11/10/97

Has anyone had experience with eliminating excessive amounts of dust in

their EM rooms?

I moved into a newly renovated suite of rooms over a year ago. Since that

time we have been plagued by dust and dirt. It has become almost impossible

to maintain uncontaminated grids or negatives and prints without dust or

scratches. For example you cannot leave a negative on a light box overnight

because it is covered by a thin film of dust by the next day.

The Engineering department had the ductwork cleaned and higher efficiency

filters installed on the fan units supplying air to the suite during

renovations. They claim the supply air is the same quality as that in my

previous location in another building (where excessive dust was not a

problem).

An outside consultant has monitored the rooms and found particle counts

between 20,000 and 50,000 in the 0.5 micron size range, per cubic foot.

They recommend installing HEPA filters on all supply air ducts to reach

class 1,000 conditions.

The Engineering department is reluctant to install 10 or more HEPA filter

units due to the cost and the necessary ductwork reworking. They claim the

supply air is clean and the filters will not solve the problem. They would

like to install electrostatic particle precipitators in each room to scrub

the air clean.

Does anybody have any experience with electrostatic precipitators and which

type is best?

Is there a standard for airborne particles within a microscopy facility?

Is the class 1,000 condition recommended by the consultant excessive?

We are about to begin renovations for our light microscope area which will

have a number of digital imaging stations in addition to film and video.

What recommendations can I make to engineering in the design of the HVAC

system to address this issue during construction?

Sorry for the excessive bandwidth of this message but I wanted to be as

explicit as possible.

Frank Macaluso tel: 718-430-3547

Analytical Imaging Facility fax: 718-430-8996

Albert Einstein College of Medicine e-mail: macaluso@aecom.yu.edu

1300 Morris Park Avenue

Bronx, NY 10461

thought occured to me. You might ask your Engineers if the return air

passes through a "plenum" above the ceiling or below the floor. This is

just a fancy word for saying that the open space (above or below) acts

as the return duct. If this is the case, and it often is, that might be

the source of a large part of your problem. You've probably seen, or

can imagine, the cleanliness of these areas! If so, regardless of

filtration systems, you should probably start with a dedicated HVAC

system for the microscopy rooms and incorporating duct work for both

supply and return sides. Then add filtration as necessary to achieve

acceptable cleanliness. It may be that, in your circumstances, Class

1000 is more than really necessary but you're probably in for some

expensive retrofitting in any event.

Stephen A. Shaffer

MicroDataware

sshaffer@microdataware.com (business)

concentrations of ozone and contribute to rapid deterioration of rubber

parts of equipment. Best test is the lifetime of a stretched rubber

band; compare to control prep'n perhaps at home.

Air-conditioner type filters treated with a polyethylene glycol spray

may stop the dirt to a great extent without going to the expense of HEPA

filters. Under the conditions you describe, the initial cost might not

be as big a burden as the cost of frequent replacements!

Jerry Freed

jjfreed@netreach.net

50K particles in the 0.5 micron range per cubic foot. From experience, I

would be surprised if you get a visible film of dust observed on a light

background overnight, but perhaps a visible film of dust on a dark shiny

negative as you describe sounds reasonable, especially when viewed by

scattered light. You definitely shouldn't get anything you can feel with

your fingers overnight.

I recommend the article "Particulate Fallout Predictions for Clean Rooms"

by Otto Hamburg in The Jour. of Environmental Sciences, May/June 1982

pp.15-20.

You can set out "witness plates" for various lengths of time and observe

the amount of fallout you get and compare these results with environments

of different load suspensions. Your witness plates should be identical to

the substrates of interest....TEM grids and negatives, and be in the same

place in your room. This can make a big difference. Particulate

characteristics, air flow, humidity, substrates, etc. will be different

from one person's environment to another's, meaning you cannot readily

expect one person's experiences to be applicable to yours.

We have a particle prep lab that ranges between 1K-50K particles >= 0.5

micron per cubic foot, depending where you are and what phase the moon is

in. We then have < Class 10 areas embedded within that where samples may

be exposed to air. (Assuming that my laser counter's calibration is still

good). Within the 20-50K areas that would be somewhat similar to yours, I

would not expect to find significant dust accumulation on negatives

overnight. However, I would guess that if the air was very dry, and there

was plenty of air flow across the negatives, perhaps they would

electrostatically collect enough by the next day to be a problem. I would

not leave TEM grids out in this air if I was concerned about contamination.

Typical unfiltered office and home air where there is no smoke has about a

few billion particles >= 0.5 microns per cubic foot. So if you have a

rough idea of how rapidly dust accumulates there, you can apply the rules

of 10 to get a VERY ROUGH idea of what you would get in the same

environment if it had a few orders of magnitude less particulate. Again, I

repeat, very rough.

I also have a story. I once worked in a cleanroom that had about 10,000

particles >= 0.5 micron per cubic foot in most areas (Class 10,000), and

some Class 10 areas. About once every several days, I would find an

enormous amount of dust on everything in the Class 10,000 areas and the

clean room would have to be thoroughly cleaned. It appeared to be caused

by "burps" in the plenum structure, caused by momentary pressure

differentials, that allowed dust that had accumulated there to burp through

seams in the ceiling. Solution: seal the seams. So your average dust

suspension measurements may not reflect momentary, but significant, surges,

for whatever reason.

Other useful references are:

FED-STD-209E Federal Standard for Clean Room and Work Station Requirements

for Controlled Environments

MIL-STD 1246 Military Standard for Product Cleanliness Levels

Hope this helps.

Cynthia J. Zeissler

Physical Scientist

National Institute of Standards and Technology

cynthia.zeissler@nist.gov

301-975-3910

They are very efficient at removing particles, but the air has to flow

through the plates, so in your case they would have to be installed in the

AC inputs. (It's different conditions when the particles are continuously

introduced than when they are stirred up off the floor.) Furthermore, the

ozone production already mentioned is a problem; activated charcoal filters

after the precipitators could solve it. If you go this route, there will

have to be regular maintenance of both the precipitators & the filters.

Good luck.

Yours,

Bill Tivol

tivol@wadsworth.org