Cleanroom classification and standardization

Cleanrooms are classified according to the number and size of particles permitted per volume of air. Large numbers like "class 100" or "class 1000" refer to FED-STD-209E, and denote the number of particles of size 0.5 μm or larger permitted per cubic foot of air. The standard also allows interpolation; for example SNOLAB is maintained as a class 2000 cleanroom.

A discrete, light-scattering airborne particle counter is used to determine the concentration of airborne particles, equal to and larger than the specified sizes, at designated sampling locations.

Small numbers refer to ISO 14644-1 standards, which specify the decimal logarithm of the number of particles 0.1 μm or larger permitted per m3 of air. So, for example, an ISO class 5 cleanroom has at most 105 particles/m3.

Both FS 209E and ISO 14644-1 assume log-log relationships between particle size and particle concentration. For that reason, zero particle concentration does not exist. Some classes do not require testing some particle sizes, because the concentration is too low or too high to be practical to test for, but such blanks should not be read as zero.

Because 1 m3 is about 35 ft3, the two standards are mostly equivalent when measuring 0.5 μm particles, although the testing standards differ. Ordinary room air is around class 1,000,000 or ISO 9.

ISO 14644-1 and ISO 14698

ISO 14644-1 and ISO 14698 are non-governmental standards developed by the International Organization for Standardization (ISO). The former applies to clean rooms in general (see table below); the latter to cleanrooms where biocontamination may be an issue.

ISO 14644-1 defines the maximum concentration of particles per class and per particle size with the following formula

${\displaystyle {\text{C}}_{\text{N}}=10^{\text{N}}\left({\frac {0.1}{\text{D}}}\right)^{2.08}}$

Where ${\displaystyle {\text{C}}_{\text{N}}}$ is the maximum concentration of particles in a volume of 1m${\displaystyle ^{3}}$ of airborne particles that are equal to, or larger, than the considered particle size which is rounded to the nearest whole number, using no more than three significant figures, ${\displaystyle {\text{N}}}$ is the ISO class number, ${\displaystyle {\text{D}}}$ is the size of the particle in ${\displaystyle \mu }$m and 0.1 is a constant expressed in ${\displaystyle \mu }$m. The result for standard particle sizes is expressed in the following table.

Class	Maximum particles/m3 a						FED STD 209E equivalent
	≥0.1 μm	≥0.2 μm	≥0.3 μm	≥0.5 μm	≥1 μm	≥5 μm
ISO 1	10b	d	d	d	d	e
ISO 2	100	24b	10b	d	d	e
ISO 3	1,000	237	102	35b	d	e	Class 1
ISO 4	10,000	2,370	1,020	352	83b	e	Class 10
ISO 5	100,000	23,700	10,200	3,520	832	d,e,f	Class 100
ISO 6	1,000,000	237,000	102,000	35,200	8,320	293	Class 1,000
ISO 7	c	c	c	352,000	83,200	2,930	Class 10,000
ISO 8	c	c	c	3,520,000	832,000	29,300	Class 100,000
ISO 9	c	c	c	35,200,000	8,320,000	293,000	Room air
a All concentrations in the table are cumulative, e.g. for ISO Class 5, the 10 200 particles shown at 0,3 μm include all particles equal to and greater than this size. b These concentrations will lead to large air sample volumes for classification. Sequential sampling procedure may be applied; see Annex D. c Concentration limits are not applicable in this region of the table due to very high particle concentration. d Sampling and statistical limitations for particles in low concentrations make classification inappropriate. e Sample collection limitations for both particles in low concentrations and sizes greater than 1 μm make classification at this particle size inappropriate, due to potential particle losses in the sampling system. f In order to specify this particle size in association with ISO Class 5, the macroparticle descriptor M may be adapted and used in conjunction with at least one other particle size. (See C.7.)

US FED STD 209E

US FED-STD-209E was a United States federal standard. It was officially cancelled by the General Services Administration on November 29, 2001, but is still widely used.

Class	Maximum particles/ft3					ISO equivalent
	≥0.1 μm	≥0.2 μm	≥0.3 μm	≥0.5 μm	≥5 μm
1	35	7.5	3	1	0.007	ISO 3
10	350	75	30	10	0.07	ISO 4
100	3,500	750	300	100	0.7	ISO 5
1,000	35,000	7,500	3000	1,000	7	ISO 6
10,000	350,000	75,000	30,000	10,000	70	ISO 7
100,000	3.5×106	750,000	300,000	100,000	830	ISO 8

EU GMP classification

EU GMP guidelines are more stringent than others, requiring cleanrooms to meet particle counts at operation (during manufacturing process) and at rest (when manufacturing process is not carried out, but room AHU is on).

Class	Maximum particles/m3
	At Rest		In Operation
	0.5 μm	5 μm	0.5 μm	5 μm
Grade A	3,520	20	3520	20
Grade B	3,520	29	352,000	2,900
Grade C	352,000	2,900	3,520,000	29,000
Grade D	3,520,000	29,000	Not defined	Not defined

BS 5295

BS 5295 is a British Standard.

Class	Maximum particles/m3
	≥0.5 μm	≥1 μm	≥5 μm	≥10 μm	≥25 μm
Class 1	3,000		0	0	0
Class 2	300,000		2,000	30
Class 3		1,000,000	20,000	4,000	300
Class 4			200,000	40,000	4,000

BS 5295 Class 1 also requires that the greatest particle present in any sample can not exceed 5 μm. BS 5295 has been superseded, withdrawn since the year 2007 and replaced with "BS EN ISO 14644-6:2007".