such as the heart and brain. Significant reproductive risk is also linked to CO. 

You may be exposed to harmful levels of CO in boiler rooms, breweries, warehouses, petroleum refineries, pulp and paper production, and steel production; around docks, blast furnaces, or coke ovens; or in one of the following occupations:

  • Welder
  • Garage mechanic
  • Firefighter
  • Carbon-black maker
  • Organic chemical synthesizer
  • Metal oxide reducer
  • Longshore worker
  • Diesel engine operator
  • Forklift operator
  • Marine terminal worker
  • Toll booth or tunnel attendant
  • Customs inspector
  • Police officer
  • Taxi driver. 


Colorless, poisonous, highly water-soluble gas with an obnoxious odor. Used in the manufacture of disinfectants, preservatives, and hundreds of industrial and consumer products such as adhesives, carpeting, decorative paneling, foam insulation, drapery, fiber and particle boards, and permanent press fabrics. Formaldehyde is a prominent factor in sick-building syndrome (SBS) as its emissions (accelerated by heat and moisture) irritate eyes and mucous membranes in nose and throat, and cause headache and dizziness. Officially named as methanal (not to be confused with methanol), it is classified as a possible carcinogen by EPA.

To capture stills/video of test locations 

Dew point is a measure of atmospheric moisture. It is the temperature to which air must be cooled to reach saturation (assuming air pressure and moisture content are constant). A higher dew point indicates more moisture present in the air. It is sometimes referred to as dew point temperature, and sometimes written as one word (dewpoint). Frost point is the dew point when temperatures are below freezing.

In simpler terms: the dew point, or frost point, is the temperature at which dew or frost will form should the air temperature fall sufficiently. Other things being equal, as the temperature falls, the relative humidity rises, reaching 100% at the dew point, at least at ground level. Dew point temperature is never greater than the air temperature, since the relative humidity cannot exceed 100%.

Wet Bulb temperature can be measured by using a thermometer with the bulb wrapped in wet muslin. The adiabatic evaporation of water from the thermometer bulb and the cooling effect is indicated by a "wet bulb temperature" lower than the "dry bulb temperature" in the air.

The rate of evaporation from the wet bandage on the bulb, and the temperature difference between the dry bulb and wet bulb, depends on the humidity of the air. The evaporation is reduced when the air contains more water vapor.

The Wet Bulb temperature is always between the Dry Bulb temperature and the Dew Point. For the wet bulb, there is a dynamic equilibrium between heat gained because the wet bulb is cooler than the surrounding air and heat lost because of evaporation. The wet bulb temperature is the temperature of an object that can be achieved through evaporative cooling, assuming good air flow and that the ambient air temperature remains the same.


By the number of channels in an air particle counter, it is defined how many various sizes of particles can be counted at a time and also defines the sizes of the various particles.

The flowrate of a particle counter is simply the rate at which its pump draws the sample air through the sample chamber. The flowrates of the particle counters commonly used in cleanroom certification traditionally have been 0.1 CFM and 1.0 CFM. 

If an airborne-particle counter is used in a dirty environment, eventually enough dirt particles will collect inside the optics that the unit will no longer function correctly. At this point the user is sometimes able to clean the unit out by running it for 24 hours with a zero filter. Often, however, the user will need to have the unit cleaned by a certified service representative. 

If you perform ISO certification in relatively dirty rooms (e.g., ISO 9), you can avoid this problem by selecting a particle counter with a high maximum concentration. This means the unit can sample from a high concentration without incurring over 5% coincidence errors. (Coincidence errors occur when the unit mistakes two small particles for one large particle.) 

The count mode defines how the particle counter displays data to the user. Concentration and Cumulative are two typical counting modes. Concentration mode samples a small volume of air then calculates the value based upon the volume setting (cm3 or ft3) in the counter. Cumulative mode allows the user to view actual particle counts as they accumulate until the end of the sample. Differential data is the particle data in a specific particle size channel; the cumulative data is the total particle data in all particle size channels. Example: Your instrument shows 200 particles in the 0.1 um channel and 35 in the 0.2 um channel. Your differential data is 200 particles > 0.1 um but < 0.2 um, and 35 particles > 0.2 um. The cumulative data is 235 total particles (200 + 35) > 0.1 um.

Using a particle counter is relatively simple; how- ever, understanding the features that distinguish counters can sometimes be a challenge. The follow- ing terms are commonly used to describe the accuracy, efficiency, and other attributes of an optical particle counter (OPC). 

Count Mode: The count mode defines how the par- ticle counter displays data to the user. Concentration and Totalize are two typical counting modes, and the Fluke 983 adds an Audio mode as well. Concen- tration mode samples a small volume of air then calculates the value based upon the volume setting (cm3 or ft3) in the counter. Totalize mode allows the user to view actual particle counts as they accumu- late until the end of the sample. Audio mode is useful when searching for areas with particle levels that exceed predefined parameters. Once levels are exceeded, the counter audibly notifies the user. 

Zero Count: Zero count is a measure of the particle counter’s accuracy, and should be taken prior to use and periodically thereafter, or when sampling error is suspected. The zero count filter is attached to the particle counter per the manufacturer’s instructions, then the counter is run for 15 minutes. The counter should not have detected more than one particle greater than 0.3 μm in a five-minute period. 

Coincidence Loss: Coincidence loss occurs when two particles cross the counter’s light beam simultaneously, creating a single pulse and resulting in a single particle count. This type of error occurs more frequently as the concentration of particles increases within the sample. Per FED-STD-209E, coincidence loss must be less than 10%. 

Counting Efficiency: The probability that the counter will sense and count a particle passing through the sample volume. Counting efficiency is a function of size up to a minimum sensitivity threshold, above which all particles are sensed and counted. A count- ing efficiency of 50% at the most sensitive threshold is typically considered optimal, and facilitates con- sistent comparisons between counts from OPCs and those of higher-resolution instruments. 

Sensitivity: A device’s ability to detect small particle sizes at a certain counting efficiency. The DT-9880/DT-9881 detects 0.3 μm at 50% counting efficiency. 

Resolution: A device’s ability to detect minute dif- ferences in particle sizes. Sensor resolution is affected by the uniformity of illumination across the sample volume, variations in flow rate, and the relative quality of the optical system. A misaligned sensor or failing laser diode will contribute to poor resolution. 

Common places where it can be of used:

  • HVAC personnel to check vents or find leaks
  • IAQ professionals to check temperature, humidity and find mold or other contamination problems
  • Filter testing
  • Checking contamination levels of spray paint booths
  • Cleanroom packaging
  • Cleanroom laundries