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14.2 Threshold Limit Values 433

Pollutant TLV-TWA Measured concentration C i /TLV i
CO 25 10 0.40
5,000 1500 0.30
CO 2
Welding fumes 5 3.5 0.70

Then the normalized concentration can be calculated as follows

C i
X
C N ¼ ¼ 0:40 þ 0:30 þ 0:70 ¼ 1:4 [ 1:0
TLV i
Since the C N > 1, it is not safe for the workers to be there daily (8 h).

14.2.2 Clean Room

Clean room is a special indoor environment where the number of airborne particles is
controlled to avoid contamination of the products. The cleanness of the clean room is
specified by the International Standard Organization (ISO) Standard ISO 146441-1.
3
The upper limit of particle number concentration (number/m )is defined as,

2:08
!
0:1
N
c ¼ 10 ð14:2Þ

p
d p
3
where c ¼ upper limit of particle number concentration, number/m

p
N ¼ the clean room class number (1, 2,…,9)

d ¼ threshold particle diameter in μm, and they are 0.1, 0.2, 0.3, 0.5, 1 and
p
5 μm.
According to ISO 146441-1, in a class N cleanroom, the number concentration

of particles greater than d cannot exceed c . To help appreciate the cleanness of a

p
p
cleanroom, one can compare it with a typical room in an office building, which is
close to a Class 9 clean room [2].
Example 14.2: Cleanroom class calculation
2
An ISO 146441-1 Class 2 cleanroom is 3 m high with a total floor area of 100 m ,
what is the maximum amount of particles that are larger than 100 nm in diameter?
Solution Substitute

d ¼ 100 nm ¼ 0:1 lm; N ¼ 2
p