Volume 3, Number 1, Feb. 1981, p.1

Air Pollution: Monitoring and Filtering

Spot Determinations

The detection and elimination of air pollutants in the museum environment and the study of their effects on works of art are topics of growing interest to the conservator.

Unfortunately, accurate continuous monitoring equipment is beyond the scope of any museum. Even a modest testing facility such as those operated by any of the many government air quality agencies such as the Air Resources Board [ARB], and the Los Angeles Air Quality Management District [LAAQMD]), can cost a quarter of a million dollars. And for this money you will be able to monitor seven pollution species. However, if one is interested in "spot determinations" test kits do exist which are very accurate.

LaMotte Chemical Products Company, Box 329, Chesterton, Maryland, 21620, produces the Air Pollution AM-61 Test Kit which costs about $400. The kit provides the capability to measure five pollution factors: total oxidants, nitrogen dioxide, sulfur dioxide, lead, and ammonia. The capability, being modular, can be expanded to twenty chemical species.

This is an ideal field testing set-up for any museum, conservation center, or interested private conservator, and finds it best utilization in detecting important upwind stationary sources of air pollution. Thus its value to a regional center is obvious to the member museums. At the Norton Simon Museum, with the AM-61 capability. we were able to follow one of the worst photochemical smog episodes in ten years (October 1-3, 1980) in the Los Angeles Basin with data nearly identical to that released by the local air pollution agency. In addition we were able to demonstrate virtually total elimination in the museum's air conditioning plant.

Air Filtering Material

There is a new air filtering material on the market from NV Verto of Rotterdam which should be of interest to conservators, building engineers and those collaborating in the planning of new environmental control systems. It is described in a 23 page booklet available from the distributor, but the major features can be summarized here.

The material known as "Filtrete" is a non-woven mat of polypropylene fibers which have been charged electrostatically to a charge density of 90 nano-Coulombs per square centimeter. The filters made from this material capture particles by a combination of electrostatic attraction and physical trapping in the case of fine and coarse particles respectively. Since they actually attract dust rather than sieve it from the airstream, such filters can be made with a very open structure which allows a higher rate of airflow and less pressure drop on passing through the filter. Air conditioning and heating systems using such filters will require less energy to recirculate the same amount of air. New systems may be designed to utilize narrower ducts allowing for less obtrusive installation in historical buildings.

The filtering efficiency for very fine dust [which is not well retained by simple filters] is similar to that of an electrostatic precipitator. However, no source of power is needed in the case of the Filtrete material. According to tests by the manufacturer, the tiny located charges are stable over a period of at least three years even in the presence of high humidity which would normally allow electrical leakage to neutralize them.

Many dust particles acquire an electrical charge through friction or other means. These are pulled to an area of localized charge on the fiber with the opposite polarity.

Those particles without a charge are electrically polarized when they enter the strong electrical field between two regions on the fiber which possess opposite charges. They cling there in a manner analogous to the attraction of an un-magnetized nail to a magnet. Small particles are very strongly influenced by this attraction as their mass becomes less significant.

When the efficiency of the electrostatic filtering is reduced by a dense coating of particles on the fiber surfaces the porosity of the mat will have bean reduced as well. At this point, physical trapping of the smaller particles becomes possible in the constricted spaces between fibers and a consistent efficiency is maintained. Generally finer dusts are less easily removed from surfaces. For this reason, it is desirable from a conservation standpoint to use filters with a high efficiency for fine particles. The Filtrete material can provide a consistently high efficiency with a greatly reduced air flow resistance or, for the same flow resistance, greater efficiency for fine particles. In addition to dust, bacteria are very effectively trapped by this filter medium. The Filtrete literature provides a useful description of the various standard test methods employed both in Europe and the U. S. along with eighteen references to original work. Their address is: Filtrete Corp., 179 Goffle Rd., Hawthorne, N.J. 07506; (201) 423-1813.

Gas Chromatography (GC) for Direct Air Analysis

The December 1980 issue of American Laboratory contains two articles which may be of interest to the conservator/chemist.

"Performance testing of ambient air analyzers for S02," by Purdue and McElroy and "A portable photoionization GC for direct air analysis" by Barker and Leveson. The article by Barker and Leveson is perhaps the more valuable since it introduces a gas chromatography unit with an improved photoionization detector (PID) that will give equally sensitive values in field usage as the best laboratory monitoring equipment. One potential value to this unit which should outweigh its moderate cost, is its applied use in the detection of toxic gases in the larger museum conservation laboratories. The Photovac model 10A10 portable photoionization GC would make a valuable contribution to a regional center's capability to serve other local conservation labs in the specific detection of threshold limit values [TLVs] of common solvents and other reagents. It is hoped that normal extraction capabilities would be adequate, but these capabilities such as fume hoods probably tend to be used for the most toxic substances while the moderate substances with high TLV values circulate in the lab.

Air Purification of Particulate Matter

For labs interested in small scale air purification of particulate matter, TEPCO, a division of Energy Resources Corporation may have a unit for your requirements.

These units are electrostatic in nature and are effective for particle sizes ranging down to 0.03 microns. This results in particulate removal of almost all ash, soot, dust, and (large particle) condensation nuclei, as well as perhaps half Aitken nuclei. This means that these units will significantly reduce even photochemical smog. TEPCO air cleaners begin with a 47 pound unit which will clean up to 400 cubic feet per minute and goes up to a 5,000 CFM model. There is even a portable model called the Rollaway for air cleaning in small areas of heavy air-particulate contamination. For further information contact: TEPCO, Inc., 3609 Marquis Drive, Garland, Texas 75042.