ISO is the International Organization for Standardization, based in Geneva Switzerland^{1}. You may have heard of ISO 9000/9001 certification, which covers general quality management systems, but there are many more ISO standards – more than 10,000 total standards available at the time of this writing (September 2013), covering everything from IT systems configuration to milk production to statistical methods.^{2}
A bummer, to me at least, is that the standards are not free, and they are not cheap. Copies of standards in electronic (PDF) or paper form can be purchased from the ISO website (store.iso.org) in Swiss Francs, or from ANSI^{3} or IEST^{4} in American Dollars. The 18 page ISO 146441 will cost you around $100. Note that the copy is for individual use only, not to be shared. Needless to say, to get yourself a complete set of ISO standards would cost you a pretty penny.
This post deals with ISO standard 146441 “Classification of air cleanliness.” There is currently (September 2013) two editions available: the official current standard 19990501 and a draft revision, which “…is a draft circulated for comment and approval. It is therefore subject to change and may not be referred to as an international standard until published as such,” available since December 02, 2010. Note that the 1999 version is 18 pages, and the revision is 37 pages. I’ll discuss the differences at the end of this post.
Overview
ISO 146441 is part 1 of 10 parts of the “Cleanrooms and associated controlled environments” standard, which includes (current revision after colon)^{5}:
 146441:1999 Classification of air cleanliness
 146442:2000 Specifications for testing and monitoring to prove continued compliance with ISO 146441
 146443:2005 Test methods
 146444:2001 Design, construction, and startup
 146445:2004: Operations
 146446:2007 Vocabulary
 146447:2004 Separative devices (clean air hoods, gloveboxes, isolators, and minienvironments)
 146448:2013 Classification of air cleanliness by chemical concentration
 146449:2012 Classification of surface cleanliness by particle concentration
 1464410:2013 Classification of surface cleanliness by chemical concentration
Note the standard’s titles changed somewhat in the revision, and 810 don’t exist in the 1999 version.
146441:1999 includes a foreword, an introduction, sections on Scope, Definitions, Classification, and Demonstration of compliance, and a bunch of Annexes. Let’s deal with each in turn.
Foreword
The foreword describes a bit about what ISO is all about and lists the parts of 14644, of which there were only 7 in 1999, and the titles were a bit different. The foreword also notes that Annexes B and C are integral to the standard, but annexes A, D, E, and F are for information only.
Introduction
Quick description of cleanroom purpose (to control airborne contamination) and scope of 146441 (assigns ISO classification levels, prescribes testing method and procedure for determining the concentration of particles). The Introduction also notes classifications are for a particular range of particle sizes only, but indicates there is a protocol for determining cleanliness levels for smaller or larger particles, if that is required by a particular process.
 Scope
Mostly a repetition of the Introduction, but does state the range of threshold (lower limit) particle sizes is 0.1 µm to 5.0 µm. The standard calls particles smaller than 0.1 µm “ultrafine” particles and larger than 5.0 µm “macroparticles” and states populations of these may be quantified in terms of U descriptions and M descriptors, respectively, which we’ll see again below.
 Definitions
Various terms are defined, notably:
 Particle: solid or liquid object which is sized 0.1 µm to 5.0 µm
 Particle size: diameter of the particle
 Particle size distribution: “cumulative distribution of particle concentration as a function of particle size”
 U descriptor: “measured or specified concentration of particles per cubic metre of air, including the ultrafine particles”
 M descriptor: “measured or specified concentration of macroparticles per cubic metre of air, expressed in terms of the equivalent diameter that is characteristic of the measurement method used”
 Classification
There are 9 ISO classes defined in the standard, as seen here^{6}:
Class 
maximum particles/m^{3} 

≥0.1 µm 
≥0.2 µm 
≥0.3 µm 
≥0.5 µm 
≥1 µm 
≥5 µm 

ISO 1 
10 
2 

ISO 2 
100 
24 
10 
4 

ISO 3 
1,000 
237 
102 
35 
8 

ISO 4 
10,000 
2,370 
1,020 
352 
83 

ISO 5 
100,000 
23,700 
10,200 
3,520 
832 
29 
ISO 6 
1.0×10^{6} 
237,000 
102,000 
35,200 
8,320 
293 
ISO 7 
352,000 
83,200 
2,930 

ISO 8 
3,520,000 
832,000 
29,300 

ISO 9 
35,200,000 
8,320,000 
293,000 
The maximum particles per cubic meter (m^{3}) is calculated for each class number N as:
C_{N} = 10^{N} x (0.1/D)^{2.08}
Where
D is the consider particle size, in µm
0.1 is a constant with the units of µm
If you’re bored and want to recreate this in Excel, you can do it like this:
N 
0.1 
0.2 
0.3 
0.5 
1 
5 
1 
10 
2 
1 
0 
0 
0 
2 
100 
24 
10 
4 
1 
0 
3 
1000 
237 
102 
35 
8 
0 
4 
10000 
2365 
1018 
352 
83 
3 
5 
100000 
23651 
10176 
3517 
832 
29 
6 
1000000 
236514 
101763 
35168 
8318 
293 
7 
10000000 
2365144 
1017625 
351676 
83176 
2925 
8 
100000000 
23651441 
10176251 
3516757 
831764 
29251 
9 
1000000000 
236514412 
101762505 
35167572 
8317638 
292511 
Copy the formula in B2 to all cells: =(10^$A2)*(0.1/B$1)^2.08
Designation: to fully designate the classification of a given room, use 1) the classification number expressed as “ISO Class N”, 2) the occupancy state to which the classification applies (either atrest or operational), and 3) the considered particles size(s) and related threshold.
E.g.: ISO Class 8; operational state; 1 µm (832,000 particles/m^{3}), 5 µm (29,300 particles/m^{3})
This section ends by noting that if you’re going to measure more than one size in testing, but sure to measure a size at least 1.5 times the size of the smaller particle. E.g. if you measure 0.2 µm particles, don’t measure 0.3 µm particles, measure 0.5 µm particles.
 Demonstration of compliance
This section includes a reference to Annex B, which covers the test method (details below), and the statement that the average particle concentrations and the 95% upper confidence limits shall be calculated. The average particle concentration shall not exceed the C_{N} value from the table and if you have at least two samples but not more than nine, you need to verify the 95% upper confidence limit is also not greater than C_{N}.
The test report should include:
a) the name and address of the testing organization, and the date the test was performed
b) the number and year of the standard used (e.g. ISO 146441:1999)
c) identification of the cleanroom tested and details on sampling locations
d) the specified design criteria, including ISO classification, occupancy states, and consider particle sizes
e) details of the test method used, including identification of the instruments employed with calibration certification
f) Test results, including measurement data for all sampling locations
Annex A Graphical illustration of the classes of Table 1 (Informative)
Includes a different way of looking at the classifications, but this was removed from the later revision.
Annex B Determination of particulate cleanliness classification using a discreteparticlecounting, lightscattering instrument
This is the annex that was reference in Section 4 as covering the standard test method. It turns out the standard test method is using a “discreteparticlecounting (DPC), lightscattering” instrument to determine the concentration of airport particles, equal to or larger than the specified sizes. The instrument should of course be calibrated prior to test.
Pretest checks may include airflow volume or velocity tests, air pressure difference tests, containment leakage tests, and filter leakage tests – i.e. standard cleanroom and HVAC IOQ tests should all be complete, naturally.
Minimum number of sample locations should be derived from the following equation (rounded up to the nearest whole number):
N_{L}=√A
where A is the area of the cleanroom in square meters
Sample locations should then be evenly distributed throughout the cleanroom and positioned at the height of work activity. If only one location is required, a minimum of three samples at that location should be taken.
The sample volume (in liters) per sample location is calculated as:
V_{s}=(20/C_{n,m})x1,000
Where C_{n,m} is the class limit (number of particles/m^{3}) for the largest particle size specified in the class (e.g. for ISO Class 5, 0.5 µm particles, C_{5,0.5}=3,520; for ISO Class 4, 0.2 µm particles, C_{4,0.2}=2,370) and 20 is the defined number of particles that could be counted if the particle concentration were at the class limit.
BUT the minimum sample volume should be at least 2 liters, and the minimum sampling time should be at least 1 min.
When the number of sample locations is less than ten, compute the overall mean of the averages, the standard deviation, and the 95% upper confidence limit. For one location or more than nine, the 95% upper confidence limit is not applicable.
The cleanroom is deemed to have met the specified air cleanliness classification if the averages of the particle concentrations measured at each of the locations and, when applicable, the 95% upper confidence limit calculated do not exceed the concentration limit.
Annex C Statistical treatment of particle concentration data
Includes equations for:
Average particle concentration at location x_{i}: = (x_{i,1}+x_{i,2}+…+x_{i,n})/n where x_{i,n} is the nth measurement at location i and n is the total number of samples taken at location i.
Overall mean of the averages: = (x_{i,1}+x_{i,2}+…+x_{i,m})/m where x_{i,n} is the average at location m and m is the total number of locations
Standard deviation s=√[(x_{i,1})^{2}+(x_{i,2})^{2}+ … +(x_{i,m})^{2}]/(m1)
95% upper confidence limit (UCL) = +t_{0.95}(s/√m) where t_{0.95} comes from the following table:
m 
2 
3 
4 
5 
6 
79 
t 
6.3 
2.9 
2.4 
2.1 
2.0 
1.9 
Annex D Worked examples of classification calculations
Includes worked examples for calculating all of the above – not going to repeat it here.
Annex E Considerations for the counting and sizing of particles outside the range applicable for classification
This annex deals with cases where your process requirements have criteria for particles smaller than 0.1 µm or larger than 5 µm. It is not terribly interesting, just defining the U descriptor and M descriptor formats and referring to IEST guidance for test methods.
The U descriptor is used for < 0.1 µm (“ultrafine”) particles: U(x; y), where x is the maximum permitted concentration per m^{3} and y is the size in µm at which the particle counter counts such particles with 50% counting efficiency. IESTGCC1002 for test methods.
The M descriptor is used for > 5 µm (“macro”) particles: M(a; b); c, where a is the maximum permitted concentration per m^{3}, b is the equivalent diameter associated with the method for measuring in µm, and c is the specified measurement method. IESTGCC1003 for test methods.
Annex F Sequential sampling procedure
Annex F covers sequential sampling, a procedure that evaluates sample results in realtime, to reduce the number of samples required for a passing or failing result.
Sequential sampling is based on upper and lower limit calculations as follows:
Upper limit: C = 3.96 + 1.03E
Lower limit: C = 3.96 + 103E
Where C is the observed count, and E is the expected count.
Graphically it looks like this:
By plotting sampling results in real time, you can theoretically get to a Pass or Fail more quickly.
2010 Revision
Foreword
Two additional parts to 14644 are noted:
 Part 8 Classification of airborne molecular contamination
 Part 9 Classification of surface cleanliness by particle concentration
ISO 14698, Cleanrooms and associated controlled environments – Biocontamination control is also noted:
 Part 1 General principles and methods
 Part 2 Evaluation and interpretation of biocontamination data
Introduction
Shortened from 5 to 2 paragraphs, but essentially the same.
 Scope
Essentially the same, explicitly calls out the use of discreteparticle airborne counting and sizing instruments now.
 Normative reference
New section – references ISO 212014:2007, Determination of particle size distribution – Single particle light interaction methods – Part 4: Light scattering airborne particle counter for clean spaces
 Definitions
Added definition for unidirectional airflow (“controlled airflow through the entire crosssection of a clean zone with a steady velocity and approximately parallel airstream”)
 Classification
No major change
 Demonstration of compliance
Removes criteria for 95% UCL
Annex A Graphical illustration of the classes of Table 1 has been moved to Annex E in the new version.
Annex A Reference method for the determination of air cleanliness classification by particle concentration
Analogous to Annex B from the 1999 revision, but with some important changes:
 Establishment of sampling locations is no longer N_{L}=√A, but based on the following table:
Which, if you compare to the previous, is quite a few more sample locations in some cases:
Area 
Sqrt(Area) 
ISO 
Diff 
1 
1 
1 
0 
2 
1 
1 
0 
4 
2 
2 
0 
6 
2 
3 
1 
8 
3 
4 
1 
10 
3 
5 
2 
24 
5 
6 
1 
28 
5 
7 
2 
32 
6 
8 
2 
36 
6 
9 
3 
52 
7 
10 
3 
56 
7 
11 
4 
64 
8 
12 
4 
68 
8 
13 
5 
72 
8 
14 
6 
76 
9 
15 
6 
104 
10 
16 
6 
108 
10 
17 
7 
116 
11 
18 
7 
148 
12 
19 
7 
156 
12 
20 
8 
192 
14 
21 
7 
232 
15 
22 
7 
276 
17 
23 
6 
352 
19 
24 
5 
436 
21 
25 
4 
500 
22 
26 
4 
The number of sample locations in Table A.1 are based on area units of 1, 2, and 4 m^{2} to achieve 95% confidence that at least 90% of the total area does not exceed the class limit.
 Instead of 95% UCL, concentration per cubic meter is calculated:
C_{i} = (* 1,000)/V_{t}
Where is the average number of particles at location i
V_{t} is the adopted single sample volume in liters
Annex B Examples of classification calculation
Contains a lot more examples, almost 8 pages worth, but I guess you’ll have to get your own copy to get access to that.
Annex C Statistical treatment of particle concentration data has been removed.
Annex C Considerations for the counting and sizing of particles outside the range applicable for classification
No signification changes to the previous versions Annex E
Annex D Sequential sampling procedure
This annex has the same basic content, but no includes 2 detailed examples. Email me for details.
References:
^{1}http://www.iso.org/iso/home/about.htm
^{3}http://webstore.ansi.org/RecordDetail.aspx?sku=ISO+146441%3A1999
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