Healthy Indoors | 31
dors and have them agree on particle density and distribu-
tion (by size) within an environment.
This process took some time, and initially a Japanese
standard (JIS) emerged. This was then supplanted by an
international standard (ISO 21501-4), which became a
requirement for anyone supplying the cleanroom indus-
try. The standard was created by the manufacturers in
response to the above pressure and codified various tests
required to create an instrument that would provide accu-
rate air quality measurements.
instrument. Unfortunately, no such standard existed.
A client could buy instruments from two separate ven-
dors and find that particle counts between these instru-
ments varied dramatically (dramatically enough to make
it impossible to reliably transfer a manufacturing process).
Even more worrisome, two instruments from the same
vendor could also vary dramatically. So, significant pres-
sure was applied to the vendors to come up with a solution
to standardizing the manufacture of these instruments, so
that a client could buy instruments from a variety of ven-
