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Why should anyone use a laminar flow system ?
The Basic reason, of course, is to make available source of air from which most of the particulate matter has been removed, as a matter of fact 99.99% of everything air-borne down to 0.3 microns should be screened out. Now you have an environment whose air supply is free of bacteria, fungi, pollen, and practically all air-borne dirt, to use for whatever purpose such an environment will do the most good.

What actually a laminar air flow system ?
A Laminar air flow systems contains three basic elements - a blower, a high efficiency air filter, and a plenum. There may be variations on this idea - many blowers, many filters, and very large plenums, but all have the same basics. The flow is called laminar because the turbulent air upstream is changed by the filter into a straight-line flow off the downstream face of the filter.

How are the HIGH EFFICIENCY PARTICULATE AIR (HEPA filters) tested ?
The HEPA filters are tested by the DOP-dioctylphthalate-method when manufactured. DOP, a liquid plasticizer is heated to the point of vaporization and reconstituted into 0.3 micron particles to form a mono disperse aerosol. These single size particles re diluted wit air until a concentration of 100 micrograms per litre is reached, and the aerosol-air mixture is passed through the filter. The amount of penetration is measured on the downstream side with a forward light scattering photometer, giving the familiar readings of 0,03% or better, The material used to make the filter material is tested in the same way by the filter material manufacturer.

What are the advantages of the DOP in-place test over attempting a leak test with a particle counter ?
To begin with, it is not possible to use DOP with a particle counter-the concentration of aerosol is so high that the counter becomes saturated and jams, requiring a trip to the factory for repairs (except where expensive high volume samples with a diluting air source, are employed). The DOP test tells the story right away-where the leaks are and how great. There are no time delays for readouts or probe recovery. The readouts relate to filter performance and not to an abstraction. The last applies to linear photometers only; log scale instruments give only relative results

What specifications or standards apply to testing the filters in place ?
Latest ISO 14644 replaces FED Std 209E

Is this in-place filter test the same one as the filter manufacturer uses ?
NO, but it is based on the same principle. The photometer is the same, but the DOP is generated by air. This is done by using a special Laskin nozzle which creates an aerosol when it is immersed in DOP liquid and air passed through it. This aerosol, unlike that used to test the filters at time of manufacture, is not a mono disperse 0.3 micron cloud, but is poly dispersed, ranging from 0.32 microns to 1 microns, averaging approximately 0.45 microns. It is also possible to create a similar poly disperse aerosol by heat atmosphere by purging with an inert gas such as nitrogen 0r argon. The concentration of DOP so obtained will be very high, and can be used for testing air streams 5,000 to 50,000 CFM

What is the procedure for in-place test ?
Actually, it is rather simple. A DOP generator introduces aerosol as a challenge agent into the upstream side to the filter system. A sample of this aerosol-air mixture is taken on the upstream side of the filter with a linear forward light scattering photometer for use as a baseline to compare the downstream readings. since the upstream concentration is known, and the photometer is linear, the downstream samples may be read out in percent of concentration, as recommended in the standards and specifications. Any leak greater than 0.01% of the upstream concentration is too much, and the spot should be marked for repairs in actuality, there should be no leaks around the filter seals, and in practice, readings at the face of the filter are usually in the 0.004% to 0.008% range.

What does percent of penetration mean ?
It is the amount of challenge aerosol, detected on the downstream side of a filter by a linear photometer, measured against the concentration of challenge aerosol on the upstream side of the filter. with a base of 100% for the upstream concentration, and a reading of 0.01% penetration on the downstream side, the indication is that the filter is 99.99% efficient with respect to that particular concentration.

Why was 0.01% established as a significant ?
Studies were made of the effects of different size leaks on typical types of work such as photo resist, micro welds, sterile transfers, and the like where it was shown that a leak of 0.01% was border at which reject rates began to increase and excessive contamination was detected.

Why can't the leak test be run with ambient air within a room ?
Because there usually isn't enough particulate matter in the ambient to be used as a challenge. The minimum specified by AACC-2T is 3x10 power 5 particles of plus 0.5 microns.

If plates to obtain bacterial counts are used, must the in-place test still be run ?
If Yes, because as in the answer above, the amount of bacteria in the ambient will vary from time to time and give erroneous data. Besides , the test should be run to eliminate any possible leaks greater than 0.001%.

Can leaks not be detected by agar plates ?
With an LAF situation this becomes chancy when a full scan procedure is adopted. with the stationary procedures both flat on the tables or normal to the air flow, detection is quite nearly impossible.

How about impingement devices ?
These are certainly effective, but a biologic challenge becomes essential if filter validation is sought. For in -house routine checks this is an approved procedure. For validation this would be both inaccurate and compromising of the sterile work area.

Is DOP aerosol harmful to the filters ?
If the number of pharmaceutical houses using the test for their filters is any criterion, the answer is most certainly yes. it is also used by the FDA, and of course by many other facets of industry around the world. For example, every atomic energy commission in the free world uses DOP and the photometer for in-place filter testing, the U.S. navy has a special adaption of the basic test equipment for shipboard use and it would seem by its universal acceptance that any-one with a high-efficiency air filtration system needs one to insure the integrity of their system.

What happens to the DOP once it is in the system ?
The DOP particles, being a liquid aerosol, impinge on the surfaces of the glass fibres making up the filter, and are trapped there, Each tiny droplet spreads out to form a mono-molecular layer on the surface, and since the surface area is extremely large, there is little chance of saturation. it would take an extremely high concentration of DOP, generated very close to the filter surface, to cause wetting out. Once the DOP is trapped, it stays on the fibres and is not released.

When DOP is used to test a system, does the room have to be wiped down ?
No, since a comparatively small amount of DOP is released into the clean area, and most of this is swept out in the air stream. some amy impinge on room surfaces, but his amount is negligible since the particles are so small they are subject to Brownian movement an remain suspended until they are finally trapped by the filter material.

Has DOP test been accepted by the pharmaceutical industry ?
If the number of pharmaceutical houses using the test for their filters is any criterion, the answer is most certainly yes. it is also used by the FDA, and of course by many other facets of industry around the world. For example, every atomic energy commission in the free world uses DOP and the photometer for in-place filter testing, the U.S. navy has a special adaption of the basic test equipment for shipboard use and it would seem by its universal acceptance that any-one with a high-efficiency air filtration system needs one to insure the integrity of their system.

with reference to Fed. Std. 209b, What is the relationship of the various classes of clean rooms ( 100, 10 000, and 100 000 ) to the 0.01% figure ?
There is no relationship at all. For example- a clean room with no one in it might be class 100, while the addition of personnel will send it into other higher categories. however, suppose the filters supplying a room were thoroughly checked for leaks, and all significant leaks were sealed, this room is operating at maximum efficiency with respect to the filters, and since the filters are mechanical in nature, there is nothing the operator can do to change conditions in this area.
The achievement of a particular class would relate to the number of air changes per hour as well as the scavenging design of the system.

Can a particle counter not be used to determine the class of the room ?
All the particle counter can do is give a reading of the number of particles per cubic foot at any one point at one time. It doesn't indicate what the particles are, or where they come from. the stepwise checkout of a clean room should be first to run the DOP test to find the significant leaks and eliminate them. If the subsequent introduction of personnel and equipment causes generation of particulate matter which may be of concern to the successful operation of the room one more step to be taken.
A sample of the room air should be taken with a millipore-type filter after which the particles collected may be sized and counted, but and this is the most important- they may now be identified. This identification will enable the room operator to establish the source of contamination is harmful to the work being done. No particle counter in existence can do this for you. As a matter of fact, to quote from paragraph 60 FED, STD, 209A referring to the DOP test," .... in most instances, this type of facility check will be more meaningful than any attempt to make an actual particle count, due to the extremely low particle concentrations expected, plus the difficulty of taking a representative sample in a laminar air flow stream".

But isn't a particle count important data ?
It is useful from the standpoint of routine surveillance. Deviations from action levels established for the facility, if not corrected by routine maintenance procedure, would signal a need for filter validations and we are back to square one -DOP.

But, why can't leak test be carried out with a particle counters ?
You can, but the basic function of a particle counter is to count and size particle, usually by sampling at a rather slow rate of 0.1 cfm with LAF, iso kinetic sampling would be mandatory. scanning a filter seal in this manner would be a slow process involving great operator skill, and what is more important the reading obtained would have no relation to the established level for a significant.

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